Research work on tuber starches

During the period under report, I have been involved in four major projects in the institute, two ad-hoc projects ( viz. physicochemical and structural characterization of minor root crop starches and potentialities of tuber starches in tablets and capsules), advanced training in starch chemistry in the UK and sabbatical at Sweden. Five students have registered under my supervision for their Ph.D degree. The work over the period covered a wide variety of aspects which include studies on the cooking quality of cassava, extraction process for starch from minor tuber crops, basic studies on the starches of tropical root and tuber crops, production and properties of starch derivatives for application in food and industry and interaction with industry and entrepreneurs for extension of technologies.

The salient findings and results are outlined below under the different headings.

  1. 1. Cooking quality of cassava

    For acceptance of cassava varieties for culinary purposes, cooking quality of the tubers is of paramount importance.. A cassava variety is considered to possess good cooking quality if the tubers are easily cooked and the cooked tubers should be soft, dry and mealy. The poor cooking ones take long time to cook and are hard, glassy and watery on cooking. Many varieties suffer from poor quality and in addition, the quality is also influenced by environmental factors. Since starch is the major component in cassava, it has a definite role in deciding the cooking quality of tuber. Hence the role of starch and its behaviour in presence of other biochemical components were examined in detail to work out possible relationship with cooking quality. For the study, selected varieties having diverse cooking quality were used. Their cooking quality, properties of the extracted starch and the behaviour of the starch in presence of other components were investigated.

    1. Physical changes during cooking.

      First of all, a physical basis for assessment of cooking quality was developed. Cooking is accompanied by imbibation of water during gelatinisation of starch leading to swelling of starch granules . Hence the changes occurring in the weight and volume of tubers during cooking was examined at definite time intervals. The softness was measured using a fruit hardness tester. The mealiness of cooked tuber was visually compared. The results from a number of varieties revealed that the cooked tubers could be classified into three broad categories based on the change in volume and weight and softness after cooking (Table 1).

      Table 1. Physical changes in cassava tubers on cooking

      Category Cooking quality Appearance of cooked tubers Δ Vol. Δ Wt. Softness*
      1 Good Soft, mealy and dry +5 -15% +5 -15% 5 – 15
      2 Poor Hard, glassy, sticky and moist 0- -10% 0- -10% >20
      3 Poor Disintegrated + >30% + > 15% < 3

      * Values in the fruit hardness tester (Scale 3 – 30)

      It is obvious that too less or too much swelling of starch granules lead to poor quality.

      The starch granules of good cooking varieties swell enough to retain the granular integrity which provides the mealiness. The poor cooking ones either do not swell to the desired level or swell too much. Too less swelling results in non-mealiness and hardness while too much swelling results in disintegration of the granular structure and release of the broken starch molecules leading to cohesiveness and non-mealiness.

      The test thus provides a physical method for determining the cooking quality of tubers.

      1. Starch content

        Since starch is the major biochemical component in cassava, the role of starch content was examined. The results with determination of quantity of starch in tubers having different cooking quality showed that varieties having low starch content suffer from poor quality. Low starch content means that enough starch molecules are not available to provide mealiness. It is also possible that since the number of granules are less, they swell too much and suffer breakdown leading to non-mealiness. This is all the more true for under-mature tubers which contain only less starch and always disntegrate on cooking. It is difficult to arrive at the minimum starch content required for good cooking quality, since some varieties having good starch content also showed poor quality. But a minimum starch of 25% starch could be considered imperative for providing cookability.

      2. Starch properties

        Since it was clear that starch content alone did not decide the cooking quality, a study of the starch properties was also undertaken. Starch extracted from different varieties by standard method was examined in detail for various properties and the results are discussed below.

        1. Granule size

          Though microscopic studies did not show any wide variability in average granule size among the different varieties, (Table 2) Coulter counter studies indicated that there is variation in the distribution pattern among the granular sizes. Thus H-1687 starch showed a higher frequency in the range of 13-16m and relatively less in the range of 6-13m compared to H-165, H-97 and S-856 starches. Variety M-4 also had a slightly different distribution pattern, but not as prominent as H-1687 starch [1]. Though the exact role of the granule size in deciding cooking quality is not clear, it is seen that these two varieties are better cooking than the others.

        2. Crystallinity

          Starch has well defined crystalline structure owing to the well ordered regions in the amylopectin mloecules. The starches from five cassava varieties exhibited ‘A’ pattern and no difference in the ‘d’ spacing was evident. The Absolute crystallinity of starch determined from the X-ray Diffractograph also did not vary very much among the different varieties. (Tab3). Similarly the flours from these varieties had nearly similar Absolute crystallinities

        3. Molecular weight

          The molecular weight determined by Ferricyanide method, alkali value and peroiodate methods did not show any definite trend and it was concluded that there is only very little difference among the varieties. (Table 2) [2].

        4. Amylose content

          The amylose content of the extracted starches determined iodimetrically exhibited only slight variation among the different varieties and thus may not be contributing very much towards quality. The soluble amylose content also showed only little range suggesting very little role (Table 2). It has been suggested by some workers that the soluble amylose present in the amorphous regions of the starch granules may be responsible for stickiness in tubers. Such an effect is not evident in the present studies. Gel Permeation Chromatographic analysis was also carried out on these starches to find out if differences exist in the chain length and Degree of Polymerisation. Starch was treated with isoamylase and the resulting debranched starch was subjected to reverse phase GPC using Fructogel columns. The fractions were collected and their carbohydrate content was determined by the phenol sulphuric acid method and the reducing value of each fraction was determined by ferricyanide method. The chain length was calculated by dividing the carbohydrate content by reducing vale and plotted against the elution volume. The figures [Fig 2] showed only slight variations in the GPC patterns and no clear relationship was evident leading to the conclusions that amylose contents and chain lengths are almost similar among the varieties.

        5. Swelling volume and solubility.

          Values for swelling volume determined by allowing free swelling of the granules in excess distilled water showed considerable difference among the varieties. Starch of varieties like M4 had lower swelling volumes compared to varieties like H-165 (Table 1). Higher swelling can lead to weakening of intermolecular forces between the starch molecules resulting in easy breakdown so that the granules do not have the granular integrity required for mealiness. Thus swelling can be one of the key factors in deciding the mealiness of cooked tubers. This is further supported by the fact that during growth period, starch of variety M4 maintains uniform swelling volume and swelling power (Fig 3), [3] and the variety has excellent cooking quality. Similarly solubility which provides insight into the strength of starch granules also was constant for starch of M4. Solubility is increased when the starch granules suffer breakdown and hence this starch appears most resistant to breakdown and thus provides consistent swelling pattern.

        6. DSC patterns

          Differential scanning calorimetry is an important tool to study starch gelatinisation and provides wealth of information on starch structure. The DSC patterns of starch from five varieties were obtained using a Perkin Elmer DSC equipment (Fig 4). The thermograms showed distinct features for some varieties. Starch of H-97 exhibited a typical pattern with a shoulder which could not be removed either by defatting or ethanol extraction showing that this pattern is genetically controlled and the starch may be containing two types of granules having difference in their intermolecular forces. Similarly the broad nature of the thermogram of starch of M4 indicates that the crystallites melt very slowly during gelatinization, again highlighting higher strength of associative forces in this variety. Intermolecular strength has a definite role to play in maintaining the starch granular integrity during cooking and this is borne out by the DSC behaviour of M4 starch.

        7. Gelatinisation temperature

          The ease of gelatinisation can also play an important role in starch swelling and hence the gelatinisation temperatures of starch of different varieties determined microscopically were compared. . The results (Table 2) indicated that H-165 starch gelatinized relatively earlier. Early gelatinisation means that the gelatinised granules are being subjected to longer periods of heating leading to higher breakdown of the starch molecules and thereby resulting in poor quality. Gelatinisation temperatures obtained from the DSC also showed that H-165 starch had slightly lower values confirming that this starch gelatinized more easily and hence possessed relatively weaker associative forces.

        8. Pasting temperatures

          Pasting temperature which indicates the temperatures at which a perceptible increase occurs was determined in the Brabender viscograph using different concentrations of the starch in distilled water. The results obtained further confirmed earlier gelatinisation by H 165 starch. M4 starch exhibited a high gelatinisation range showing the stronger intermolecular bonding. Thus slow and steady gelatinisation of starch is preferable to early and rapid gelatinisation so that the starch is able to maintain its structural integrity.

        9. Viscosity

          Viscosity in an important property of starch and was determined for the different varieties using a Brabender viscoamylograph at different starch concentrations. There was clear difference in the patterns for the starch of different varieties. Broadly the patterns could be classified into three

          1. Single stage gelatinisation with high peak viscosity and high viscosity breakdown
          2. Two-stage gelatinization with high peak viscosity and breakdown
          3. Broad two-stage gelatinization with medium viscosity and medium breakdown

          It observed that H.1687 starch had a medium peak viscosity, low viscosity breakdown but high set-back viscosity. M4 starch had slightly lower peak viscosity and setback viscosity compared to H-1687 starch. On the other hand, H 165 starch had a very high peak viscosity and the breakdown was also quite large. The viscographs clearly indicated that for H-165 starch, the set-back viscosity was much lower compared to peak viscosity, whereas for H-1687 starch, the reverse was true (Fig.5). The results indicate that the starch of H-165 undergoes rapid increase in viscosity and under shear and heat suffers breakdown. This leads to cohesiveness for the paste brought about by the broken starch molecules and hence poor quality. On the contrary, starch of M4 and H-1687 do not exhibit high viscosity rise and hence suffer lower breakdown. These two varieties have better cooking quality and hence viscosity behaviour can play a major role in deciding cooking quality. These patterns seem to be genetically controlled as they were maintained by these starches irrespective of the environmental factors, though there was variation in the viscosity values. Similarly Redwood viscosity values did not differ much among the varieties since it is not a dynamic viscometer and hence the starches do not suffer breakdown by the shearing forces.

      3. Effect of other ingredients

        Often it is found that tubers having high starch content and desirable rheological characteristics do not cook well and others factors like environmental conditions and age of the crop influence the cooking quality. Again the starch properties determined after extraction need not truly represent the real condition in the tubers . The starch granules have to swell in lower water regime and also in presence of various other components. So this aspect also has relevance in deciding the cooking quality. The major factors present in the tubers are fibre, sugars and much smaller quantities of proteins and lipids and minerals. These factors were determined and their effect on the starch properties were studied. Calcium which is known to have a firming effect on potato tubers and phosphorus being always present in starch were also estimated.

        1. Distribution of starch and sugars in different parts of the tubers

          Congo red staining of cooked tubers of different varieties indicated variation in starch content in different portions of the tubers. A more uniform distribution was observed in better cooking varieties. In order to confirm the results, the starch, sugar and dry matter contents of different portions of the tubers were determined. In addition, the starch was extracted from these areas and their swelling property examined. . The portions selected for their determinations were (i) proximal (A) (ii) middle (B) (iii) distal (C). From each portions, 3 regions viz., outer (O) (1-2 mm inside of the skin) is Interior (I) and Core (C) (5-10 mm diameter around the cortex). The results (Tab) clearly indicated large variation in starch, sugar and dry matter contents between the regions in some varieties whereas the variation was much less in some varieties. There was also considerable difference in the distribution of starch between tubers of good cooking quality compared to tubers of bad quality of the same variety itself. It is evident that M4 which shows good mealiness and softness on cooking has not only a good starch content, but also a good distribution of starch especially in the Interior and Core, whereas variety like H-165 which cooks hard and non-mealy has a much lower starch content in the core portion compared to interior. In fact the starch content can be as low as 6-8% and the sugar content increases to even higher than this value. Similar trend is observed in the case of H-226 also. H-2304 which exhibits a high starch content shows over 40% in the interior region falling to less than 20% in the core. It is possible that when the starch content is lower in the core, its expansion is not enough to exert an outward pressure towards the middle portion which contains more starch, this leading to different type of swelling in the core and interior. In varieties, where the core starch content is more of nearly equal to that in the interior, the pressure of the swelling granules is more uniform leading a more uniform texture. This is confirmed by the observation that these varieties increase their diameter on cooking, whereas the poor cooking one decrease in diameter. The varieties H-165 and H-226 also lose their starch content especially in the core regions as the age increases leading to a poorer quality. In these varieties the sugar content often exceeds the starch content at these stages. Either starch gets degraded at the core or there is reduction in starch synthesizing activity. The swelling volumes of starch extracted from different regions also show interesting trends. For M4 of good softness and mealiness, the swelling and volume of starch extracted from any region shows only slight variation, whereas the variation between the values in case of H-165 and H-226 is considerable. However the reducing value is not increased for these samples. This shows that those starch granules which have been retained broken down into smaller pieces but their associative forces have been considerably weakened. Lower associative forces indicate their easier tendency to disintegrate under heat and stress. The results clearly bring out that associative forces play a good role in determining starch property which in turn affects the cooking quality. Starch of variety M4 possesses strong associative forces while H-165 starch has only associative weak forces. This is further confined by the studies on starch properties in relation to age of crop of different varieties. Since starch swelling is of importance in determining quality, a study of swelling in different concentrations was also carried out. It was found that when the starch content reaches ‘critical concentration’, the solubility falls. This explains the absence of mealiness when the starch content is low, especially in the core portions of many varieties. The high swelling possible in excess water reduces the associative forces which hold the starch molecules in the granule leading to breakdown of starch molecules. Hence it is clear that of Other biochemical constituene and hence other factors also decide the quality. There was no swelling. Next the biochemical principles and their role were examined. For the study, the tubers of eight varieties having different cooking quality were selected and the starch, sugar, fibre contents were determined. In addition, the Ca and phosphorus contents were determined. The starch and sugar contents in the outer, middle and core portions of the tubers were also determined. It was evident that starch content is an important factor in deciding the quality. Tubers having low starch content did not cook well. The Calcium and phosphorus contents showed only very minor variation and there was no noticeable relationship between their content and cooking quality. Use of Calcium sequestering agents like during cooking also did not bring about any improvement in cooking quality of poor cooking varieties confirming that Ca may not have an important role in deciding cooking quality.

        2. Effect of other ingredients on starch swelling.

          In order to check the effect of the non-starchy components on the starch properties, the properties of the flour obtained from the different varieties were examined. These included DSC pattern, XRD pattern, Swelling volume and viscosity parameters. The results clearly brought out the influence of fibre present in the tubers on starch gelatinisation. Whereas absolute crystallinity, XRD and DSC patterns were hardly affected, the viscosity behaviour and swelling characteristics were considerably influenced by the fibre. There was noticeable reduction in swelling and peak viscosity of the starches in presence of fibre. The fibre acts as a partial barrier to free entry of water molecules during gelatinisation and hence allows only restricted swelling. The viscosity patterns are modified by the fibre in imparting lower viscosity breakdown by cementing the starch molecules against breakdown under shear and temperature. It was also seen that defatting by hexane or extraction with methanol to remove lipids and sugars hardly affected the swelling and viscosity patterns again confirming that it is the fibre rather than fat or sugars that exert higher influence on the starch properties. This fact is further confirmed by the swelling and viscosity behaviour of the starchy flour obtained from inoculum provided fermentation of tubers. The starchy flour thus obtained contains large quantity of fibre and they modify the starch properties favourably. In fact it was observed that food products made from flour possess less stickiness compared to those made from starch alone. So fibre has a definite role in deciding the starch properties.

