Tropical Tuber Crops

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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 are soft, dry and mealy. The poor cooking ones take long time to cook and are hard, glassy (non-mealy) 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 behavior 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 behavior 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 imbibition of water during gelatinization of starch leading to swelling of starch granules . Hence the changes occurring in the weight and volume of tubers during cooking were examined at definite time intervals. The softness was measured using a fruit hardness tester. The mealiness of cooked tuber was visually compared. The results derived from the tests on a number of varieties revealed that the cooked tubers could be classified into three broad categories based on the change in volume , weight and softness after cooking (Table 1).

      Table 1 : Physical changes in cassava tubers on cooking

      Category	Cooking quality	Appearance of cooked tubers	Vol.	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)

      Thus tubers undergoing 5-15% increase in weight and volume can be considered good cooking ones, whereas those which undergo reduction in these parameters or too much increase in weight and volume come under the poor cooking group. Swelling of starch is the major process during cooking. 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 an easy and convenient physical method for determining the cooking quality of tubers.

   2. Factors responsible for cooking quality

      In order to understand the factors responsible for imparting good quality to cassava tubers, starch content, starch properties and interaction of starch with other ingredients were examined for a number of varieties having diverse cooking quality.

      1. Starch content

         Since starch is the major biochemical component in cassava, the role of starch content in deciding cooking quality 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 implies that enough starch molecules are not available to swell and impart mealiness. It is also possible that since the number of granules is 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 disintegrate 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 content of 25% on fresh weight basis could be considered imperative for providing good cooking quality.

      2. Starch properties.

         Since it was clear that starch content alone did not decide the cooking quality, a comparive 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, (Tab 2) Coulter counter studies indicated that there is variation in the distribution pattern among the granular sizes (Fig 1). Thus H-1687 starch showed a higher frequency in the range of 13-16 mm and relatively less in the range of 6-13 mm 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. 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 molecules. The starches and flours from five cassava varieties exhibited \u2018A\u2019 pattern and no difference in the \u2018d\u2019 spacing was evident (Fig. 2). The Absolute Crystallinity of starch determined from the X-ray Diffractographs also did not vary very much among the different varieties (Tab 3]. 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]

         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 (Tab.2). The soluble amylose content also showed only little range suggesting very little role [Tab 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 Polymerization. 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 plots [Fig 3] showed only slight variations in the GPC patterns leading to the conclusions that amylose contents and chain lengths are almost similar among the varieties.

         5. Swelling power 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 and H-1687 had lower swelling power compared to varieties like H-165 (Tab. 4 ] . 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 providing 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 4], 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 M4 starch appears most resistant to breakdown and thus provides consistent swelling pattern.

         6. DSC patterns

            Differential scanning calorimetry is an important tool used to study starch gelatinization and provides wealth of information on starch structure. The DSC patterns of starch from five varieties were obtained using a Perkins Elmer DSC equipment. The thermograms showed distinct features for some varieties (Fig 5, Tab.5). . 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 thermo gram of starch of M4 indicates that the crystallites melt very slowly during gelatinization, again highlighting the 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 behavior of M4 starch.

         7. Gelatinization temperature

            The ease of gelatinization can also play an important role in starch swelling and hence the gelatinization temperatures of starch of different varieties determined microscopically were compared. The results (Tab 2) indicated that H-165 starch gelatinized relatively earlier. Early gelatinization means that the gelatinized granules are being subjected to longer periods of heating making them more susceptible to breakdown and thereby resulting in poor quality. Gelatinization 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 gelatinization by H 165 starch (Fig. 6). M4 starch exhibited a high gelatinization range showing the stronger intermolecular bonding. Thus slow and steady gelatinization of starch is preferable to early and rapid gelatinization so that the starch granule is able to maintain its structural integrity.

         9. Viscosity

            Viscosity is an important property of starch and was determined for the different varieties using a Brabender Viscograph at different concentrations. There was clear difference in the patterns for the starch of different varieties (Fig. 6).
            It was observed that H.1687 starch had a medium peak viscosity, low viscosity breakdown but high set-back viscosity. M4 starch possessed 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. The results show that the starch of H-165 undergoes rapid increase in viscosity and under shear and heat suffers breakdown. The collapse of the swollen granules leads to cohesiveness for the paste brought about by the broken starch molecules and hence may be responsible for the poor cooking 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 behavior 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 (Tab.2).

