Keywords: Tropical tubers; Starches; Physicochemical properties; Rheological properties.

1. Introduction
2. Extraction Techniques
3. Other components in Starch
4. Colour and Appearance
5. Granule Shape and Size
6. Spectral Features
7. X-Ray Diffraction Pattern
8. Molecular Weight
9. Amylose Content
10. Thermal Characteristics
11. Gelatinisation and Pasting Temperatures
12. Viscosity
13. Swelling Power
14. Solubility
15. Clarity
16. Sol stability
17. Digestibility
18. Conclusions

Introduction

Dioscorea rotundata is an important edible root crop grown widely in Africa and forms a prestigious component of the West African diet. Its taste is regarded as superior to that of other root crops [1]. However, there is little or no cultivation of D. rotunda in Asia [2]. Recently this crop has been introduced into India by the Central Tuber Crops Research Institute, Trivandrum, and has been found to adapt and yield well in different areas of the state of Kerala and promises to be a potential crop. Two selections, viz. Sree Subhra and Sree Priya, have recently been released by the Institute and they have been found to give a tuber yield of 35 to 40 t/ha and have acceptable quality.

The major component of the tubers is starch which is found to be over 75% of the dry matter content. There have been some studies on the D. rotundata starch, but systematic investigation on the properties in relation to varietial differences is lacking. The shape and size of the starch granules have been examined by some workers [3, 4]. Rasper found that the starch of most of the varieties he studied, possessed oval shape with an average length of 52 ?. The average granule size reported by Rodriguez- Soza and Parsi Ros is 33 ?m [4]. The amylase content was found to be around 21% [3]. The paste viscosity studies using Brabender amylograph have been carried out in detail by Rasper [3] and Rodriguez-Soza and Parsi Ros [4] and Rodriguez-Soza et al. [5]. They found that the starch has good viscosity stability and gel strength and also found that swelling power increased with temperature, reaching very high values at 95°C. There is possibility of wide variation in the properties of starch of different clones of D. rotundata. This paper deals with the study of various properties of starch of six clonal selections of D. rotundata. The results can throw various applications in addition to other tuber crop starches.

Material and Methods

The six clonal selections used for the study include two released varieties, viz. Sree Subhra (I-146), Sree Priya (U-195 (2)); two promising selections. (T3-(1) and 212), and two darf clones, 183 and 184. The crop was cultivated according to standard practices and harvested at 10 months stage.

Starch was extracted according to standard procedure [6]. The tubers were washed free of mud and peeled to remove the thin outer skin. The washed tubers were sliced and 1 kg of the material was crushed in a Waring Blender at room temperature using a large excess of water. The starch was allowed to settle and the supernatant decanted off. The starch cake was powdered and dried at 40-60°C for 24 h. The starch was accurately weighed and transferred to polythene covers for further studies. Microscope observations on the starch granules were carried out according to MacMasters [7]. Reducing values were obtained by Schoch’s procedure [8] and expressed as Ferricyanide numbers. The total amylase content was determined by method of Sowbhagya and Bhattacharya [9] and soluble amylase using procedure by Shanty et al.[10]. For 2% solution viscosity data, ISI procedure was followed and the same solution was used for determining the clarity and sol stabililty. Clarity was based on the absorbance by the solution at 500nm compared to water = 0, while sol stability was the time taken, in hours, after which the starch gel starts settling.

Brabender viscographic pattern 5% and 6% starch pastes was obtained on a Brabender viscoamylograph (model 80102) using 350 cm g cartridge. The speed of the rotor was fixed at 75 rpm and the rate of heating and cooling was 1.5°C/min. The viscosity at 97°, the viscosity after 20 min. stirring at 97°, the viscosity after cooling to 25° and the peak viscosity were read out from the curves and expressed as Barbender Units (B.U.). The pasting temperature was also obtained from the curves Swelling volume of the starch was determined at 95°C by standard procedure [11]. Six concentrations, 0.5, 1.0, 1.5, 2.0, 3.0 and 5% w/v were used in order to understand the effect of concentration on swelling properties. The phosphorus content of the starch samples was determined by the vanadomolybdate method [12].