          The examination of the effect of the tuber extracts on the starch properties also provided interesting results. Fresh tubers of different varieties were crushed and squeezed out to provide the extracts. The isolated starch from different varieties was allowed to swell freely in the extract and the swelling volumes and viscosity studied. Whereas the extract of M4 enhanced swelling of all extracted starches, the extract of H165 tuber brought about a reduction in swelling and viscosity. The sugar content in the extract of H-165 was found to be relatively higher. It is well documented that sugars have the capacity to increase the solubility of starch and hence may be contributing to breakdown of starch and thus lowering the cooking quality. The observation that tubers harvested immediately after a rain following a drought contain higher sugars and suffer from poor cooking quality further confirm this fact..

          The main conclusion from the study of cooking quality was that the cooking quality of cassava tubers cannot be attributed any single factor, but an interplay of many factors. The major contributing factors are as follows

          1. Starch content. There should be reasonable starch content. An exact amount cannot be fixed but tubers having less than 25% will be hard to cook. Uniform distribution of starch in the tubers and stability of starch during growth period are desirable.
          2. Presence of other components. Fibre and sugar affect swelling of starch. Whereas fibre restricts swelling and thereby prevents viscosity breakdown, sugars impart higher solubility to starch. So some small quantity of fibre along with starch can lead to better quality.
          3. Starch property. Starch property is an important criterion in deciding the quality. Whereas very low swelling is not desirable, too much swelling can lead to granular structural disintegration and thus poor quality. Stability of viscosity and reasonable setback are desirable.

          The physical parameters useful to characterize cooking quality developed for the first time can be useful to Cassava Researchers as a quantitative measure of quality. The data generated on role of starch quantity and quality and influence of other ingredients can be utilized by the Cassava Breeders and Biotechnologists to produce varieties having desirable traits for cooking quality.

          Var Cooking Quality Portion of the tuber Sugar % Starch % Sw. Vol
          M4 Soft, Mealy O 7.6 14.8 32
          I 2.26 40.0 30.7
          C 1.27 43.0 31.3
          Kalikalan Soft Mealy O 3.62 21.2 49
          I 1.66 41.5 55.3
          C 1.07 33.2 45.3
          Ichyapuram Local Soft Mealy O 1.6 26.5 50.8
          I 1.4 33.9 42
          C 1.2 26.7 41.3
          H-226 Med Soft, Med. Mealy O 2.1 27.3 37.7
          I 1.8 31..8 40.9
          C 1.02 27.3 35.6
          H-2304 Med Soft, Med. Mealy O 2.42 28.8 34.7
          I 1.78 43.8 36.0
          C 2.4 31.1 32.6
          H-165 Non-Mealy, nonsoft O 3.0 30.0 48.3
          I 4.66 28.2 42.6
          C 6.76 11.1 33.0

          The data can be useful to the breeder to produce varieties having desirable characteristics.

        3. Extraction of starch

          Unlike the cereal and potato starches which have been used in industries since long, the root starches have not received much attention except cassava and to some extent sweet potao starches. This can be attributed to two main factors. One is the difficulty in extraction of starch from the tubers other than cassava and second factor is lack of knowledge about the properties of these starches. Hence both these factors were taken into consideration for studies in detail.

          Whereas extraction of starch from cassava is simple and the isolated starch is pure white in colour and relatively free from other impurities, starch extraction from other tuber crops is not so easy. The settling of starch granules is hindered by presence of various components like mucilage, latex etc. and this leads not only to loss of starch, but also lowering of quality of the extracted starch. The long residence time can bring about microbial contamination leading to breakdown in starch and resultant loss of starch quality. In addition, the colour of the starch is also affected so that the acceptability of the starch in applications like food – especially sago – and textiles suffers. Work was carried out at CTCRI on use of various chemicals in improving the yield of starch from various tubers. It was observed that among different chemicals tried, ammonia gave best results, (Tab. 4) [5]. The extraction of starch from different tubers using ammonia (0.03M) not only improved the yield, but also the functional characteristics like paste viscosity and swelling. Ammonia acts by reacting with the mucilaginous compounds allowing the starch granules to settle fast. Use of lactic acid and citric acid improved the yield of starch from sweet potatotubers and also the colour of the extracted starch [6]. A detailed study on use of five chemicals in varying concentrations revealed that the yield varied considerably for the different starches at different concentrations of the chemicals examined, but ammonia appeared more effective in improving the yield of starch from aroids and yams. Table [7, 8] The quality of sweet potato starch extracted using the enzyme combination of pectinase and cellulase was studied and the results reveled that the enzymatic extraction did not bring about any detrimental effect on the starch properties. [Pap]. Though tubers of some varieties of Amorphophallus are yellowish, the extracted starch is white in appearance.

          Thus it has been possible to develop a convenient method for extraction of starch from aroids and yams. With the realization of the special properties of these starches and increasing demands for speciality starches, the method will offer a good technique to obtain the starches in good quality and quantity.

        4. Basic studies on the tuber starches.

          Once the extraction of starches in good quantity and quality from most of the tubers was achieved, a detailed study of the physicochemical and functional properties of the different tuber starches was carried out in the Institute and the results are outlined below

          1. Other components in starch

            Even though starch extracted from the tubers appear white and pure, still it harbours many other biochemical components like moisture, fibre, lipids, sugars, minerals which influence the starch properties. The moisture content varies from 6-16% depending on the process used for drying the starch. The fibre content can show much higher variability depending on a number of factors like the sieve used for removal of the fibrous material, varietal variation and age of the crop. These factors are all the more important in cassava and sweet potato, where the fibre content increases with the maturity The presence of the fibre modified most of the rheological and functional properties of the starches [14]. The effect of fibre on starch properties is also clear from the fact that cassava flour (containing 2-3% fibre) had different swelling and viscosity properties compared to the isolated starch (having 0.1-0.15% fibre) and neither defatting nor ethanol extraction brought about any major change in the properties of the starch or flour [15]. The total dietary fibre in cassava flour was found to vary between 4.7 to 5.5%. For Coleus starch 0.4% fibre content was observed [16]. It has been found that the fibre content in the extracted starch from cassava tubers subjected to fermentation with inoculum provided culture contained considerable quantity of fibre [11]. The tuber starches contain much lower quantities of lipids in them and so the effect of lipids on starch properties is not so pronounced compared to cereal starches. The lipid content in different cultivars of cassava was found to vary from 0.11 to 0.22% in starch and 0.27-0.45% in flour of five varieties [15]. Lipids and surfactants form complexes with amylose chains and the free chains of amylopectin and thus influence starch properties. Based on this principle, it has been possible to improve the viscosity stability of cassava starch by treatment with surfactants It has also been established by Modulated DSC studies that there is no hindrance for the tuber starches to complex with surfactants or lipids even though the native starches do not contain much lipid in them [18,19]. The tendency of amylose to form complexes with lipids and surfactants has also been made use of in determination of amylose content in starches using Modulated DSC [21, 22]. Another important component invariably present in starch is phosphorus, since P is involved in starch synthesis. Studies on the P content in cassava starch during growth period did not reveal any major variation with age of the crop for six cultivars over the growth period of 2- 18 months [23]. The P content in different Colocasia cultivars varied from 0.006 to 0.013% [24]. Studies on the P content in six accessions of D. rotundata showed only very minor variability (0.011-0.015%) [25]. Examination of starch from three cultivars of Canna edulis from CTCRI revealed that the P content ranged from 0.05 to 0.08% which is even higher than that found in potato starch [26]. The high phosphorus content can impart high viscosity to starch and also improve the gel strength. The high viscosity is attributed to the repulsion between the ionic phosphate groups. This was evident in the case of Canna starch which has high viscosity and good gel strength. Biscuits made from the starch were having good texture and crispness. These starches can also be very useful in food applications requiring good gel strength like jellies etc.

        5. Colour and appearance

          Colour is an important criterion for starch quality, especially for use in sago and textile industries. Use of organic acids also was helpful in improving colour of sweet potato starch. The starch extracted from Colocasia and Dioscorea tubers by normal process always has an off-colour which is difficult to remove. However use of ammonia during extraction was found to improve considerably the colour of the starch from aroids especially Colocasia [5]. Though tubers of some varieties of Amorphophallus possess yellow colour in their flesh, the resultant starch is pure white in colour. If the extraction is proper, the colour remains white even during storage for six months.

        6. Granule size and shape

          Cassava starch granules are mostly round with a flat surface on one side containing a conical pit, which extends to a well defined eccentric hilum. Some granules appear to be compound [3]. Under polarised light, a well defined cross is observed. Sweet potato starch is polygonal or almost round in shape and has a centric distinct hilum. Polarisation crosses are less distinct compared to cassava starch. Yam starches have a large variability in shape, round, triangular, oval and elliptical. Some of the elliptical granules are found to possess truncated ends [3]. Colocasia starch granules are mostly round or oval and the hilum is observed only with difficulty. Amorphophallus starch granules are mostly polygonal or round in shape and have faint hilum. We obtained oval and polyhedral shape for Canna edulis starch granules [26]. The size of the granules is also quite variable among the tuber starches (Tab. 5). Cassava starch was found to have a size range of 5-40m. Studies on the starch granule size variation with age of the crop starting from 2nd month till 18th month for six varieties revealed that size increase was observed upto 6th month, but then remained steady [23]. Colocasia granules are much smaller (range of 1-10m) and are among the smallest of starches observed in the plant kingdom. The small granule size makes the starch useful in various applications eg. as a filler in biodegradable plastics, in toilet formulations, aerosol etc. Unlike other tuber crops, which do not exhibit any significant variability in size among varieties, Colocasia starch was found to exhibit varietal difference. Studies on 10 varieties revealed a significant difference in average granule size (Fig 6, Tab. 6). Variety C-9 was found to have the highest value of 5.19m and C-46 the lowest (2.96m) [24]. The granules extracted from corms and cormels of four cultivars of Colocasia were compared and it was found that there was very little difference between them [30]. Even the distribution of the granule sizes showed only minor difference between the corms and cormels. It was also found that though there was variation among the cultivars, there was no significant variability during the growth period. Such varietal difference was not observed in Dioscorea alata, D. rotundata and D. esculenta starches. These starches showed an increase in granule size upto 5 months and thereafter remained steady. One noticeable feature is that D. esculenta granules are very small, while the D. alata granules are very large. The granules of D. esculenta possessed an average size of 2-15m, close to Colocasia starch. The small granule size of D. esculenta may be useful in applications similar to those of Colocasia and cereal starches. Starch granules of D. alata and D.rotundata are much bigger, the range is between 6-100m for D. alata (average 35m) and 10-70m for D. rotundata (average 33m) starches. No significant difference in granule size among the different varieties of three yam species studied viz. D alata, D. esculenta and D. rotundata was observed.. There was only minor variation in starch granular size among 10 accessions of Amorphophallus studied.(Tab. 7) [31] . Xanthosoma starch granules range from 10-30m in size with an average value of 17m, and no difference among accessions was observed. Coleus starch granules had a a size range of 5-20m [16]. Pachyrrhizus starch was found to have a size range of 7-40m, and very little difference among varieties. However the largest granule size was observed for Canna edulis starch The average granule size was found to be over 35m for three varieties of Canna [26].

        7. Spectral features

          The infrared spectrum of starch of different varieties of cassava was found to be similar with peaks at 3600-3200 (broad), 2800 (medium), 1660 (weak), 1480-1250 (medium) and a number of peaks between 1150 and 710 cm-1

          FT-IR of the different tuber starches revealed only very minor differences, in spite of their different crystallinity and granule sizes. The Raman spectra of the tuber starches indicated distinct differences in the peak pattern in the region 800-200 cm-1 [32].

          THE CP- MAS13 C NMR of the different starches showed typical pattern for the starches and three main peaks were observed. The first peak was at 101-102 ppm corresponding to C1,2,3,5 and appeared as a singlet or doublet depending on the source of the starch. The next peak appeared at 75-80 ppm corresponding to C and was a singlet. The final peak was at 64 and a singlet. The peaks were observed only if the starch granules had 8-10% moisture in them. There was clear correlation between the XRD pattern and the NMR peak pattern. Whereas the starches having pattern ‘A’ showed a doublet for C1 peak in NMR, those with ‘B’ pattern had clear singlet C1 peak (Tab. 8; Fig. 7). Thus the structural difference between the two types of starch is evident.

        8. X-ray Diffraction Pattern

          Starch has a definite crystalline nature and the crystallinity has been assigned to the well ordered structure of the amylopectin molecules inside the granules. Cassava starch has been found to posses ‘A’ pattern with three major peaks at 2Ø=15.3,17.1 and 23.50 [3]. The absolute crystallinity values of five varieties were found to be in the range 8-14% (Tab2) [15].. The flour also possessed similar XRD pattern and absolute crystallinities. Sweet potato starch was found to possess ‘A’ pattern. Colocasia, Xanthosoma, Pachyrrhizus, Arrowroot and Amorphophallus starches also possessed ‘A’ pattern [3] . But the edible Dioscorea starches (viz. D. alata, D.esculenta and D. rotundata) possessed ‘B’ patterns similar to potato. It was found that the XRD pattern of extracted starch is same throughout the growth period of D. rotundata (Fig. 4). Starch of Canna edulis and Curcuma sp. exhibited ‘B’ XRD pattern. A detailed study of the XRD parameters of the starch extracted from Amorphophallus and Xanthosoma tubers subjected to pre-treatment using different chemicals was carried out. The ‘d’ spacing, angle intensity and peak intensity were found to be similar for control (water) and chemically pretreated samples of Amorphophallus samples. However shift occurred for the peaks indicating partial change in the crystalline phase [7,8, 33]. For Xanthosoma starch, higher concentration brought about more significant changes especially with potassium metabisulphite. Heat moisture treatment did not change the XRD pattern of D. rotundata starch [34].

        9. Molecular weight

          The molecular weight of cassava starch showed only minor differences existed among the different varieties (Tab. 2). D. rotundata starch showed only minor changes in the molecular weight over the growth period [25]. Colocasia esculenta and Amorphophallus paeoniifolius starch from different varieties had almost the same range of reducing values, showing that the tuber starches have nearly equal molecular weights. Studies on the yam and aroid starches extracted from tubers subjected to treatment with different chemicals also indicated only very minor differences in the reducing values among the different treatments [7, 8, 33]. For coleus starch, the reducing value was 1.71, indicating the same range as for other tuber starches [16].

        10. Amylose content

          The linear component of starch, viz., amylose, imparts definite characteristics to starch and therefore amylose content is an important criterion in determining the starch properties. Amylose content is found to vary considerably among different starches and even genetic modifications have been carried out to obtain starch of amylose contents varying from 0- >75% amylose.

          It was found that the Blue Values corresponding to total amylose varied from 0.50 to 0.55 for seven cassava varieties, indicating only very minor variation among the varieties [29]. When the amylose content of six varieties of cassava was compared during growth period, there were only insignificant differences in the amylose content [23]. . Examination of the effect of surfactants on the amylose content in cassava starch revealed that though the surfactants reduced the Blue Values, there was no correlation between concentration of surfactant and reduction in Blue Values [17]. Maximum reduction was obtained with cetyl trimethyl ammonium bromide while lowest reduction was with potassium stearate. Sodium lauryl sulphate and cetyl trimethyl ammonium bromide, having bulky hydrophilic groups, might be blocking the entry of the iodide into the amylose helix.