            Table 2 : Varietal differences in starch

            Varieties	Granule size (mm)	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 3 : XRD patterns  and absolute crystallinities of starch/flour of different varieties of cassava

            Varieties	XRD pattern	Absolute Crystallinity (%)
            		Starch	Flour
            H-97	A	8.8	8.4
            H-165	A	11.27	11.9
            H-856	A	10.16	10.0
            H-1687	A	11.47	10.98
            M 4	A	8.91	9.10

            Table 4 : 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 local	41.8	54.3	24.4

            Table 5 : DSC data on starch from varieties of cassava

            Variety	T onset 0C	T max 0C	T end 0C	H cal g-1
            H-97	69.36	72.29	77.13	3.43
            H-165	65.35	69.22	74.86	3.27
            S-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

      3. Effect of other ingredients

         Often it is found that tubers having high starch content and desirable theological characteristics do not cook well and others factors like environmental conditions and age of the crop influence the cooking quality. In addition, the starch properties determined after extraction need not truly represent the real situation in the tubers. 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 besides starch are fiber, sugars and much smaller quantities of proteins, lipids and minerals. These factors were determined and their effect on the starch properties was studied.

         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 these determinations were (i) proximal (A) (ii) middle (B) (iii) distal (C) . From each portion, 3 regions viz., outer (O) (1-2 mm inside of the skin) ; Inner (I) and Core (C) (5-10 mm diameter around the cortex) were examined. The results (Tab 6) clearly revealed large variation in starch, sugar and dry matter contents between the regions in some varieties whereas the variation was much less in some other varieties. There was also considerable difference in the distribution of starch between tubers having good cooking quality compared to tubers showing bad quality of the same variety. 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 Inner 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 inner. In fact, the starch content was found to be as low as 6-8% and the sugar content exceeded the starch content . H-2304 which has a high starch content ,shows over 40% in the inner region while 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 inner. In varieties, where the core starch content is more or 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 tubers of these varieties increase their volume on cooking, whereas the poor cooking ones show decrease in volume. The tubers of varieties H-165 and H-226 undergo rapid decrease in 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. The rapid reduction of starch in the Core may be due to starch degradation or reduction in starch synthesizing activity.

            The swelling volumes of starch extracted from different regions also show interesting trends. In tubers of M4 having 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. 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 weaker associative forces. This is further confined by the results obtained in 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 \u201ccritical concentration\u201d, 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.

         2. Effect of other ingredients on starch 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, fiber contents were determined. In addition, the content of Calcium which is known to cause a firming effect on potato tubers and phosphorus which always accompanies starch were determined.

            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 fiber present in the tubers on starch gelatinization. Whereas absolute Crystallinity , XRD and DSC patterns were hardly affected, the viscosity behavior and swelling characteristics were considerably influenced by the fiber. There was noticeable reduction in swelling and peak viscosity of the starches in presence of fiber. The fiber acts as a partial barrier to free entry of water molecules during gelatinization and hence allows only restricted swelling. The viscosity patterns are modified by the fiber 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 respectively hardly affected the swelling and viscosity patterns again confirming that it is the fiber rather than fat or sugars that exert higher influence on the starch properties. This fact is further confirmed by the swelling and viscosity behavior of the starchy flour obtained from inoculum provided fermentation of tubers. The starchy flour thus obtained contains large quantity of fiber and they modify the starch properties considerably. In fact it was observed that food products made from flour possess less stickiness compared to those made from starch alone. So fiber has a definite role in deciding the starch properties.

            The calcium and phosphorus contents did not show any noticeable difference among the starch of different varieties and use of calcium sequestering agents like metaphosphates did not bring about any change in cooking quality and these may have not much role in deciding cooking quality.

            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. 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.
            3. Presence of other components. Fiber and sugar affect swelling of starch. Whereas fiber restricts swelling and thereby prevents viscosity breakdown , sugars impart higher solubility to starch. So a small quantity of fiber along with starch can lead to better quality.

            The physical parameters 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 like uniform distribution of starch in the tubers, lower breakdown tendency for the starch and lower sugar content.

            Table 6 : Distribution of starch and sugar in different portions and swelling volume of starch

            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