Table 1 : Yield, Granule Size, Reducing Value and Amylose Content of D. rotundata Starch.

Results and Discussion

The percentage of starch that could be recovered from the peeled tubers is given in Table 1. It is seen that the recovery of starch is quite high for all the cultivates, being above 20%. The highest value was for I-212 (23.9%). The values can be considered as good recovery, since the starch content of D. rotundata is around 23-25% on fresh weight basis, and, hence, there is little loss during extraction. The high recovery on overnight settling indicates that mucilaginous substances present do not interfere in starch settling. The results point out that the starch of D. rotundata can be easily and profitably extracted for various purposes. This can also give an impetus to the cultivation of the crop on a large scale.

The shape of the starch granules of the different cultivars was the same, viz. round to oval. Most of the granules were intact and single. This is similar to that reported by Rasper [3], who found oval granules in most of the varieties he studied. The size of the granule showed at wide variation, in the range 7.5-75?m (Table 1). There was no significant variation between the different samples. The average granule size varied from 27-34 ?m, with highest value being recorded for cultivar U-195 (2) and lowest for 183. These values are in the same range as reported earlier by other workers [3, 4]. Rasper found the size range to lie between 10-70 ?m, with a length 40-52 ?m and width 23-27 ?m [3]. Rodriguez-Soza and Parsi Ros obtained a value range 13-52 ?m with an average value of 33 ?m. The starch granules are thus smaller in size compared to D. alata and much bigger than D. esculenta starch granules [13]. The reducing values of the starches were found to be below 1.0 (Table 1) for all the cultivars indicating that the average molecular weight is high and similar to other tuber crop starches.

The total amylase content of the different cultivars was found to vary between 20.9 to 24.6%, with highest value for U-195 (2) (Table 1). The results are in conformity with the earlier reported values, 23-25% by Rasper [3] and 21-23% by Kay [14]. The amylase content is slightly higher than in cassava and D. esculenta starches and similar to D. alata starch. In spite of the higher amylase content, the starch does not behave as high amylase, being in the neighbourhood of 15% of the total starch. The soluble amylase content in D. rotundata starch has not been reported earlier.

The 2% viscosity of starch of the different cultivars showed variations between 37 and 46 s, highest being for U-195 (2) and lowest for 183 (Table 2). The Redwood viscosity values compare well with cassava starch, which usually has a value of 45-50 s. ISI has fixed a minimum value or 44 s for cassava starch for use as sizing agent in textile industries, and the starch of Dioscorea has viscosity very near to that of cassava.

The paste viscosities at 5% and 6% concentrations are given in Table 2. Notable variation in peak viscosity was observed between different cultivars with values ranging from 325 B.U. for starch of 184, to 550 B.U. for starch of U-195 (2) at 5% concentration. At 6% concentration, the values rose to 650 to 920 B.U. The data on viscosity values at 97°C, show that the complete gelatinization occurs in most cases after 97°. This indicates that the associative forces in the starch are strong and require prolonged heating to be disrupted before water enters. The viscosity after holding period is only a few units less than peak viscosity reflecting the strength of the starch paste. The gel strength is observed for all the cultivars and has been highlighted by earlier workers [3, 4]. The high gel strength is desirable in many food applications, since an unstable gel leads to a cohesive texture. Such high viscosity stability is not observed for cassava and sweet potato starches. The strong associative forces in the starch granules not only restrict the easy entry of water, but hold the swollen starch molecules together preventing breakdown.

Table 2 : Viscosity Properties and Pasting Temperature of D. rotundata Starch.

Table 3 : Swelling Volume, Clarity and Sol Stability of D. rotundata Starch

The pasting temperature as given in Table 2 show only minor variation in the initiation point. However, I-212 showed a slightly earlier rise in viscosity, but it is not significant enough to infer that the starch of this cultivar has weaker associative forces.

The cooling curves exhibit only a slow and steady rise and no sudden rise characteristic of high amylase starches. This shows that the starch has only low retrogradation tendency similar to other tuber crop starches. This is, however, in contrast to the earlier report by Rodriguez-Soza who found a notable retrogradation tendency in Dioscorea starch. Rodriguez-Soza et al. found that the viscosity patten of D. rotundata starch is also pH dependent. The peak viscosity increased when pH was increased from 3 to 5, decreased till pH=7.0. The retrogratdation tendency was highest pH 5.5 and lowest pH 3[5].