          Colocasia esculenta starch had wide range in the amylose content and noticeable relationship between the amylose content and granule size was noticed. The variety C-9, which had the largest granule size also possessed the highest amylose content [24]. . Amylose content in six D.rotundata varieties ranged from 21 to 24.6% [25] while that in D. esculenta and D.alata in the range 20-26%.

          Thus the amylose content also varies considerably among the Dioscorea starches. The starch of D. esculenta, D. alata and D. rotundata was also examined for total amylose content in relation to age of crop. The results indicated only very little variation in the amylose content with age of the crop. Xanthosoma starch also had a similar amylose content viz. 15-25% and very little varietal variation was observed. The amylose content of ten cultivars of Amorphophallus paeoniifolius was found to vary very little and ranged from 21.9-23.5% [31].

          Starch from three accessions of Canna edulis was found to have an amylose content ranging from 24-30%, the highest being observed for purple accession [26]. Coleus starch had an amylose content of 33% [16] . Thus among the different tuber crops, Canna edulis and Coleus starches have highest amylose contents (Tab.8).

          Soluble amylose is an important component in starch which plays a significant role in deciding the textural properties. The amylose molecules in the amorphous regions are supposed to make up this fraction and so are easily leached out and hence responsible for cohesiveness in cooked tubers The soluble amylose contents (Tab.9) in the tuber crops starches determined using iodimetry ranged from 10-40% of total amylose. Soluble amylose content in different varieties of cassava was found to be almost equal, even during the growth period.

          Similar trend was observed for Colocasia, D. alata and Xanthosoma starches. For Amorphophallus starch from different accessions, the soluble amylose content ranged from 9-11% forming nearly 45% of the total amylose content [31]. The soluble amylose content in Coleus starch was 12.8% [16] while it ranged from 10-12% for Canna edulis starch [26].

          Results on the complexation of tuber starches with the surfactants were quite interesting in that the soluble amylose was suppressed to different levels with different starches and different surfactants. In case of cassava starch, the surfactants had variable effect on soluble amylose portion. The anionic surfactants potassium palmitate and potassium stearate and the neutral surfactants glyceryl monosterate had little suppressive action but the cationic surfactants cetyl trimethyl ammonium bromide and sodium lauryl sulphate had significant effect. Increase in concentration of these surfactants led to reduced blue values for soluble amylose[17].

          The effect of the cationic surfactant cetyl trimethyl ammonium bromide on the other tuber starches is even more interesting (Tab. 10). Though the surfactant reduced the blue value for soluble amylose of all the starches, the effect on

          Colocasia and D. esculenta starches was more prominent indicating that the soluble amylose of these two starches may be more anionic in nature or that the amylose helix of these two starches are such as to be able to sterically favour the complex formation. It is worthwhile noting that these two starches have the lowest granule size, but no correlation between complexing properties and granular size could be observed for the other starches

        11. DSC characteristics

          Differential scanning calorimeter (DSC) has become an important tool in studying starch gelatinisation. The method is simple, fast, requires only small quantities of sample and gives reproducible results. Now the advanced version of DSC, viz., Modulated DSC can give more valuable information regarding gelatinisation, glass transition and starch-lipid complexes.

          Comparison of the DSC parameters among different tuber starches revealed not only large variation among the different starches, but also between different samples and experimental conditions used like moisture content, rate of heating etc. used [37,38]. The DSC data obtained by us by using normal DSC and MDSC is presented in Tab. 11 and Fig 8. The effect of reheating the sample after one cycle has also been studied. The general trends which emrged from the study of 11 tuber starches are outlined below. Among the different tuber starches, cassava and sweet potato starches generally had lower Tonset and Tend values. Highest values were noticed for Colocasia starch and the other starches had values in between. The range of gelatinization varied from 8-100C for the starches. Thus wide variation is evident among the different studies.

          Thus it is evident that there is considerable variability in the gelatinisation temperatures among the different tuber starches. The difference in the gelatinisation temperatures can be traced to the variation in the starch intermolecular bonds. High temperature of gelatinisation can be indication of higher stability of the starch crystallites in the starch molecules which means more heating is required to swell the granules. In addition, a number of other factors like varietal differences, environmental conditions and the experimental protocols like level of moisture, sample preparation, rate of heating and instrument used – all contribute to the values.

          The range of gelatinisation is also quite different among the different starches. In our study, we obtained highest range for cassava starch (12.900) and lowest for Xanthosoma starch (4.70) [38]. However here also lot of ambiguity exists among different reports and can be attributed to a number of factors . Higher range has been attributed to higher level of crystanillity which imparts higher structural stability so that the water molecules need longer time to penetrate the crystalline areas .

          There does not appear to be any relation between the XRD pattern and gelatinisation range, as both cassava and Xanthosoma have ‘A’ pattern but the range of gelatinisation are far different. An ‘A’ XRD pattern indicates closer packing and should have a higher range of gelatinisation, but such an effect is not observed. In addition, the Tonset does not appear to be influencing the range of gelatinisation in any regular pattern. Similarly granule size and gelatinisation range do not show any relationship. In the Brabender viscographic curves, the yam starches show a longer range which does not appear in the DSC. The difference in the water: starch ratio between Viscography and DSC may be reason for this anomaly.

          Gelatinisation enthalpy is another important parameter obtained by DSC. The value depends on a number of factors like crystallinity, intermolecular bonding etc. For cassava starch values of 12.4 and 16.6 J g-1 were obtained by us. The enthalpy of gelatinisation of five varieties of cassava varied from 10.6-13.8 J g-1 [15]. Gelatinisation enthalpy was also found to depend on the genetic and environmental factors . Effect of variety and environmental conditions was also evident.

          The gelatinisation enthalpy was 12.9 J g-1 for Colocasia starch and 16.6 J g-1 for Amorphophallus starch [37] . For D. alata starch our values were 11.6 and 15.4 J g-1. For D. rotundata starch values of 10.28 and 15 J g-1 were recorded . For Xanthosoma starch the enthalpy was 9.1 and 15.22 J g-1 in the two studies . We obtained enthalpy value of 17 J g-1 for Canna starch . Here also there does not appear to be any relationship between the enthalpy and other factors like amylose content, granule size and XRD patterns. Since the amylopectin has a more crystalline nature, higher amylopectin content has been considered to contribute to higher enthalpy of gelatinisation. However such an effect is not evident in any of the studies. Again the differences do not reflect the structural differences among the tuber starches. The results also show that unlike the gelatinisation temperatures which show wide variation, the enthalpy of gelatinisation is within in a small range for the tuber starches.

          DSC has been quite useful in studying retrogradation properties of starches. Starch retrogradation is another aspect in which lot of vagueness exists. Originally it was assumed that retrogradation occurs by association of the amylose chains. But using waxy starches, it has been established that amylopectin can also take part in the retrogradation by the association of the outer chains Retrogradation parameters of tuber starches have been examined by DSC, but the results were quite erratic and hence difficult to arrive at definite conclusions [38] . The values for Tonset showed very wide range from 370 to 580, highest and lowest being for arrowroot and Xanthosoma starches respectively. It is well known that retrogradation brings about drastic reduction on Tonset of starches. The highest reduction was observed for Amorphophallus and lowest for D. esculenta starches. The range also varied widely from 12.20C for cassava to 430C for D. alata. The wide range in values indicates that the retrograded starch contains recrystallised amylopectins of different crystanillity. Among the starches D. alata starch appears to have the maximum variability in crystalline structure.

          The reduction was to the extent of 2.5 fold to 7 fold. No relationship could be derived from the properties of retrograded starches with those of the nonretrograded starches [37].

          MDSC in starch gelatinisation

          The tuber starches were studied in detail using MDSC. MDSC uses a continuous heating-cooling cycle which is helpful in identifying reversible and non-reversible processes occurring in polymers. Gelatinisation of starch is a non-reversible process whereas melting of the amylose-lipid complex is reversible one. In the normal DSC, the latter is studied by subjecting the starch to cooling after the first run and then re-heating it so that the gelatinisation peak does not appear during the second heating. Since MDSC separates the two processes, MDSC should be better in studying the two processes. It was with this presumption that MDSC studies were carried out on various starches.

          MDSC was run on a Seiko SII 6200 DSC equipment provided with an built-in software. The samples were directly weighed into coated aluminium pans. Double distilled water was added to get a water-starch ratio of 2:1 and empty aluminium pan was used as reference. The heating cycle used was as follows: first heating from 150C to 1500C at the rate of 30C /min, cooling to 300C at 300C/min; second heating from 30 to 130C at 30C/min and final cooling to 300 at 300C /min. Using the built-in software the thermogram was resolved into reversible, irreversible and total peaks. The gelatinisation onset (To), Gelatinisation end(Te) and gelatinisation enthalpy (Δ H) were determined from the thermographs and corrected for dry weight. The results were expressed as mean of at least three values.

          Some of the graphs obtained for the different starches are given in figures. The graphs corresponding to total contains both the reversible and irreversible changes taking place during gelatinisation.. The starch gelatinisation is an endothermic process which involves absorption of water by the starch and swelling and it is an irreversible process. . The second peak corresponds to the melting of the starch-lipoid complex which is formed between starch and lipid and the peak depends on a number of factors like concentration, lipid type and heating conditions.

          In modulated DSC, the heating takes place in stages, in which there is continuous cycles of heating and cooling and it is claimed that it is able to separate the reversible and irreversible processes taking place during the runs. It is expected that therefore it should be possible to separate the starch gelatinisation and starch-lipid complex melting. So during the splitting of the total peak into K (kinetic) and C modes, the irreversible transformation viz. gelatinisation should appear in the K graph while the reversible viz. starch-lipid melting should be only present in the C graphs. However the result shows that such a clear separation does not take place. Whereas the H for K is invariably lower compared to S(Total), the C graph invariably contains some peak corresponding to the starch gelatinisation which should not be there . This indicates that the separation of S into K and C does not strictly follow the reversible and irreversible pattern. It is also quite possible that the so-called gelatinisation of starch is not fully irreversible and may contain reversible processes also. This is invariably true for all the starches examined The absence of peak for starch-lipid melting during the second heating cycle proved beyond doubt that the tuber starches do not contain any naturally bound lipids in them. The result is interesting in that Pacchyrrhizus plant produces pods and so it can also be considered as a leguminous crop. The starches from leguminous crops usually contain native lipids and show the starch-lipid melting endotherm. The absence of such a peak in the DSC of Pacchyrhizus starch shows that the starch has more similarity to tuber starches rather than legume starches. Starch-surfactant complexation in determination of amylose content in various starches.

          The starch-surfactant complexation has been used for determination of amylose in the tuber starches. The principle behind this method is comparison of the enthalpy values of the starch-lipid melting with standard amylose. The method has many advantages over the other common methods. Iodimetric method is very sensitive to pH and often provides inconsistent results due to the difficulty experienced in dissolving starch to get uniform solution.. GPC method is long and is also not so reliable. The DSC method is very simple and requires only very small quantities of the sample . The principle behind the method is that when a lipid or surfactant is added to starch, it binds with the amylose fraction of starch. The measure of enthalpy of melting of complex can give the estimate of amylose content in starches. It has also observed that whenever the complex is cooled and reheated, the enthalpy during second heating is higher compared to first heating. In addition the Tonset is also elevated. This indicates that during first heating when starch gelatinises, the amylose portion is released and hence can form complex with the lipid or surfactant. So if the process is repeated, the complexation becomes more strong. In modulated DSC, there is continuous heating-cooling cycle during each run and hence it can be considered as a series of heating and cooling operations. So this method should give better results. This is the principle behind use of MDSC in amylose determination. Another drawback in the earlier method which uses DSC is use of the lipid lysolecithin which is quite costly. So we tried use of easily available surfactants viz. Sodium dodecyl sulphate(SDS) and cetyl trimethyl anmmonium bromide. (CTAB)

          The experimental technique involves weighing out 2-5 mg starch into the pans, addition of 5% solution of the surfactant, so that the starch : water ratio is 1:2. After sealing, the pans are allowed to equilibrate for 1 hour. The pans are transferred to the equipment and an empty pan is used as reference. The heating sequence is as follows. First heating from 15 to 1500C at 30 C /minute, cooling to 300C at 300C/min, reheating from 300C to 1500C at 30C/min and final cooling to 150C at 300C/min. The amplitude was fixed at 20C/min and frequency 0.017Hz so that effectively, the sample is heated continuously by 30C, immediately cooled by 20C and so on. Each run takes nearly an hour and half. A sample of potato amylose was used as standard. The enthalpy of gelatinisation was calculated using standard software. The results were calculated on the basis of at least three runs.

          Since the starches were obtained from different sources and it is not always safe to rely on reported data, the amylose contents in the starches were determined for all samples by standard iodimetric method. This involved dissolving the starch in DMSO-Urea solution, followed by determination of absorbance of the iodine-starch complex obtained from the solutions. The total amylose content was determined by precipitating out the starch from the solution using alcohol followed by re-dissolving in DMSO-urea solution .and determination of absorbance. In order to compare with data obtained by GPC, the analysis of all samples was carried out at the Food Science department of Uppsala university based on the standardised procedure developed there., Starch was debranched using α-amylase followed by fractionation in a Sepharose column. For debranching, 5 μl isoamylase from Pseudomonas amyloderamosa was used and after reaction, the enzyme was inactivated in a boiling waterbath for 5 min.. The samples were injected on a Sepharose CL 6B column (1.6 X 70 cm. ) using 0.25 M KOH as eluent at a flow rate of 13 ml per hour. Two ml. Fractions were collected and the elution profile was obtained by the phenol- sulphuric acid method.

          The results are presented in Table. The results obtained by the MDSC method have been compared with those obtained using iodimetry and Gel Permeation Chromatography. The starch samples used in the study are of different origins including cereal, root and tuber and pea starches. In addition genetically modified starches from barley, maize and potato have been compared. So a wide range of starches has been examined. Among the cereal starches, there was good agreement between values obtained by GPC, iodine staining and the present method. The values obtained for CTAB were higher than those obtained on using SDS. For tuber starches, both the surfactants gave very good results. However in case of some tuber starches, the apparent amylose was much higher than the total amylose which is not easily explainable. The absence of lipids may a contributing factor. On the whole, the results indicate a very good match between the amylose contents determined by iodimetry and GPC with those obtained by the present method. A comparison between the two surfactants do not show any major difference though it appears that CTAB is more sensitive for low amylose samples. The length of the hydrophobic chain is very important in deciding the complex formation. It has been found that 12 to 18 carbon chain length is optimal for complexation with amylose. Both the surfactants have this range and the role of the hydrophilic head needs further examination. The method thus appears quite efficient since it gives acceptable vales for a wide range. The surfactants used are widely available. Unlike iodine staining which requires defatting to get more accurate results, the present method does not need any such step.

      4. Influence of chain length on the starch gelatinisation

        The influence of chain length of the lipid used for complexation with starch on the gelatinisation characteristics was studied using MDSC. It is well-known that for efficient complexation a minimum chain length of the lipid or surfactant is required. Though complexation has been reported for even 6 carbon system, the optimum length is 12 to 18. The strength of the complex is can be gauged from the gelatinisation temperatures as well as gelatinisation enthalpy. The higher the gelatinisation temperature the stronger will be the complex. Similarly a higher enthalpy of gelatinisation indicates stronger complex. Earlier work has been mainly on pure amylose and complexation was mainly studied using Glyceryl derivatives of various fatty acids and data has often been compared with the results from different lipids or surfactants.. It was decided that a comparison will be more relevant if the surfactant used is same and for this purpose lysolecithin having different chain length of fatty acid was examined. A large number of starches from different sources was examined. These included cereal starches, root and tuber starches and genetically modified ones. Lysolecithin having the fatty acid residues (C6:0), (C-10:0), (C-14:0) and (C-18:0) were used for the study. A 1% solution of the surfactant was prepared in double distilled water. Starch was weighed accurately (1-5mg) into the pans, 2 times the weight of the surfactant solution was added, the pans were sealed and MDSC was run as described earlier. The Tonset and Tend and H were determined from the thermograph after the resolution of the graphs into reversible and irreversible graphs. At least three runs were made for each sample.

        The results are presented in tables.

        No Starch C-6 C-10 C-14 C-18
        Tonset Tend H Tonset Tend H Tonset Tend H Tonset Tend H
        1. Cassava 51.6 60. 1 11.7 63.4 76.0 11.6 65.4 77.4 10.5 64.3 76.5 8.8
            93.7 103.4 1.88 106.0 114..8 1.8
            73.5 81.0 1.3 98.4 104.9 2.4 108.7 116.4 2.75
        2. Xanthosoma 74.2 81.5 13.13 72.0 79.2 12.8 75.6 82.6 11.38 74.1 81.5 9.3
            92.3 101.5 2.3 105.0 115.8 3.0
            72.7 79.8 1.86 98.2 103.5 2.82 108.5 116.4 3.75
        3. Colocasia 76.7 80.9 11.7 77.2 85.1 11.6 79.5 86.6 9.36 79.5 86.9 12.0
            95.1 102.0 0.53 104.2 112.5 0.5
            80.2 87.3 0.38 99.5 105.2 0.98 108.4 115.4 1.15
        4. D. alata 71.2 80.3 12.9 70.3 80.0 12.3 72.2 80.6 8.72 71.5 80.2 10.6
            94.1 102.0 0.09 104.2 115.5 3.35
            72.1 78.5 1.52 94.1 104.2 2.36 109.2 116.5 3.6
        5. D.esculenta 66.1 71.1 13.13 63.2 75.2 9.18 62.4 73.0 6.5 64.9 73.3 6.6
            94.5 105.2 1.35 104.0 114.0 1.75
            71.1 78.6 1.21 98.6 104.5 1.5 109.8 115.4 1.48
        6. C.edulis 64.2 71.6 9.08 63.1 71.2 12.91 64.3 71.2 8.12 63.9 71.5 12.67
            92.4 103.3 3.78 104.4 116.5 4.7
            73.3 78.8 1.6 98.2 104.2 2.98 110.5 117.5 3.98
        7. Amorphophallus 78.0 84.8 12.26 75.1 83.0 14.48 77.9 85.0 12.16 77.15 84.3 9.38
                94.1 102.3 2.0 104.5 116.7 3.38
            71.2 78.6 2.28 98.0 104.3 2.9 109.1 116.8 3.5
        8. D.rot 74.5 83.1 11.8 72.1 81.9 13.2 73.3 84.7 9.65 72.4 81.3 8.45
            93.9 101.3 1.25 103.8 115.5 2.52
            73.7 80.2 1.49 97.5 103.0 2.71 109.1 116.2 3.2
        9. Pacchy. 67.0 73.1 8.83 62.6 72.9 9.47 64.4 74.5 9.45 63.7 75.3 8.10
            94.2 103.1 2.37 104.7 113.8 2.35
            75.7 85.1 0.66 99.0 105.4 2.53 106.7 118.2 3.24
        10 Arrowroot 69.1 77.3 12.31 68.8 78.0 12.61 71.3 79.5 7.85 70.2 78.6 8.37
            97.3 102.4 2.15 106.3 115.9 3.0
            71.8 79.2 1.82 97.5 103.9 2.62 109.1 116.6 3.4

        Various interesting points emerge from the results. As is evident from the table, the chain length really influences both the gelatinisation temperatures and the enthalpy. The peaks corresponding to starch gelatinisation, starch-lipid complex melting during first heating and during second heating are affected. For wheat starch the values are 48-58, 102-114, and 107-1160C for C18 lysolecithin. For C14, the corresponding values are 50-64, 93-102 and 98-1050C . When C10 is used the values are 50-59, 69-87 and 77-880C respectively. However for C6 only peak for starch gelatinisation was observed. The following conclusions can be drawn. There is no effect of chain length on the starch gelatinisation temperature since there is only very small difference between the values. Increase in chain length leads to higher gelatinisation temperatures for the starch-lipid complex melting indicating the strength increases with increase in chain length. This is further confirmed by the results for the C6 system where there is no peak at all for the starch-lipid complex melting. A comparison of the values for cassava starch is as follows 64-76 for starch gelatinisation, 106-114 for starch-lipid complex melting for first heating and 108-1160C for second melting with C-18. For C-14, the values are 65-77, 92-101 and 98-1030C. In case of C-10, the gelatinisation temperatures is.63-760C. Though there is no peak corresponding to starch-lipid melting during first heating. There is a peak for melting of starch complex. during second heating . The absence of the peak for the first heating is due to the masking of this peak by the starch gelatinisation peak. This is confirmed by the enthalpy values for the starch gelatinisation.

        Similar results were obtained for all the tuber starches. It was also observed that there was no peak for starch-lipid complex melting when C6:0 lysolecithin was used showing that there is no effective complexation when the chain length is only 6 and this effect is clearer when tuber starches are used. The peak for starch-lipid complex melting was observed only for Glacier variety of Barley during first heating and this result with this starch can be explained on the basis of high amylose content in this variety.

        The results with the enthalpy also showed similar trends. The Δ H values for gelatinisation of wheat starch were 5.6 J/g for C-18, 4.95 for C-14, 6.3 for C-10 and 7.49 J/g for C-6 lysolecithin respectively. For the starch-lipid complex melting during first heating, the values were 4.2, 3 and 2.2 J/g and those for second heating the values were 4.17,3, and 2.2 for C-18, C-14 and C-10 respectively. It is noted that there is no peak for the starch-lipid complex melting indicating that the complexation of lipid with C-6 system is negligible or absent. This can be expected because minimum 6 carbon is required for complexation and the bulky hydrophilic head may be inhibiting complex formation. The enthalpy for starch gelatinisation for C18 is higher compared to C14. However, for C10, the enthalpy is again higher and this is not explainable. Similar trend is observed for the other cereal starch samples. For tuber starches, the Δ H showed a steady increase with reduction in chain length. Such difference between the starches is not clear. The ΔH values for starch-lipid complex melting show reducing values with reduction in chain length for most starches. However for the tuber starches, the first starch-lipid complex melting peak is absent and hence there is no Δ H values for these. The values for the second peak follow clearly the trend of reduction with reducing values of chain length.

        The data thus clearly indicate the following. The longer the chain length, the complexation is stronger. This is shown by the higher gelatinisation temperature and gelatinisation enthalpy of the starch-lipid complex melting . These show higher values during second heating as expected proving that amylose leaches out during gelatinisation of starch. The complexation formed with C-6 is very low or absent or even if formed, it is very weak. The role of the hydrophilic head is also important. Some differences exist between the cereal and tuber starches in their complexation with C-6 lysolecithin indicating differences in their internal structures.

        Effect of native lipids on the gelatinisation of different starches.

        It has been observed that durum wheat provides better quality needles compared to normal wheat flour. The reason behind this is not clear. It was felt that the native lipids present in these may be influencing the starch gelatinisation differently, since it is basically the starch which gives the texture, structure and shape to the needles. To examine this aspect, it was decided to study the effect of native lipids from wheat, durum and rye on the gelatinisation characteristics of starch of D.alata and pachyrhysus. These two root starches were taken since D alata has a B XRD pattern and Pacchyrhysus A pattern and so the study can throw some light on the starches with different structural properties.

        The total lipids were isolated by extraction of the corresponding flour at room temperature with ethanol and removal of the solvent in vacuum, The polar lipids were separated from the total lipids by column chromatography. Accurately weighed quantity of The lipids (200mg) was allowed to equilibrate with water, starch (200 mg) was added and the mixture was thoroughly mixed to ensure uniform distribution. For DSc studies, a small quantity of the material was accurately weighed and transferred into the pan, taking care that all the material is completely inside the pan, it is sealed and placed into the sample holder. The DSc was run as follows Heating 15 to 150 at 5C/, cooled to 30, reheated to 130 at 5C/min and cooled again. The gelatinisation parameters were directly read out from the thermograph. Each sample was run at least three times using polar lipids and total lipids.

        The results are summarized in Tables. It is evident that the lipids form strong complex with the starches examined. However the results do not show any higher value for polar lipids compared to total lipids. It was expected that the polar lipids should form stronger complexes and should give higher values for both gelatinisation temperatures and enthalpy . Such result was not observed . But the results clearly showed that the tuber starches can form complexes with lipids and there is no inhibition for such complex formation and hence lipids can be used to modify the starch properties.

      5. Gelatinisation and Pasting temperature

        Gelatinization of starch takes place over a definite range of temperature known as gelatinization temperature. Gelatinisation temperature can be measured microscopically, by DSC and also by using a viscograph. However use of viscograph provides the pasting temperature rather than gelatinisation temperature. The former may be defined as the temperature at which a perceptible increase in viscosity occurs and is always higher than gelatinisation temperature. Among different tuber starches, cassava starch has the lowest gelatinisation temperatures. No relationship between granule size and gelatinization temperature was observed [29]. Pasting temperature of H-165 starch determined using a viscograph was slightly lower than those for most of the other varieties, and M4 starch had the highest range of pasting temperature [15]. These values are quite close to DSC values of 66 and 780C for Tonset and Tpeak respectively. When cassava starch was subjected to steam pressure treatment at different pressures, there was progressive increase in pasting temperature by 2 to 90C depending on the time of treatment and pressure used. The pasting temperature rise was higher for longer time of treatment and higher pressures [39]. Increase in pasting temperatures was also observed on treating the starch with surfactants [17]. It was found that different types of surfactants generally increased the pasting temperatures, but most pronounced effect was noticed on treatment with potassium palmitate and potassium stearate. On esterification to acetate, the pasting temperatures were reduced due to weakening of associative forces [40]. Examination of gelatinization temperature of cassava starch in non-aqueous solvents indicated that the values were enhanced tremendously in glycerol and ethanol, while in DMSO and formalin, only slight increase was noticed. The large increase in the first two solvents can be traced to steric factors [41].

        The pasting temperature of sweet potato starch varied between 66.00C to 86.30C.

        Pasting temperatures of different cultivars of Colocasia esculenta and Xanthosoma sagittifolium starches have been determined using the Brabender Viscoamylograph.. There was only very slight difference between varieties but the pasting temperatures of these two starches were distinctly higher than those of cassava and sweet potato starches.

        The gelatinisation temperature of starch of three accessions of Canna edulis was 74-85 to 80-950C by the Brabender Amylograph, but 74-750C by the RVA [26]. Pasting temperatures of starch of

        Amorphophallus paeonifolius extracted from 10 accessions were compared and again no significant variability was noticed and the starch gelatinised in the same range as the other aroid starches viz., 81-85 to 82-850C [31]

        However the range was lower compared to cassava or the yam starches. For Pacchyrrhizus starch, we obtained values of 74-790C and only minor variation existed among varieties [3]. The values for curcuma starch was 810C by RVA [27] while it was 65-850C for coleus starch determined microscopically [16].

        Yam starches generally gelatinised over a temperature range of around 200C and gelatinisation continued even after 950C showing strong intermolecular linkages. The large range for yam starches may be attributed to the presence of phosphate linkages in these starches (similar to potato starch) since it was found that the yam starches invariably contained higher quantity of phosphorus.

      6. Viscosity

        An important and useful property of starch is that it provides a viscous paste when heated in presence of water. It is this viscosity that accounts for the use of starch in textile, paper, adhesive and food industries.

        Detailed Brabender viscographic studies on cassava starch from different varieties and at different starch concentrations have been carried out and the results indicate variation in the viscosity parameters. There was no correlation between viscosity and granule size. peak viscosity and setback viscosity.

        In a Redwood Viscometer No. 1, a 2% paste of cassava starch had a value over 50 seconds and only minor variation among varieties was observed. When cassava starch was subjected to steam pressure treatment, the viscosity fell steadily with increase in pressure and time of treatment. At a pressure of 15 psi for 150 minutes, the viscosity fell from 58.5 seconds to 30 seconds. The same trend was observed in the Brabender Viscosity results also, where the peak viscosity dropped from 430 BU to just 30 BU. It was also observed that the reduction in viscosity was linearly related to the severity of treatment. [39].

        It was observed that different surfactants affected the viscosity of cassava starch differently [17]. Whereas sodium lauryl sulphate increased the peak viscosity, especially at higher concentrations, the effect of potassium stearate and potassium palmitate was not so pronounced. Glyceryl monostearate was found to reduce the peak viscosity at higher concentrations. Similar effect was observed for the Redwood 2% viscosity values also. Viscosity of cassava starch in various non-aqueous solvents has been compared [41]. Ethanediol depressed the Redwood viscosity from 55 seconds to 17.5 seconds, while glycerol increased it to 175 seconds. DMSO and Formalin also increased the viscosity, but to a much lower extent.

        In addition to peak viscosity, the breakdown in viscosity is another important criterion that decides the applicability of starch in food and industry. In this respect, cassava starch is considered inferior to maize starch, because its viscosity is rapidly reduced on heating under shear leading to a ‘long’ and cohesive texture for its paste, which is not desirable in food and textile applications. Much effort has been paid to strengthen the starch paste viscosity. Steam pressure treatment can improve the paste stability, but it is accompanied by a corresponding reduction in peak viscosity. However, surfactants were found to have a more desirable effect. On incorporation of potassium stearate or potassium palmitate, even at 0.02% mole concentration/100 g starch, the viscosity was maintained and also stabilised [17]. Since the surfactants are easily handled and work up is easier the method may be used to modify starch viscosity.

        In view of the presence of a large number of hydroxyl groups in starch, cross linking has been used to stabilise starch viscosity. Various di- and tri-functional chemicals have been used to crosslink the starch. The most effective chemicals have been epichlorhydrin, phosphorus oxychloride and sodium metaphosphate. These chemicals bind the starch molecules in the granules, increasing the associative forces, rendering the starch granules stronger and preventing breakdown during heating and stirring The textural properties of cassava flour could be improved by treatment with phosphoric acid and > however the latter imparted a bitter taste to the product.[42]. . Recently, it has been found that some Lewis acids are able to liquefy starch at low concentrations of the salt without affecting the basic properties of the starch and this treatment may be effective as a method for thinning starch. Many salts can influence starch properties depending on the type of salt used [44,45].