The swelling volume of the starch of different cultivars as determined at 95°C for various concentrations is presented in Table 3. The swelling volume goes on increasing with increasing concentration until it tapers off above 2% concentration. At higher concentrations there is not enough water for the granule to imbide and swell. The largest swelling volume was obtained for I-146, at 0.5% concentration but at the higher concentrations, U-195(2) and I-146 possess higher swelling volumes. This is in line with the higher peak viscosity obtained for these starches. Lower swelling volumes were observed for starch of 183 and 184, which have lower peak viscosity. These starches may be possessing stronger associative forces. The values reported earlier [4] for the swelling power of D. rotundata starch at temperatures above 85°C were much higher with a correspondingly higher solubilitys. However, we could not obtain such high swelling volumes or solubility even at 95°C.

The clarity of the starch paste lies between those of cassava and potato starches. There is only minor variation between the different cultivars (Table 3). The clarity of the starch is similar to other tuber starches and unlike high-amylose starches.

The sol stability was low, but within acceptable ranges. Only very little difference was noticed among the different samples, the values being between 24-36 h. The retrogradation tendency observed in case of high amylase starches is not noticed in D. rotundata starch.

In view of the finding that the starch exhibits high gel strength, it was attempted to correlate the phosphorus content of D. rotundata starch with the gel strength. As seen in Table 3, the P content of this starch was three to four times that of cassava starch, but much lower than potato starch. It is already reported that the high gel strength and peak viscosity of potato starch is due to phosphate crosslinkages which are more difficult to break [15]. Probably, Dioscorea rotundata starch has also such phosphate crosslinkages, which render its properties like high gel strength, and viscosity stability different from those of cassava starch. However, this needs further confirmation.

Bibliography

1. Horton, D., J. Lynam and H. Knipscheer: Proc. Sixth Symp. Intern. Soc. Trop. Root Crops, CIP, Lima, Peru, 1984, p.21.
2. Onwueme, I.C.: Tropical Root and Tuber Crops, John Wiley and Sons, New York, p.234.
3. Rasper, V.: Proc. First Intern. Symp. Trop Root Crops, Univ. West Indies, Trinidad, 2, 1967, p.48.
4. Rodriguez-Soza, E.J., and O. Parsi-Ros: J. Agr. Univ. Puerto Rico 66 (1982), 27.
5. Rodriguez-Soza, E.J., and O. Parsi-Ros, and M.A. Gonzlez: J. Agr. Univ. Puerto Rico 65 (1981), 154.
6. Badehuizen, N.P.: In: Methods in Carbohydrate Chemistry, Vol.4, Ed. R.L. Whistler, Acad. Press, New York 1964, p.14.
7. MacMasters, M.M.: In: Methods in Carbohydrate Chemistry, Vol.4. Ed. R.l. Whistler, Acad. Press, New York 1964, p.233
8. Schoch, T.J.: In: Methods in Carbohydrate Chemistry, Vol.4. Ed. R.l. Whistler, Acad. Press, New York 1964, p. 64.
9. Sowbhagya, C.M., and K.R. Bhattacharya: Starch/Starke 23 (1971), 53.
10. Shanty, A.P., K. R. Bhattacharaya, and C.M. Sowbhagya: Starch/Starke 32 (1980), 409.
11. Sair, L.: In: Methods in Carbohydrate Chemistry, Vol 4. Ed. R.L. Whistler, Acad. Press, New York 1964, p.284.
12. Jackson, M.L.: Soil Chemical Analysis, Prentice Hall India Ltd., New Delhi 1968, p.498.
13. Gallant, D.J., H. Bewa, Q.H. Buy, B.Bouchat, O.szylit, and L. Sealy: Starch/Starke 34 (1982), 255.
14. Kay, D.E.: Root Crops Digest, TPI, London 1973, p.235.
15. Leach, H.W.: In: Starch – Chemistry and Technology R.L. Whistler and E.F. Paschall, Acad. Press, New York 1965, p.297.