        Sweet potato starch behaves almost similar to cassava starch in its viscosity characters, viz., peak viscosity, viscosity breakdown and setback viscosity. The rheological properties of sweet potato starch extracted using an enzymatic process did not vary among the different concentrations of enzyme used upto 0.1% [9]. Breakdown in viscosity was observed only at higher concentration of enzyme The rheological properties of various tuber starches have been compared using the Bohlin rheometer and wide variability in the values of G’ and G” was observed. However all the starches exhibited uniformity in their elastic behaviour predominating over viscous nature, Table 12, Fig. 9[47].

        Data on the viscosity characteristics of the other starches are also widely variable . The viscosity of Colocasia starch extracted from ten cultivars was less than that of cassava starch and close to cereal starches . Considerable difference in peak viscosity values among the accessions was observed. . At 5% concentration, the viscosity ranged from 130 to 350 BU. The lowest and highest values at 6% and 7% concentrations were 200 and 600 (6%) and 320 and 880 BU (7%) respectively [24]. It was also interesting to observe that C-9 starch having the highest granule size and amylose content had the highest peak viscosity. As earlier stated, there was only nominal breakdown in viscosity even at the highest concentrations pointing out the possibility of using this starch in various applications which require paste stability.. The viscosity values were nearly same for the corm and cormel starches [30].

        Variability in the viscosity properties of Amorphophallus paeoniifolius starch extracted from ten accessions was quite minor [31]. The values ranged from 280-380 BU at 5% concentration, 450-620BU at 6% concentration and 770-920 BU at 7% concentration. The viscosity breakdown for the starch samples was very low, similar to other aroid starches and in the same range as the cereal starches

        Starch extracted from seven varieties of Xanthosoma sagittifolium has been compared for its viscosity. There was only a slight difference in the viscosity values among the varieties. The highest value was 360 BU and the lowest 280 BU at 6% concentration. The breakdown varied from 0 to 30 BU at 7% concentration, showing the stability of the starch paste.

        All the yam starches showed a characteristic pattern of slow rise in viscosity and even after 950C an increase in viscosity was noticed. This implies that all the granules do not gelatinise at 950C and some of them gelatinise only during the holding period. In this respect the yam starches resemble potato starch.

        The viscosities of starch of four varieties of D. esculenta extracted using water and ammonia solution were compared. There was only minor difference in the values between the varieties (800-950 BU), but the viscosities of starch extracted using ammonia solution were much higher than those obtained by water extraction.

        When the viscosity of D.alata starch from five varieties extracted with water and ammonia was compared, the results were not so conclusive. Mostly, there were no peak viscosity values. As observed for the D.esculenta starch, there was no observable breakdown in viscosity on heating and stirring.

        The 2% viscosity of six varieties of D. rotundata starch varied from 37.5 to 46 seconds . As far as the paste viscosity is concerned, the values ranged from 325 to 550 BU at 5% concentration and 680 to 920 at 6% concentration [25]. The viscosity breakdown was quite low in spite of the high viscosity levels. The yam starches contain three to four times as much phosphorus as found in cassava and aroid starches. It has been reported that the phosphate linkages in potato starch is responsible for its high viscosity and such effects may also be important in the yam starches. Steam pressure treatment of the D.alata and D.rotundata starch has been found to reduce the viscosity. The reduction in viscosity was found to be directly related to the pressure and time of treatment (Fig. 10). The peak viscosity came down to nil value at 15 psi for 60 minutes for both the starches [34]. This method can be tried as a method for modification of starch, since the process is simple and does not involve use of any chemicals and so workup is simple

        The Redwood viscosity of Coleus starch was found to be 37 seconds at 750C for a 2% solution which increased to 56 seconds on cooling to room temperature [16].

        Rapid Visco Analyser has become more popular in studying viscosity and many publications have come out recently on the studies on tuber starches . . The RVA profiles starch of some varieties of D. alata, D.esculenta and D. rotundata harvested at different maturity have revealed that maturity did not affect the rheological properties to any major extent. Pacchyrrhizus starch had similar viscosity as arrowroot and there was only very little difference among 10 varieties examined. Results in our laboratory, however do not show such high setback for Canna starch. The peak viscosity values for the starches from three accessions of canna varied from 3887 to 4187 cPs. The RVA patterns of the starches from the three accessions do not coincide exactly with the Brabender data. The results indicate noticeable breakdown for all the three starches but is not widely different among them [26].

        Rheological studies on tuber starches

        Rheological studies of the starches extracted from Amorphophalus, Xanthosoma, Colocasia, D.rotundata, D.alata and D. esculenta tubers treted with selected concentrations of SHMP, NaCl, KMS, GMS and NH4OH and control starches were conducted in a Bohlin rheometer system (oscillation set). The experimental conditions were:

        1. torque element 1.542gcm.
        2. Measurement interval – 60s.
        3. Thermal eqlbm. Time – 10s.
        4. d. Sensitivity – 1%
        5. Amplitude – 3%
        6. Heating rate – 1.50C/min

        Auto strain off.

        The cup in which the starch slurry had been poured into was a C25 measuring system. Frequency, phase angle, viscosity, storage modules(G’) loss modules (G”), range, strain and correlation were determined at each one minute time interval. The ratio G’/G” was calculated a t 950C 750C, 600C, 450C and at 350C at different time intervals.

        From the date obtained, the rheological properties of different starches were examined.

        In the case of D.rotundata tuber starches, GMS treated starch had grater G’ value and G’/G” ratio compared to other treatments. This indicates a greater elastic behavior of the starch. For all the treatments and control G’ and G” are maximum at 950 C. Thereafter, they decrease and on cooling beyond 350C, G’ and G’/G” values increase. That is the elastic behavior is gradually decreasing upto 350 C and then an increase in elastic property takes place.

        For control D. alata starch, the G’ value ranges from 105 Pa to 90.4 Pa and G’/G” value from 11.4 to 6.41. SHMP, NaCl, KMS and NH4OH treated starches have lower G’/G” and G” values compared to that of control starches. But all the chemically pretreated starches showed similar trend in the G’ and G’/G” changes.

        D. alata control starches have lesser G’ values than D.rotundata starches at higher temperature. Hence these starches possess more elastic properties than D .rotundata. At higher temperature. But at 350C, after 45 minutes the values were higher for D.alata starch. The G’ values were lowest for NH4OH treated starches and the highest for SHMP treated starches. The G’ values of control sample ranged from 59 to 146 Pa. G’ value decrease up to 45 C and there after increases on cooling. That is liquid characteristics were more significant at higher temperature while at lower temperature the elastic (solid) characteristics become more prominent.

        G’ values of the D. esculenta control starch ranged from 19 to 25.3 Pa. The value decrease upto 600C and thereafter increased and on cooling to 350C the values remain constant. All chemically pretreated starches behaved in a similar manner as that of the control. SHMP treated starches had the highest G’ value but the ratio of G’/G” value was highest for GMS treated starches.

        Amorphophallus tuber starches possess almost same G’/G”values at different temperatures. A small decrease in the G’/G” ratio was observed upto 350C and thereafter an increase was noticed.. The G’/G” value were found to be highest foe GMS treated starches. Colocasia tuber starch had higher G’/G” values in the range of 1.4 to 6.8. G’ values increased when the temperature was decreased from 95 C to 35 C. Hence the elastic behavior was more prominent at lower temperature for Colocasia starch. NaCl,GMS and KMS treated starches behaved in a similar manner as that of the control samples but cooling after 35 0C did not make any significant change in G’/G”. The G”/G” were higher for GMS treatment than control.

        In case of Xanthosoma control starches. G’/G” value decreased after 45 C and increased on further cooling. But all the chemically treated starches showed decrease in G’/G” upto 60 C and thereafter increase with decreased temperature. At higher temperatures GMS treatment provided the highest G’/G” value. But on cooling at 350C for 60 minutes, G’/G” values were more or less similar for GMS and KMS treated starches.

        Thus it is clear that different pretreatments bring about different effects on rheological properties. However the results indicate that the treatments do not bring about any major changes in the elastic characteristics of the starches and hence the treatments do not bring about any deleterious effect on the starch properties. A comparative study of different tuber starches was made using the Bohlin rheometrer. Three concentrations were examined viz. 3,4 and 5%. From the data, three factors were compared including viscosity, elastic modulus and phase angle. The results indicated wide variability in these among the different starches.

        The viscosity data obtained from the rheograms do not yield any conclusive results. For most of the starches, no clear trends could be observed except that increase in concentration led to increase in viscosity, but the increase was not quantitative. However, the difference in the viscosity among the starches was obvious. Canna edulis and D rotundata starches had higher levels of viscosity at all the three concentrations while Colocasia and D. esculenta had low values. The aroid starches appear to provide more reliable results compared to the yam starches There is good agreement with the Brabender results that Canna edulis, D. rotundata and D. alata starches possess high viscosity while Colocasia and Xanthosoma have low viscosity levels. Breakdown in the viscosity and setback observed with the Brabender viscographs are not very evident. The results are presented in Figs.1

        Storage modulus

        The storage modules (G’) for the different starches at the three concentration are given in Figs. Wide variation between the different root starches could be observed. The range of values for Xanthosoma starch at 3% concentration was 0.1 to 0.2 Pa during the heating-cooling cycle; which increased to 5-7 Pa at 4% and 12-33 Pa for 5% starch concentration. During cooling, a slight decrease was observed for 3% concentration. The results indicate low gel strength at this concentration. At 4%, the increase in G’ during cooling was more apparent and this became more significant at 5%, showing high gel strength at 5% concentration.

        For D. alata, the G’ values were quite high especially at higher concentrations. The range at 3% was 6-12.6 Pa . There was a small increase during holding at constant temperature (950) and cooling to 350C. At 4% concentration, the values were much higher (36-53 Pa) with a small increase during cooling and holding at 350C but was not very significant. But at 5%, the increase during later stage was very evident. Earlier experience on this starch had shown that the starch has a good gel strength and is confirmed by the present study especially at higher concentration.

        For Pacchyrrhizus starch, the G’ was quite low at 3% during heating and till last stages of cooling, when there was a noticeable increase. At 4% concentration, a discrepancy was evident during initial phase, but the increase in cooling was obvious. This was further confirmed by the large increase in 5%, The results indicate strong elastic nature and gel strength for the starch at high concentration.

        Colocasia starch had very low G’ at 3% and hence very low elastic character. When concentration was increased to 4% the values were much more perceptible and there was a sharp increase during cooling . For 5% concentration the G’ was nearly 30 times that at 3% and increase on cooling was very evident. Thus elastic character becomes clear at 5% and though the gel strength was not high, tendency to gel was obvious. Colocasia starch has been found to exhibit low viscosity and relatively poor gel strength (Moorthy, 1994). These characteristics may be related to the small granule size of this starch.

        Arrowroot starch at 3% concentration had a detectable G’ at 3% concentration, which increased with concentration, two fold at 4% and three fold at 5% concentration. The storage modulus followed the same pattern as the viscosity, a fall during holding at 950C, than a slight increase on cooling and then remaining steady. This was true for all the three concentrations and the increase was nearly same at these concentrations.

        For cassava starch, the trends were clear. At 3%, the values were insignificant, but at 4%, they became evident (5-7) while at 5% they were much more significant. . However at 5%, there was a steep fall during holding period and then a small rise, very similar to arrowroot starch. In this respect G’ resembled viscosity pattern at 5% concentration. The rapid fall at 5% concentration during heating and holding reflects the breakdown of the starch which is also evident in the Brabender Viscograph. Though the starch exhibits good elasticity, the gel strength doesn’t appear to be very high on cooling.

        D. esculenta starch had very low G’ at 3% Even at 4%, the G’ was low, but during cooling some increase was evident. At 5%, the G’ was quite detectable and showed a rapid raise during cooling. No further change was observed. The viscosity results also had shown a similar trend. D. esculenta starch has very small granules and thus similar to Colocasia starch. For these starches swelling is also not very high and hence the observed low values for viscosity and G’

        Canna starch has been found to possess very high gel strength both in the Brabender Viscographic measurements and also during preparation of various food products This was confirmed by the G’ data in the present study . At 3% concentration itself, the values were highest during the starches examined (10-22 Pa). At 4% the increase was two fold which further rose 10 fold at 5% concentration. Though there was a tendency to increase during cooling, it was not very high. The starch thus possess high elasticity and gel strength and can be useful in many food applications. Canna starch is known to possess high phosphorus content and perhaps this helps in its possessing high G’ and gel strength.

        D. rotundata starch, was similar to canna starch in having high G’ . At 3%, the value was 10-35 Pa increasing to 20-35 for 4% and further to 45-65 Pa for 5% concentration . D. rotundata starch has also been found to be possess high gel strength and elasticity and is confirmed by the present results . However a fall during cooling is difficult to explain and may be due to experimental error.

        For Amorphophallus starch trends were more evident. The G’ values were 1.4-3.5 Pa for 3%, increasing to 8-10 for 4% and further to 14-23 Pa at 5% concentration. There was also an increase during latter phase of cooling. This indicates that the starch has higher elasticity compared to the other aroid starches.

        The results on storage modules indicate that Canna edulis and D.rotundata starches possess high elasticity and gel strength. The aroid starches have lower G’, but definite trends could be observed for these starches. A 6.4% paste of sweet potato has a reported value of over 150 Pa at 850 falling to less than 100 Pa on further heating and then cooling. The values appear quite high compared to the starches used in the present study even taking into account the higher concentrations used (Garcia et al, 1998). 4%cassava paste has been found to have a G” value of 16.57 Pa at 200 after cooling. (Hansen et al, 1991)

        Phase angle

        The phase angle change due to shear gives an indication of the strength of the starch gel. It reflects the ratio of elasticity to viscosity of the samples.

        Figs. provide the trend of the different starches. The phase angle values for Xanthosoma starch were in agreement with the G’ values. At 3%, the phase angle values were in the range 27-65 which dropped to 12-20 at 4% and 2-24 for 5% concentration. There was a steep fall at the initial stage of heating at 3%, which may be due to very low values for G’ at this stage . For 4% concentration also, there was a small increase in phase angle during cooling, which is not explainable. However at 5%, the behaviour was as expected and there was a steady decrease during cooling and then remained steady

        For D. alata starch, the data was rather irregular and though the trend indicated a decline with increase in concentration, the data was not in consistency with G’ values. G’ though low, may also be influencing the deformability of the starch paste.

        Pacchyrrhyzus starch exhibited definite trend of a reduction with increase in concentration. Lowest phase angle of 13-28 was observed for 5% concentration as expected. Compared to other starches, the values are quite high and in conformity with G’ values. Though the starch does not possess high elasticity, the phase angles were on the higher side.

        The data for Colocasia starch was quite inconsistent at 3%, but for 4% and 5% the values were more reliable and as expected there was a reduction with increase in concentration. However there was no decrease in the values on cooling, -indicating that gel strength does not increase with cooling.

        The values for phase angles were nearly same at 3, 4 and 5% concentrations for arrowroot starch. Though G’ increased steadily with concentration, the phase angles were nearly equal . The range was 16-70 or 3%, decreasing to 14-48 at 5% and there was no decrease in the phase angle values during cooling showing poor gelling tendency.

        Cassava starch had a phase angle values of 38- at 3% which fell to 10-25 at 5% concentration . The high value at 3% is due to the low G’ at this concentration. The values were nearly same at 4 and 5% concentration.

        Cassava starch thus appears to achieve high elasticity at 4 and 5%. The gel strength cannot be considered very high but still large.