Dr. S.N. Moorthy and S.G. Nair, Scientist S2 (Organic Chemistry) and Scientist S2 (Genetics), respectively, at the Central Tuber Crops Research Institute, Sreekaryam, Trivandrum- 695 017, India.

Central Tuber Crops Research Institute, Trivandrum 695 017, India

Abstract : Starch was extracted from the tubers of ten accessions of Amorphophallus paeoniifolius of the same maturity, with a yield varying from 7 to 14 % on a fresh weight basis. The starch was pure white in color. There was no significant difference in granule size between the different accessions. The starch granule size and viscosity values were lower in A. paeonifolius than in Dioscorea spp. And cassava, but the viscosity was higher than that of cereal starches. The viscosity stability was good, indicating suitability for many food applications.

Keywords : elephant yam, aroids, Amorphophallus paeoniifolius, starch properties.

Introduction

Amorphophallus paeoniifolius is an important tropical tuber crop grown in many parts of India, Philippines, Malaysia, Indonesia and Sri Lanka (Ramachandran 1977; Ghosh et al. 1988). The subterraneous tuber is hemispheric, globose and weighs 2-8 kg at harvest. The flesh of the tuber is often yellow in color and normally contains 8-18% starch ( fresh weight basis) (Rajendran et al. 1977). The tubers are consumed as a vegetable after cooking well to remove the acrid properties present in them. There have been few reports on the properties of the starch of Amorphophallus (Wankhede and sajjan 1981; Soni et al. 1985) and the studies are restricted to a single local variety. There is little morphological variation among the different accessions of Amorphophallus available in the CTCRI Germplasm.

Materials and Methods

The accessions used (Am 2, Am 5, Am 14, Am 15, Am 27, Am 32, Am 36, Am 43 and Am 51) were grown at the CTCRI farm under standard practices and harvested after 10 months. Starch was extracted from the tubers by the method of Moorthy (1991). Tubers were washed, peeled and cut into small pieces, then 1 kg was washed again and disintegrated in a mixer fro 1 min with 0.03 M ammonia solution. The starch milk was passed through 80- and 260-mesh sieves and the suspension allowed to settle overnight. The supernatant was withdrawn and resettled fro 6-8 h. The combined residues were resuspended in water and allowed to stand for 6 h. The resulting wet starch cake was crushed manually, spread out to dry in the sun for 8 h and then oven dried at 50⁰C for 12 h. The sampling was duplicated to give the yield of starch.

Granule size was measured at 10x and 45x magnification using an ocular micrometer. A total of 100 granules were randomly selected from five fields ( 20 granules per field ) and the mean was calculated. Distilled water was used for mounting and 0.1% iodine solution for staining the granules.

The total amylose and soluble amylose contents were determined by the procedures of Sowbhagya and Bhattacharya (1971) and Shanthy et al. (1980), with six replications. Swelling volumes were obtained by Schoch’s method (1964) at concentrations of 0.5, 1.0, 1,5, and 2.0% w/v; three replications were carried out.

The paste viscosity was monitored on a Brabender viscoamylograph, Model 801002, using a 350 µg cartridge. The starch suspension was heated from 50⁰C to 97⁰C at the rate of 1.5⁰C/min . Concentrations were 5, 6 and 7%. After holding the paste at 97⁰C for 30 min, it was allowed to cool to 50⁰C. The pasting temperatures were read from the viscosity curves. Viscosity of 2% starch solutions were also determined using Redwood viscometer (ISI 1969) in triplicate.

Clarity and paste stability were obtained from a 2% solution using the absorbance of the sample at 500 nm compared to water and the time taken by the starch to start settling from the solution respectively. The X-ray diffraction pattern was obtained on a Philips PW 1730 X-ray system using monochrome CuK? radiation.

Results and discussion

The yields of starch (Table 1) varied from 7.0 to 14.3%, the highest being for Am 51. The starch content in Amorphophallus is reported by Kay (1973) and Rajendran et al. (1977) to be in the range 4-8%. This is low compared with cassava and Diosorea spp., but the extraction is easy and settling of starch is not hampered by presence of mucilage in the tubers, unlike Colocasia and Dioscorea alata tubers. The starch yield is also comparable to the 10-12% of potato. The colour of the starch is pure white and is not tinged with the yellow colour present in the raw tubers.

The granule size of all the accessions was between 1 and 10 ocular divisions (3.34-33.4 µm) with 53% in the classes 6.68 and 10.02 µm. Grains above 30 µm below 5 µm were few. Average grain size of the different accessions was between 9.6 and 13.03 µm, the variety not being statistically significant (Table 1). Wankhede and Sajjan (1981) obtained 7-30 µm length and 5-24 µm width for Amorphophallus starch, while Kay (1973) gave a range of 5.5-18.7 µm. The starch granule size is thus less than that of Dioscorea spp. or cassava, but higher than that of Colocasia. The granules were mostly round in shape and no fissures were observed.

The total amylose content of starch of different accessions showed only very minor variation, from 21.9 to 23.5% (Table 1). Wankhede and Sajjan (1981) reported 24.5-2.50% while Soni et al. (1985) found 18.6%. There were no earlier reports on the soluble amylose content of the Amorphophallus starch. The soluble amylose, whish is starch of Amorphophallus paeoniifolius supported by amylose, which is supposed to be present in the amorphous regions of the starch granules is considered responsible for the undesirable cohesive character of many cooked starches (Hoover and Hadziyev 1982). The soluble amylose content formed 40-50% of the total amylose content, similar to other tuber crop starches.

Table 1 : Yield of starch, granule size and amylose content of Amorphophallus starch

*Mean of five replications
† Mean of six replications

Supported by amylose, which is supposed to be present in the amorphous regions of the starch granules is considered responsible for the undesirable cohesive character of many cooked starches (Hoover and Hadziyev 1982). The soluble amylose content formed 40-50% of the total amylose content, similar to other tuber crop starches.

No noticeable differences were seen in the swelling volumes of the starch of different accessions. The values increased steadily with increase in concentration and showed no abnormal behaviour at higher concentrations (Table 2). The viscosity properties (Table 3) are similar to those of Xanthisima and Colocasia starches. Generally they had lower viscosity values than Dioscorea and cassava starches, but were slightly higher than cereal starches. The different accessions did not vary significantly. The increase in viscosity with concentration was regular, with no noticeable change in viscosity pattern. The viscosity values at 97⁰C and after holding for 30 min indicate very low break down of viscosity at 5% and 6% concentrations. At 7% concentration, the breakdown was around 100 Brabender units. The low breakdown in viscosity is a very desirable property of the starch since it gives a short non-cohesive paste suitable in many food and industrial applications.

Table 2 : Swelling volume of Amorphophallus starch

*Mean value of three replicates

Table 3 : Rheollogical properties of Amorphophallus starch

Table 4 : 2% Viscocity, clarity and paste stability of Amorphophallus starch

*Relative to water = 0

The pasting temperature was nearly the same for all the accessions (81-85⁰C), higher than cassava or potato starch but similar to cereal and Colocasia starches. The gelatinization temperature determined microscopically by the earlier workers in 73-80⁰C (Wankhede and Sajjan 1981; Soni et al. 1985). The higher gelatinization temperature for this starch can be attributed to the strong associative forces found in the granules. The delayed gelatinization will provide a uniform paste.

The 2% solution viscosity (obtained in Redwood viscometer) was 37-46 s. Although the Redwood values do not reflect the viscosity stability, they are higher than cereal starches but lower than cassava starch. The clarity of the 2% solution was less than cassava or Dioscorea starches, though the amylose content is almost the same (Table 4). Intermolecular associative bonds may contribute to a large extent, and may also be responsible for the good viscosity stability. The paste stability of the starch of the different accessions was 24-30 h (Table 4). The starch possessed lower stability than cassava or Colocasia starches but higher than cereal starches. The gel strength at 6% was quite high, indicating that association between the starch molecules becomes quite strong on cooling.

The X-ray diffractionpattern of the starch of Amorphophallus wasan ‘A’ pattern. Peaks were observed at 7^o 40′ , 8^o 30′ and 12^o. The pattern follows the other aroid starches, which also posses a typical ‘A’ pattern.