        For D. esculenta starch, the values were very low at 3% concentration but at 4 and 5% levels, the values were in line with G’ values. At 5% concentration, a high gel strength is indicated by low value for phase angle.

        Though G’ had risen very rapidly at later stages, this effect was not visible in the phase angle values.

        Canna edulis starch had exhibited high G’ at 4 and 5% concentration, but the phase angle values do not reflect the same effect. Though there was a progressive decrease with increase in concentration, it was not following a definite relation. Even at 3% concentration, the values were quite low indicating high elastic nature of the starch paste.

        For D rotundata starch, the values were quite low even at 3% concentration showing high elasticity, but at 5% concentration, the values did not reflect the high G’ observed. Generally high gel strength was noticed for this starch.

        For amorphophallus starch, the results do not show any special trend, 5% values being lower than 3 and 4% values. There was no decreasing trend on cooling. In contrast to G’ which showed high values, the effect was less prominent.

        Strain sweep tests

        The results of the strain sweep experiments are presented in the figures. The following trends emerged . Higher the G’, the greater is the tendency to breakdown under shear. This was found to be true for all high G’ starches, viz., D. rotundata, D. alata and Canna edulis. For medium G’ starches the thinning was evident at the highest concentration, while it was quite negligible for the low G’ starches. Amorphophallus starch however did not show such tendency. Even among the different concentrations, the same effect is very clear. At higher concentrations, the intermolecular collisions will be more and this can lead to breakdown in the G’. In addition for Colocasia and D. esculenta starches, their smaller granular size may also be contributing to the stability.

        The summary of results is presented in Table 18. The highest viscosity values were observed for Canna edulis, followed by D.alata and D. rotundata. Colocasia, Pcchyrrhyzus and Xanthosoma starches had low values. Cassava starch also exhibited unexpected low values. The general tendency of increase with concentration increase was evident. The storage modules also followed the same trend, with Canna edulis having high storage modulus and in line with Brabender viscosity values and the observed gel strength of their pastes. Cassava starch has Arrowroot had reasonable high values at 5%, while also exhibited high values at 5% concentration. The values were nearly low for colocasia, D. esculenta and Pachyrrhyzus. Starches. The trends were quite clear for Viscosity values for Amorphophallus, Cassava, Arrowroot and Canna starches. For Colocasia and D.esculenta starches, the values were prominent only at 5%. Most of the increase was found during cooling.

        For storage modulus, trends were quite evident for Canna, arrowroot, Amorphophallus, Colocasia and Pachyrrhyzus starches. Only D. alata exhibited variation. Colocasia and Xanthosoma starches had higher G’ during cooling, while these were less for most of the other starches which showed almost equal values. The high gel strength of C. edulis, D.esculenta, and D.rotundata starches is evident from the values.

        The phase angle values also follow the pattern of G’. Though most of the starches showed a decreasing trend during cooling, it was not observed for all starches.

        Thus the results clearly point out that viscosity measured by the Bohlin rheometer is not very reliable at low concentrations and also for starches which gelatinise slowly beyond 950C . It may not be correct to compare the Brabender values with the Bohlin data. The G’ values are consistent with the gelatinisation profile for most of the starches and most of the starches show elastic characteristics especially at higher concentration. The phase angles do not follow the same trend as G’ indicating that the effect of variation in loss modulus (G’) may be important in many cases.

      7. Swelling power and solubility

        Starch swells on heating in water and the extent of swelling depends on the origin of the starch. Swelling power and solubility provide evidence of non covalent bonding between starch molecules. Factors like amylose-amylopectin ratio, chain length and molecular weight distribution, degree / length of branching and conformation decide the swelling and solubility [20, 48]. Cassava starch has swelling power which is in between those of potato and cereal starches- a property in conformity with its observed viscosity. The swelling volume of different varieties of cassava varied from 25.5 to 41. 8 ml g-1 of starch (Tab.2). During the growth period, starch of two varieties H-2304 and M4 maintained their swelling volumes within small ranges, while that for some varieties such as H-165 expressed wide variations which indicate that these varieties are very much susceptible to environmental influences [23] . Swelling volumes also depend on the presence of various chemicals and any treatments carried out on starch. A high amylose content and presence of stronger or greater numbers of intermolecular bonds can reduce swelling . Formation of lipid-starch complex can also affect the swelling volumes as also presence of naturally occurring carbohydrate and noncarbohydrates along with starch [20]. This has been amply illustrated in the effect of fibre on the swelling volumes of different varieties. The fibre acts as barrier to free swelling of starch. It was also found that extraction with ethanol or defatting of flour did not change the swelling volumes showing that the suppressive effect is more due to the fibrous material rather than lipid or sugars present in the flour [15]. The starchy flour extracted from fermented tubers also exhibited the same trend [49]. Sodium sulphite was found to have noticeable effect in suppressing the swelling volume of cassava starch. The swelling volume dropped to very low values at definite concentrations The values dropped nearly to zero at 0.05 and 0.1% concentration of the salt (Fig 12). At higher concentrations, the swelling volumes increased to nearly the same level as the native starch. The effect has been attributed to the oxidative-reductive depolymerisation brought about by the sulphite ions. The effect was nullified at higher concentrations due to destruction of the sulphite ions. Similarly the effect of the sulphite was neutralised by addition of propyl gallate which is an oxygen scavenger [43] . The effect was not unique to cassava starch and swelling volume of Dioscorea starches was lowered by sulphite at similar concentrations also. Thus the effect can be attributed to the salt. In order to check whether is effect is only with sodium sulphite alone, other salts were tried including potassium sulphite, sodium chloride, sodium sulphate and sodium phosphate. Potassium sulphite gave similar results showing that the suppressive effect is due to sulphite ions while other salts failed to produce such a trend. Another salt that brought about a similar effect was sodium thiosulphate at the same concentrations. [50]. The swelling vlomes were lowered at 0.01% and then increased. The solubility on the other hand experienced a drastic increase at the same concentration and this salt also be acting similar to sodium sulphite. Surfactants also affect the swelling volume of starches. The swelling volume was reduced by half by potassium palmitate and potassium stearate even at lowest concentrations, while glyceryl monostearate affected only to a small extent. In contrast sodium lauryl sulphate and cetyl trimethyl ammonium bromide enhanced the swelling volume considerably [17].

        Steam pressure treatment also lowered the swelling volume by compressing the starch molecules and thus restricting the free swelling of starch [39].

        Considerable variation in swelling volume of different varieties of Colocasia was noticed. The values ranged from 26.5 to 60 ml g-1 – which indicates a high degree of variability. For C-9 starch, having highest granule size, the swelling volume was the least [24]. Inverse relationship was noticed between the granule size and swelling volume of ten accessions of taro [30].

        Swelling volume of starch of six clonal selections of D. rotundata has also been examined at different concentrations and only slight differences were noticed among the selections. At 1% concentration the values ranged from 15 – 25 ml g-1 and at 5%, the range fell considerably due to deficiency of enough water to swell all granules [25]. Starch from different varieties of D. esculenta, D. alata, Xanthosoma sagittifolium and Amorphophallus paeoniifolius had much lower ranges. The relatively lower swelling of Dioscorea starch compared to potato starch has been attributed to the higher lipid content in the starch and also higher inter-associative forces compared to potato starch. Swelling volume of Canna edulis starch was observed to be 11-14 ml g-1 for three accessions of canna starch. In general, the aroid starches had rather low swelling volumes. Starch from ten accessions of A. peoniifolius had swelling volumes ranging from 11.7 – 13.0 ml g-1 at 0.5% concentration and 21.5 to 24.4 ml g-1 at 1.0% concentration [31]. For Coleus starch, the swelling volume was around 25 ml g-1 [16]. For Xanthosoma starch, Glycerol monostearate (GMS) and ammonia enhanced the swelling volumes to a small extent [33].

        Solubility of starch also varies with origin of starch. Solubility depends on a number of factors like inter-associative forces, swelling power, presence of other components etc. Cassava starch has a higher solubility compared to the other tuber crops and the higher solubility can be attributed partly to the high swelling it undergoes during gelatinisation. The solubility values ranged from 25 to 48% . The solubility of starch of different cassava varieties varied from 17.2 to 27.2%. However no direct correlation between swelling and solubility could be observed [3]. Solubility data of starch from different varieties during the growth periods also showed that starch of varieties H.2304 and M4 had good stability in their solubility, whereas the others had medium or poor stability [23]. The solubility was enhanced by heat moisture treatment.

        Solubility of cassava starch in various non-aqueous solvents has been examined. It was found that maximum solubility was obtained in DMSO and formalin, while in glycerol it was moderate. Starch was insoluble in anisole and methyl cellosolve (Tab.14). The solubility data indicate that starch is more soluble in polar solvents or solvents with affinity towards water [41]. The solubility of starch of yams and aroids pretreated with various chemicals was affected to different extent by the chemicals used and also the concentration [8]. The values were between 18-32%. Solubility of the other tuber starches varied from 10-30% and the aroids had usually lower solubilities.

      8. Clarity and Sol stability

        Transparency of starch paste varies tremendously among the different starches. The high clarity has much relevance in food and textile applications. Clarity depends on the associative bonds between the starch molecules in the granules. Cassava starch, having weaker associative forces compared to cereal starches has thus better clarity. When derivatives of cassava starch were compared, acetylated and propylated derivatives had better clarity, though only to a small extent. The clarity was best when pyridine-acetic anhydride was used for acetylation. This reagent gives highest clarity probably due to the high D.S achieved. . Esterification tends to weaken the associative forces by reducing the available hydroxyl groups [40]. In contrast, heat moisture treatment reduced the clarity by strengthening the associative forces.

        Dioscorea starches have almost equal clarity as cassava starch, indicating that their associative forces are similar. Variation was not observed among different varieties of Dioscorea alata, D. esculenta and D. rotundata starches. Steam pressure treatment was found to decrease the clarity of D. alata and D. rotundata starches and the reduction was directly proportional to the pressure used and time of treatment. The reduction in clarity on pressure treatment can be attributed to the strengthening of associative forces [34] The clarity of aroid starches is, as expected, poor and the values are closer to that of cereal starches. The clarity of starch pastes of ten accessions of Colocasia esculenta was nearly equal [24]. Similarly no significant variation among starch of ten different accessions of Amorphophallus paeoniifolius was observed [31]. The clarity of three accessions of Canna edulis was found to be much higher than the aroid starches. [26]. Generally the ‘B’ starches appear to have higher clarity compared to ‘A’ starches.

        Sol stability or paste stability reflects the retrogradation tendency of starch pastes. Cassava and sweet potato starches have low retrogradation tendency and therefore high paste stability.

        The lower retrogradation tendency of cassava may be due to the higher weight-average molecular weight of the amylose fraction in cassava . Dioscorea starches also have good stability, while the aroid starches have poor stability [3]. The results also reflect the differences in associative forces among the different tuber starches.

        The sol stability of cassava starch in various non-aqueous solvents has been studied. The values varied from 3 hours in ethanediol to over 20 days in formalin. Formalin may be preventing parallel association of the starch molecules, especially the amylose chains, by forming some complexes [41]. Sol stability was also affected by added surfactants. Sol stability was enhanced by derivatisation also, though there was no correlation between degree of substitution and sol stability. The paste stability of starch of different varieties of D. alata, D. rotundata, D. esculenta, X. sagittifolium, and A. paeoniifolius was nearly same. Heat-moisture treatment (steam pressure treatment) of Dioscorea starches decreased the paste stability and the observed reduction at higher levels of treatment was so high that the starch gel started settling within 2-3 hours, indicating that the starch molecules come so close to each other by the compressive treatment that they associate themselves very easily leading to fast settling [34].

      9. Digestibility

        Digestibility of starch by enzymes is of importance for evaluating nutritive value and also in industrial applications. Cassava starch is one of the least resistant root starches. In vivo digestibility of different tuber starches was studied on albino. The results indicated that in case of raw starch, digestibility of cassava, sweet potato, Colocasia, Xanthosoma and Amorphophallus starches was quite high (65-75%), comparable to corn starch (76%) but that of the Dioscorea was low (15-25%) similar to potato (10%) . On cooking, the digestibility was substantially increased for all the starches including yam and potato starches. The large increase in digestibility on cooking can be attributed to the change in starch structure on gelatinisation [51]. This was confirmed by the observation that the XRD patterns of all the cooked starches were similar, unlike the uncooked starch in which aroid starches have ‘A’ pattern, and yams posses ‘B’ pattern. A comparison of the digestibility was also carried out using pancreatic α-amylase under in vitro conditions. Under in vitro conditions, maximum activity was obtained on cassava starch while Colocasia suffered breakdown to much lower extent indicating that in vivo and in vitro digestibility cannot be directly compared. The digestibility could not be related either to the amylose content or the soluble amylose contents in these starches. Earlier reports had indicated that popping of sweet potato improved starch availability and nitrogen digestibility. . In a study of digestion of aroid and yam starches, effect of pre-treatment influenced α-amylase digestibility [33].

        The results are summarized in Tables. It is evident that the lipids form strong complex with the starches examined. However the results do not show any higher value for polar lipids compared to total lipids. It was expected that the polar lipids should form stronger complexes and should give higher values for both gelatinisation temperatures and enthalpy . Such result was not observed and it is presumed that more experiments should be conducted to verify this. There does not appear to be any distinct results for any of the starches examined. All this warrant further work in this area.

        The cereal starches have intrinsic lipids present on them. It was desired to find out if added lipases modify the gelatinisation characteristics of starches. It is wellknown that these lipids affect the starch gelatinisation to a large extent. The removal of lipids may also be helpful in reducing the lipid flavour associated with cereal starches. Wheat starch was used for the study. Lipase at three different concentrations (viz 1,3 and 5%) was added to a starch suspension in water (1:1) and the pans incubated at 370C for, 15, 30, 45 and 60 min and DSC run at the following conditions. First heating 15-150 at 50C/min, second heating upto 1300C at the same rate. The Gelatinisation temperatures and gelatinisation enthalpy starch-lipid complex melting temperatures and enthalpy were calculated using inbuilt software. The results do not indicate any major change in the starch gelatinisation temperatures or enthalpy for any of the concentrations examined. However some slight increase in starch-complex melting enthalpy was observed at 3% concentration of the enzyme for 30 minute treatment.. There was also corresponding decrease in the temperature of melting. It is not clear why the values again tend to change at higher period of treatment. It was expected that the lipase would act upon the native lipids present in the starch and weaken the complex and hence reduce the enthalpy of melting. But such a phenomenon does not happen and may be the lipase is unable to break the starch-lipid complex. Further work using higher levels of lipase and longer.

      10. Conclusions

        The studies on the different tuber crops reveal the vast variability available among them. Such differences are not observed in case of cereal starches . The high viscosity of cassava and Canna starches makes these starches very useful in many food and industrial applications especially where high thickening power is desired. The low viscosity of aroid starches can be exploited in paper industries which prefer lower viscosity and good film forming capacity. The small granular size of Colocasia and D. esculenta starches make these ideal as filler in biodegradable plastics, and in aerosols and talcum powders. The clarity of cassava, Canna and yam starches can be very useful in many food applications. Similarly the good gel strength of these starches, especially Canna starch can be utilised in a wide array of food products. The easy gelatinisation of cassava and sweet potato starches can make them suitable in manufacture of hydrolytic products derived from starch . The range of characters observed makes the tuber starches amenable to different applications based on their properties in place of chemically modified starches. An awareness of their potential uses can help in large scale cultivation of these crops and extraction of starch from them. It is also possible to modify the starch properties by simple physical methods like hydrothermal or steam pressure treatments Latest developments in biotechnology can also be tried to modify the starches. These include fermentation of starch by use of selective organisms or enzymatic modifications which can bring about specific substitutions. Lot of work has been done on fermentation of cassava and its effect on starch quality ] whereas the use of enzymes in starch derivatisation has not been exploited and offers very good scope for value addition.

        1. Modification of starches

          Blending cassava starch with maize and potato starches modified the viscosity properties with the resulting blend having properties in between those of the starches [46].

          The studies on the bsiac properties of different tuber starches show a wide varuiability in their physicochemical and functional properties. However they have some undesirable properties. Therefore attempts have been made to modify the undesirable properties while maintaining the desrable ones. These include physical and chemical modifications. The physical methods tried is steam pressure treatment and blending with other starches. . Chemical modifications include complexation with surfactants and derivatisations like esterifications, crosslinking and oxidation .

          • Steam pressure treatment.
          • Blending with other starches.
          • Chemical derivatisation

          Starch derivatives Since some properties of tuber starches like cohesive texture and poor viscosity stability of cassava starch, poor clarity and sol stability of colocasia starch are undesirable in food products attempts were made to improve the properties by chemical modifications In addition, production of some products by degradation of starch was attempted. Starch is chemically a polymer of glucose units joined together by α(1,4) linkages and partly α(1,6) linkages. These linkages render the starch susceptible to breakdown by various chemicals and enzymes unlike cellulose which is much more stable to many chemicals and enzymes. Thus it is possible to derive various partially degraded products with special properties suited to various applications in food and industry. In addition the presence of a large number of hydroxyl groups makes it possible for reaction with various chemical reagents and physical treatments which can modify the starch properties. These treatments can be degradative or nondegradative and a number of physical and chemical treatments were attempted. These include steam pressure treatment, complexation with lipids and surfactants, and chemical modifications viz. oxidation, derivatisation, crosslinking, etherification and fermentation. The effect of these treatments on the starch properties described below.

          Various surfactants and emulsifiers are added to starch to improve their functional characteristics and hence a study was conducted to examine how the surfactants affect cassava starch properties. The results revealed that different surfactants affected the viscosity differently. Whereas sodium lauryl sulphate increased the peak viscosity, especially at higher concentrations, the effect of potassium stearate and potassium palmitate was not so pronounced. Glyceryl monostearate was found to reduce the peak viscosity at higher concentrations . Similar effect was observed on the 2% viscosity also but to a much lower extent. In addition to peak viscosity, viscosity stability is important criterion for applicability of starch in food and industry. In this respect, cassava starch is inferior to maize starch, because its viscosity is rapidly reduced on heating under heat and shear which leads to a ‘long’ and cohesive texture for its paste, which is not desirable in food and textile applications. Much effort has been paid to strengthen the starch paste viscosity. Steam pressure treatment can improve the paste stability, but it is accompanied by a corresponding reduction in peak viscosity. However, surfactants had a more desirable effect. On incorporation of potassium stearate or potassium palmitate, even at 0.02% mole concentration/100 g starch, the viscosity was maintained and also stabilised. Since the surfactants are easily handled and work up is easier the method may be used to modify starch viscosity.

          It was found that sodium sulphite has a special effect on the swelling of cassava starch. In an examination of effect of the salt at various concentrations, at the concentration of 0.05% and 0.1% sulphite there was a dramatic fall in the swelling volume. Above this concentration, the swelling volume recovered to original value. The anomalous behaviour has been attributed to superoxide induced oxidative reductive depolymerisation of starch . At higher levels of the salt and in presence of oxygen scavenger propyl galate the effect was absent. Similarly the effect was absent in the Brabender Viscograph showing that the reaction is very specific to levels of oxygen. This type of thinning can have some applications which require starch solutions which have low viscosity levels.

          Ester derivatives were prepared from starch by different procedures. Various acids and anhydrides were used for esterification. The reaction was attempted using the following systems.

          1. Direct reaction with the acid
          2. Reaction of acid/anhydride in presence of alkalies
          3. Reaction of acid with starch in presence of catalysts
          4. Reaction with acylating agents
        2. Direct reaction with acid

          The esters prepared by direct action of acid were (a) formic (b) isobutyric (c) glycolic (d) thioglycollic derivatives.

          The degree of substitution (D.S) obtained by using different acids are given in Table 18. Lactic acid did not give an ester on direct reaction with starch due to its weakly acidic nature.

        3. Reaction of acid/anhydride in presence of bases

          The acid or its anhydride was reacted with the starch in presence of different bases. The bases tried included dilute alkali, pyridine, triethanolamine and triethylamine. In case of dilute alkali, though the yield of the products was good, the degree of substitution was low. Further substitution could be achieved by treating the samples again with acid anhydride and dilute alkali. However such repeated treatments could not increase the degree of substitution above 0.3. It was observed that a high degree of substitution was obtained when pyridine was used and the maximum possible D.S= 3.0 was achieved when pyridine in combination with anhydride was used. By controlling the amount of acid/anhydride used, desired level of D.S could be obtained. The other bases tried for the reaction, viz. triethanolamine and triethylamine did not give good D.S. indicating that their basicity is not enough to form stable complexes with the acid.

          Table 19: Yield, nature and properties of different ester derivatives of cassava starch

            Yield g 25-1gStarch D.S Viscosity(Seconds) Mode of preparation
          1. Formyl 20.7 0.23 50.0 Direct reaction with acid
          2. Glycollic 18.8 0.20 44.0 Do
          3. Thioglucollic 18.0 0.19 44.0 Do
          4. Isobutyric 19.3 0.22 51.0 Do
          5. Citric 24.2 0.05 58.5 Do
          6. Malic 22.5 0.06 56.5 Do
          7. Tartaric 22.9 0.02 60.0 Reaction with acid/anhydride in dilute alkali
          8. Succinic 23.5 0.10 57.0 Do
          9. Stearic 22.0 Very low Do
          10. Phthallic 23.2 Very low Do
        4. Catalytic esterification

          The esterification of cassava starch was tried with various catalysts. Metal halides were used in the reaction of the starch with acetic anhydride in acetic acid. The catalysts tried were ZnCl2, SnCl2, MnCl2and AlCl3. The results showed that SnCl2 gave highest D.S. followed by ZnCl2.

          The catalytic effect of perchloric acid on acylation of cassava starch was tried The results showed that the optimum temperature for the reaction to give good yield and a reasonable level of substitution was 300C. Propionic anhydride in presence of perchloric acid gave similar results. The D.S of the esters could be determined by finding out the intensity of the 1680 cm-1 absorption in the IR spectrum.

          The gelatinisation temperatures were slightly lowered by increasing the substitution. The associative forces are weakened by substitution of the hydroxyl groups and hence the earlier gelatinisation. Viscosity at 750C also showed a fall with increasing substitution, again due to the reduction of associative forces. Clarity was improved to a small extent by substitution by acetyl or propionyl groups. The paste stability is as high as 10 days when the D.S is around 0.10. This property is desirable for food purposes, where the tendency of starch to retrograde, especially on freezing and thawing, poses a problem.

          Ferrous sulphate and hydrogen peroxide catalysis did not give any notable level of substitution.

        5. Reaction with acylating agents

          Attempts to prepare esters by using sodium acetate-acetic anhydride gave a D.S of 0.05. The viscosity of the starch was 46.0 seconds (Redwood No.1), gelatinization temperature was 48-650C and it had acceptable clarity and good sol stability.

          The different ester derivatives prepared, their method and D.S are summarised in Table 4. It was generally observed that higher levels of substitution were obtained when pyridine was used as the base and the anhydride of the acid was used for the reaction. However, as noted for acetyl derivatives, the sol stability was reduced when pyridine was used for the reaction.

          Among the various derivatives prepared, acetyl derivative was found to be the easiest to prepare and had desirable properties. It was found that phthallic and stearic esters could be obtained only in low levels of substitution, probably due to steric factors.

          Chloroacetic ester of starch was prepared by reaction of chloroacetic acid with starch suspended in KOH solution at low temperatures. However D.S was low and the product did not possess good thickening property.

        6. Cross linking

          Starch granule strength could be improved by cross linking with poly functional reagents. These reagents bridge the starch molecules and prevent the breakdown under heat and hence reduce the cohesive nature of starch and minimise viscosity fall during holding period.

          The crosslinking agents used were epichlorhydrin, phosphoric acid and phosphorus oxychloride. The crosslinked products were obtained by standard procedure and showed improved stability. A sample of phosphate crosslinked starch had higher viscosity and less cohesive texture. Though higher crosslinking levels are achieved by using epichlorihydrin or phosphorus oxychloride, these reagents are difficult to handle and the reaction conditions are very selective (pH and temperature should be kept within specific ranges). The phosphate ester on the other hand can be prepared easily using phosphates and has no toxic effects. Hence this reagent is preferred to bring about crosslinking.

        7. Oxidative reactions

          Oxidation of starch can lead to various products depending on the oxidising agent used. Oxidised starch gives a clear fluid and adhesive paste which does not form a hard gel on cooling but retains its free flowing, adhesive nature. Films formed from oxidised starch pastes are strong, tough and horny, in contrast to the weak and brittle films of acid modified starches or dextrins. Oxidised starch has maximum use in paper industry and also in drilling muds as dispersant. Dialdehyde starch which is obtained by cleavage of 2,3-diol bond is useful for we-end application in paper industry. The basic polymeric structure is maintained and hence dialdehyde starch can be useful for synthesis of polyols.

          Oxidation of starch was tried with bromine under steam pressure. The amount of bromine used, the pressure and time of treatment were varied. The final product obtained was analysed for reducing value and viscosity. It was found that starch is degraded to a large extent during the oxidation by bromine as indicated by increased reducing values and decreased viscosity. There was no effect of sodium peroxide or benzoyl peroxide on the products under different conditions showing that the reaction is not radical initiated.

          Dialdehyde starch was prepared by oxidation of cassava starch with potassium metaperiodate The dialdehyde starch did not give blue colour with iodine, indicating that it loses the helical structure required for iodide-starch complex. The loss in the coiled nature is probably due to absence of enough hydrogen bonds to stabilise the coiled structure and also due to formation of hemiacetal type of structure by internal bond formation. This is confirmed by the fact that the dialdehyde starch shows only a very weak peak at 1680-1700 cm-1 in its IR spectrum. However, the dialdehyde starch gave condensation derivatives with hydrazine, urea and hydroxylamine, though to a small extent. These condensation products were formed in gelatinous form and were difficult to crystallise.

          Oxidised starch for possible use in drilling muds was made by permanganate and chromic acid oxidation. The starch was oxidised by using aqueous 0.05M permanganate and 0.5 M HNO3 or 0.1 M sodium dichromate and concentrated HNO3. The products stained blue with iodine and exhibited weak carbonyl absorption in the IR spectrum indicating that no 2-3 diol bond cleavage had taken place.

        8. Pregelatinised Starch

          Pre-gelatinised starch is used in various instant foods, since it is more miscible in water or milk compared to raw starch. Pre-gelatinised starch was prepared by heating, with continuous steady stirring the starch with minimum amount of water required to gelatinise the starch, until the starch was completely gelatinised (as evidenced by the translucency of the paste). After heating was stopped, the slurry was spread out into a thin film and dried in the sun or in an oven at 60-650C. The dried sample was powdered while hot and stored by keeping out of contact with moisture. The finely ground material was used for various preparations. A formulation including the pregelatinised starch, cocoa powder and sugar was found to be quite acceptable in taste and quality and was miscible in hot and cold milk and serve as an infant food.

        9. Modification by fermentation

          Six varieties of cassava having varying HCN contents were subjected to fermentation by a mixed culture inoculum comprising of Lactobacilli, Corynebacteria and yeast cells. The properties of the starch extracted from the fermented tubers were studied for possible modification during fermentation. Apparent reduction in total and soluble amylose contents was observed. Differential Scanning Calorimetry of the samples indicated that the enthalpy of gelatinisation was reduced, but the gelatinization temperature was enhanced. Marked reduction in Brabender viscosity values of starch from fermented tubers was observed, but he X-ray diffraction patterns remained unaffected. All these changes could be attributed to the presence of fibrous material and consequent reduction of starch content in unit volume rather than any major change in the granular structure of starch.

        10. Value added products from starch

          1. Starch adhesives

            The simple starch adhesive was made by dissolving starch in dilute alkalie with continuous stirring and chemicals like urea and borax were added to improve tack. Finally a small quantity of formalin was added as preservative. Use of Carboxymethyl cellulose and /or sodium silicate enhanced the viscosity. Use of glycerol did not improve the properties. A dry adhesive was made by the following technique. Starch containing 35% moisture was mixed thoroughly with 0.1 part of phosphoric acid and 1 part of urea. The moist mixture was dried under vacuum to 7% moisture level and mixed with 0.5% tri-calcium phosphate. The slurry was heated to 1250C with vigorous stirring and maintained at this temperature for 1 hr. The yellow product when mixed with 4 parts water and boiled, gave a transparent paste, which did not settle or become thick. Anti-fungal and antibacterial agents and stabilisers were added. This paste can be conveniently used as remoistening gum. Starch paste was also made by dissolving the starch in calcium chloride solution. The stability could be improved by adding some glycerol.

            Adhesives were prepared using cassava starch with polyvinyl alcohol (PVA). PVA was dissolved in cold or hot water, pregelatinised starch was added to give a ratio of starch:PVA 1:1 and stirred thoroughly for 2 hours until a uniform paste was obtained. To this material a plasticiser was added to maintain the uniformity of the paste. An antimicrobial agent (either copper sulphate crystals or 40% formalin) was introduced to ensure good storage life for the sample. The samples were tested for their efficiency in the following systems : paper and paper; paper and cardboard; plywood and hardwood; wood and wood ; ceramic and wood and ceramic and ceramic. The procedure adadpted for testing the samples was based on the one used at Forest Research Institute, Dehradun The paste was applied to the surfaces of the two materials and they were placed together and a weight of 2 or 3 Kg was kept on the sample for 3-6 hours. Afterwards the weight was removed and the pieces were tried to be separated by pulling using hand. A good adhesive is indicated by the surfaces sticking together not peeling away. The paste had excellent adhesive property in all the systems except in plywood and wood;ceramic. It was found to be most superior for ceramic:ceramic system and thus holds promise in the building sector. The drawback in the paper; wood and plywood; paper system was the hygroscopicity affected the stability and slowly there was decrease in the adhesiveness. For the ceramic:ceramic system, there was no letup in the stickiness even after prolonged storage even in humid conditions. This should serve as a good substitute for the synthetic pastes similar to fevicol. etc. In order to improve the resistance to moisture, hot water soluble PVA was used instead of coldwater soluble PVA and poly vinyl acetate was used in place of polyvinyl alcohol. Dioctyl phthalate was found to give good appearance and also good stability to the paste. It was also found that use of urea or borax in small quantities was helpful in increasing the pastiness of the samples.

          2. Carboxymethyl starch

            The conditions for the production of carboxymethyl starch were tsandardised at the laboratory level. The basic process consists of treating starch with alcoholic alkali to obtain a granular suspension to which the monochloroacetic acid or sodium salt of the acid is added and stirred for 4 hours. The product is precipitated in excess ethanol or methanol, filtered and dried to obtaina slight cream coloured product having high solubility and viscosity. The viscosity obtained was three times higher than that of starch at the same concentration. This product has high demand in textile and adhesive industries. The process was further modified to reduce the quantity of alcohol required for precipitation of the derivative. Three entrepreneurs are interested in the technology and it is expected that the know how will be transferred in the near future.

            Cold water miscible starch was prepared by solubilising the starch in dilute alkali and precipitaion in alcohol. The resulting granular product is transparent, miscible in water and possesse good viscosity. The product has potential as instant starch in textile applications.

            Starch phosphate was prepared by reaction of starch with phosphates in presence of urea and borax. The required quantity of the chemicals was dissolved in minimum quantity of water and the starch was thoroughly mixed. After drying overnight, the powder was subjected to heat treatment for 4 hours at 1450C and then washed with water-alcohol mixture to remove dextrins. A final washing with excess methanol yields a product with high viscosity, stability and solbility.

        11. Fructose syrup

          Table 1. Yield and total amylose content for starches extracted with ammonia solution and water

          Species Extraction medium Yield (%) Total amylose ( blue value)
          Cassava Water 21.8 ± 0.54 0.37 ± 0.012
          Cassava NH3 22.2 ± 0.37 0.37 ± 0.019
          Cololcasia Water 6.2 ± 1.79 0.28 ± 0011
          Cololcasia NH3 16.2 ± 0.37 0.26 ± 0.017
          Dioscorea alata Water 17.0 ± 1.43 0.45 ± 0.008
          Dioscorea alata NH3 18.3 ± 1.0 0.44 ± 0.010
          Dioscorea esculenta Water 17.7 ± 1.06 0.29 ± 0.004
          Dioscorea esculenta NH3 18.7 ± 1.14 0.28 ± 0.008
          Dioscorea rotundata Water 18.8 ± 0.85 0.40 ± 0.010
          Dioscorea rotundata NH3 19.5 ± 1.16 0.40 ± 0.013
          Sweet potato Water 13.0 ± 1.02 0.34 ± 0.008
          Sweet potato NH3 10.9 ± 1.10 0.35 ± 0.013
          Xanthosoma Water 20.0 ± 0.32 0.38 ± 0.014
          Xanthosoma NH3 20.5 ± 1.76 0.36 ± 0.015

          Table 2. Granule size and shape of different starches

          Starch Granule shape Granule Size (m)
          Cassava Round, Truncated, Cylindrical, Oval, Spherical, Compound 5 -40
          Sweet potato Round, Polygonal, Oval 5-35
          Col0Casia esculenta. Round, 1-10
          Xanthosoma saggittifolium Round, variable 10-50
          Pacchyrhizus erosus Round, Cupoliform or convex-biconcave 6-35
          Arrowroot Round, polygonal 5-50
          Amorphophallus paeoniifolius Round, Polygonal, 3-30

          Canna edulis

          Oval, polyhedral


          D. alata Oval 16-100
          D. esculenta Round, Oval, 2-15
          D. rotundata Oval, Polyhedral 10-70
          Coleus Round, oval 5-20
          Curcuma sp.   14-46

          Table 3. Table Varietal differences in starch

          Varieties Granule size (m) Alkali number (ml of 0.1 N alkali) Reducing values (Ferricyanide No.) Formic acid released on periodate oxidation ( ml of 0.01 N Ba(OH)2 Amylose content (Blue value at 660 nm0 Pasting temp. (0C) Viscosity of 2% paste (seconds)
          M-4 5.4 – 35.1 7.2 1.8 6.5 0.530 60.70 58.0
          Kalikalan 5.4 – 40.5 9.2 1.8 6.6 0.550 63.70 58.0
          H-1687 5.4 – 40.5 8.8 1.4 6.3 0.540 55.68 58.0
          H-2304 5.4 – 43.2 8.0 1.4 6.7 0.525 52.68 55.0
          H-226 5.4 – 43.2 3.4 1.8 6.65 0.500 55.66 56.0
          H-97 5.4 – 43.2 6.2 1.2 7.1 0.535 58.70 55.0
          H-165 8.1 – 48.6 7.2 1.6 6.9 0.505 52.65 54.0

          Table 4. Starch granule size, total and soluble amylose contents in Colocasia starch

            Granule size* (m) Total amylose content (%) Soluble amylose content (%)
          C-9 5.19 31.1 15.6
          C-62 2.96 26.3 15.9
          C-46 4.27 22.1 10.4
          C-149 3.06 23.4 15.6
          C-189 3.30 20.8 7.8
          C-216 3.51 22.1 10.4
          C-218 3.39 24.7 14.3
          C-220 3.55 29.2 17.1
          C- 226 3.16 24.7 16.5
          C-304 3.20 26.3 13.0

          * CD- 0.3011

          Table 5. Yield of starch, granule size and amylose content of Amorphallus starch

            Yield (%) Average granule size (m) Total amylose(%)
          Am 2 7.0 13.03 23.2
          Am 5 10.0 12.49 23.5
          Am 14 8.1 10.32 22.9
          Am 15 12.3 11.12 23.3
          Am 27 11.1 9.62 23.3
          Am 32 9.9 10.62 23.2
          Am 34 12.2 11.69 22.9
          Am 36 10.5 10.35 23.9
          Am 43 10.5 10.19 21.9
          Am 51 14.3 9.85 23.2

          Table 6. XRD patterns and Chemical shifts for C1peaks of different tuber starches

          Starch source Nature of C1 NMR peak XRD pattern
          Cassava Doublet A
          Col0Casia Doublet A
          Amorphophallus Doublet A
          Pacchyrhizus Doublet A
          Xanthosoma Doublet A
          Arrowroot Doublet A
          D. alata Singlet B
          D. esculenta Singlet B
          D. rotundata Singlet B
          Canna edulis Singlet B

          Table 7. XRD patterns and absolute crystallinities of starch/flour of different varieties of cassava

          Varieties XRD pattern Absolute crystalinity (%)
          H-97 A 8.8
          H-165 A 11.27
          H-856 A 10.16
          H-1687 A 11.47
          M 4 A 8.91

          Table 8. Amylose content (%) in tuber starches

          Cassava 18-28
          Sweet potato 16-27
          Colocasia esculenta 10-19
          Xanthosoma 15-28
          D. alata 16-25
          D. esculenta 15-27
          D. rotundata 18-27
          Pacchyrhizus 17-27
          Arrowroot 16-27
          Amorphophallus 15-25
          Canna edulis 18-29

          Table 9. Effect of cetyl trimethyl ammonium bromide on the amylose of different tuber starches

          Starch Total amylose blue values Solubleamyloseblue values
          Cassava 0.32 0.18
          Cassva+ctab 0.27 0.13
          Colocasaia 0.28 0.18
          Colocasia + CTAB 0.20 0.07
          D. esculenta 0.29 0.14
          D. esculenta + CTAB 0.22 0.04
          D. alata 0.43 0.18
          D. alata + CTAB 0.38 0.11
          D. rotndta 0.38 0.18
          D. riotunddata + CTAB 0.35 0,12
          Sweet Potato 0.38 0.13
          Sweet Potato + CTAB 0.34 0.09
          Xanthosoma 0.38 0.21
          Xathosoma+ctab 0.33 0.15

          Table 10. DSC data on starch from varieties of cassava

          Variety 0C Tonset Tmax Tend H cal g-1
          H-97 69.36 72.29 77.13 3.43
          H-165 65.35 69.22 74.86 3.27
          H-856 65.62 70.14 74.94 2.65
          H-1687 67.12 71.45 75.39 3.15
          M-4 68.20 73.24 78.54 2.95

          Table 11. DSC parameters of different tuber starches

            Tnset Tmax Tend H J g-1
          Cassava 68.5064.0 71.2ND 74.7276.9 12.416.6
          Sweet Potato 61.3 73.2 84.5 15.3
          Col0Casia 83.2379.9 85.68ND 90.0285.0 12.8810.6
          D. alata 77.2173.74 81.52ND 85.4480.2 11.615.4
          D. esculenta 75.9265.7 79.75ND 85.6875.35 13.6413.25
          D. rotundata 79.0272.17 83.12ND 87.9580.8 10.2815.01
          Xanthosomasaggittifolium 83.1174.8 85.72ND 90.4179.5 9.0815.22
          Pachyrrhizuserosus 63.6 ND 76.6 13.65
          Canna edulis 65.35 ND 70.85 16.04
          Amorphophalluspaeoniifolius 77.8 ND 83.53 16.6
          Arrowroot 68.5 68.5 85.0 15.6

          Table 12. Rheological properties of tuber starches in a Bohlin Rheometer

            P (3%) P (4%) P (5%) V (3%) V (4%) V (5%) G (3%) G (4%) G (5%)
          Cassava 38-56 17-29 10-25 0.03-0.14 0.3-0.45 0.5-0.8 0.8-1 3.5-6.5 8-22
          Col0Casia 19-90 35-46 22-33 .003-.01 0.05-0.19 0.24-0.47 0.01-0.16 1.3-1.6 2.9-5.8
          C. edulis 11-21 8-14 9-15 0.7-1.1 1.2-2.9 2.2-3.2 8-25 35-49 70-93
          D. alata 12-24 10-20 6-16 0.16-0.33 1.1-2.2 1.6-2.6 6-12 36-53 54-92
          D. esc 14-70 21-35 14-27 0.01-0.07 0.02-0.08 0.04-0.21 0.02-0.1 0.1-0.28 0.6-5
          D. rot 9-24 7-15 9-16 0.03-1.8 1.1-2.6 1.2-2.6 12-35 23-39 48-66
          Amorph 25-29 17-24 17-22 0.12-0.28 0.4-0.8 0.7-0.14 1.4-3.5 7-11 14-23
          Arrowroot 16-20 14-19 14-18 0.25-0.3 0.45-0.68 0.6-1.05 5-8 8-16 13-23
          Pacchyr. 30-73 21-42 13-28 0.01-0.02 0.07-0.11 0.14-0.2 0.04-0.53 0.7-1.7 1.3-3.9
          Xantho 27-65 12-20 5-24 0.02-0.1 0.2-0.34 0.1-0.57 0.1-0.9 5-7 1-33
          Units of V and G Pascals

          Table 13. Swelling volume, swelling power and solubility of cassava starch of different varieties

          Varieties Swelling volume ml/g starch Swelling power Solubility%
          M4 30.5 38.5 22.8
          Kalikalan 38.8 51.4 24.8
          H-1687 25.5 35.1 23.6
          H-2304 30.5 39.5 24.8
          H-226 33.8 42.6 27.6
          H-97 30.5 34.6 17.2
          H-165 37.8 51.8 27.2
          Ichyapuram l0Cal 41.8 54.3 24.4

          Table 14. Solubility, gelatinisation temperature and viscosity of starch solutions

            Solubilityg/100 ml Gelatinisation temperature 0C Viscosity (2%) Redwood sec. Viscosity (2%) after cooling sec.
          Glycerol 10 130-145 175 1800
          Ethanediol 2 110-125 17.5 38
          DMSO 25 75-85 85 130
          Formailin 25 70-85 78 139
          DMF Nil
          Methylcellosolve Nil
          Anisole Nil

        What is the potential value of the results in increasing the production, productivity, profitability and substantiality of agricultural enterprises in the relevant field ?

        The tropical tuber crops have serves as subsidiary and subsistence food for millions all over the humid tropics. However, these crops have lost their glory due to a number of factors like preference for cash crops, changing food habits, ready availability of cereals and lower profit from the crops. This has led to decline in interest in the crop and in order to regain the past status it is imperative that people are provided with good culinary varieties and steady market for the produce. The study on cooking quality have revealed the major conditions necessary for the best quality. With rapid strides being made in Biotechnology and Computer modeling, the breeder can develop suitable varieties with desired quality based on the results in the present study. It is possible to modify starch properties, and other biochemical influencing the starch characteristics and arrive at the correct combination.

        Concerted efforts are on way to popularise the tuber crops in tribal areas and the people can be made aware of the food and industrial applications of these crops so that they get acceptance as cultivated crop.

        The methodology for extraction of starch from minor tuber crops can open up a new vista for these crops. Commercial cultivation will become a reality giving a boost to their cultivation.

        The wide variability available in the tuber starches can be exploited for specific applications and they can replace some of the chemically modified starches. This is all the more relevant since there is growing resistance to use of chemically modified products. Some of the special properties like small granule size of colocasia and D. esculenta tubers make them suitable for use as filler in biodegradable plastics and in aerosols and toilet formulations. The high viscosity of canna and yam starches can be exploited for food and industrial applications.. The gel strength of Canna starch makes it particularly suitable for food applications. The bland taste, white colour, clarity and sol stability of cassava renders this starch highly valued in various food products like puddings, gravies, pie fillings etc. With widespread awareness and applications of these characteristics, there will be high demand for these starches and thereby for the crops .

        Since now the basic information on all the properties and applications of tuber starches have been deciphered, it can serve as first step towards the concept of Starch Data Bank for ready reference and Starch Bank for possible mutual exchange among scientists and industrialists interested in starch and starch products.

        The modification of starch by physical methods like steampressure treatment and blending are environment friendly methods and will receive lot of attention in the future. The tuber crop farmers can grasp this opportunity by acting as a source of special qualitry starch.

        The complexation of starch with lipids and surfactants can be exploited to produce the so-called Resistant starch which have come into prominence recently due to their lower calorific value and hence suitability a s food for the diabetics and the obese. It has been proved that there is no inhibition for the tyber starches to complex qwith these compounds.

        The development of starch derivatives is a prime area because there is a lot of demand for specaility starches for specific applications. Here we can tap the export market also since the modified starches are in high demand in many western countries and Japan. Advances in Biotechnology can also lead to production of starch derivatives by use of enzymes or microorganisms

        Products like ecofriendly detergents and surfactants made by derivatising starch with long chain fatty acids have much potential.

        Value addition is the watchword of the present day and production of various value added products is therefore all the more relevant. Products like fructose syrup, pregelatised starch are examples of high level of value addition benefiting both the cultivator and the industrialist. This is all the more relevant in the WTO regime.

        That has been the actual impact of these findings on production, productivity, and sustainability of agricultural enterprises in the relevant field ?

        The demand for starch is expected to grow sharply in the near future. Survey conducted by our Institute has revealed that m tonnes of cassava starch will be required by 2020 compared the present consumption of m tonnes. The textile industry will be needing m tonnes from the present m tonnes, while the paper and cone industries are expected to need an additional quantity of m tonnes by 2020. Similarly adhesive industry will also require much larger amounts in the future. Other industries producing pharmaceuticals, detergents, explosives, drilling muds etc. will also steadily demand higher starch quantities. Therefore the present results are very important in the industrial scenario of the country especially of the states, which grow these crops.

        There is ample scope for export of starch and starch derivatives. The experience of Thailand is worth emulating. It has earned the pride of place as the highest exporter of starch and derivatives. India can also compete in this field by producing quality and speciality starches for export market.

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