The study indicates that Amorphophallus starch can be used in many starch based foods. It is easily extractable and possesses a pure white colour and good viscosity stability, suitable for many applications in the food industry.

Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India.

Iodine azide is known to undergo addition to alkenes and alkynes with a remarkable high degree of regio- and stereoselectivity¹. However, there are no reports of such additions of iodine azide to allenes. In this Communication a regio- and sterospecific addition of iodine azide to 1, 2-cyclodecadiene and 1, 2-cyclotridecadiene is described. The results are summarized in the Table.

Table : Addition of IN3 to C-9 and C-13 cyclic allenes.

Iodine azide was prepared in situ in the manner described by Hassner and coworkers³ by addition of iodine monochloride (9.1 g, 0.055 mol) to a stirred slurry of sodium azide (7.5g, 0.125 mol) at ca -10°C in acetonitrile (50cm³). Allene (0.06 mol) in acetonitrile (10cm³) was added slowly over a period of 0.5 h and stirred for 15 h at room temperature. Normal work-up procedure followed by chromatography over neutral alumina gave 1:1 pure liquid adducts.
The results suggest that the addition of IN3 to C-9 and C-13 cyclic allenes is not only regiospecific but also stereospecific. Any one of the following possible reaction intermediates (I-III) could explain the regiospecificity of the addition. However, the authors feel strongly that the stereospecificity of the addition could be explained better via the resonance stabilized planar allylic cation (I). In that event, the stereochemistry of the adduct from C-9 or C-13 cyclic allene must be dictated by the stability of (I).

At present, work is in progress with acyclic allenes, and also optically active cyclic allene to check the validity of this proposal.

The C.S.I.R., New Delhi, is thanked for an award of a S.R.F. to S.N.M., and Dr P. Balram, Carnegie Mellon University, Pittsburg, Pennsylvania, USA, for measurement of the 250MHZ ¹H n.m.r. spectra.

References

1. Hassner, A., Accounts Chem.Res., 1971, 4, 9.
2. The identity of the products was established by elemental analysis, ¹H.n.m.r. and infrared spectral measurements.
3. Fowler, F.W., Hassner, A. & Levy, L.A., J. Am. chem. Soc., 1967, 89, 2077.

Central Tuber Crops Research Institute (CTCRI), Sreekariyam, Trivandrum 695 017, India

(Received 20 October 1989; revised version received 4 September 1990; accepted 28 September 1990)

Abstract

Ammonia solution(0.03 M) was used to extract starch from various tuber crops by the conventional settling method. It was found that there was noticeable improvement in the yield of starch from Colocasia (6-16%), while it fell for sweet potato starch and remained almost the same for the other starches. The various properties of starch, thus extracted, were compared with those for starch obtained by water extraction. It was found that total amylose of all starches were unaffected while the ‘soluble amylose’ was slightly suppressed for Colocasia starch extracted with ammonia solution. Peak viscosity was found to be increased to a large extent for Colocasia and Dioscorea esculenta starches by ammonia extraction, while it was lowered for sweet potato starch. The swelling volume of Colocasia starch extracted with ammonia was similarly enhanced by 25%, but the Dioscorea esculenta starch did not show such a tendency. Sweet Potato starch suffered a reduction in swelling a reduction in swelling volume, Phosphorous content was found to be independent of the extraction medium.

Introduction

The tropical tuber crops are important food in the humid tropics because of their high carbohydrate content which is mainly in the form of starch (Cillie & Joubert, 1950; Onweume, 1978; Gallant et al., 1982).

*CTCRI Publication No. 572.

Industrial applications based on starch for these crops have been also increasingly recognized. Though cassava has been processed to give starch for many years, extraction of starch from the other tubers has not received much attention. The main reason is the difficulty experienced in extraction of starch from tuber crops other than cassava. Cassava is unique in that it contains over 80% of starch on a dry weight basis with little protein, fibre or other polysaccharides. Hence the starch settles fast and can be easily obtained in a pure white form. Sweet potato starch often has a dull colour, probably due to colouring matters present in the skin and hence its extraction involves the use of dilute alkali which flocculates the impurities and dissolves colouring matter (Paine et al 1938; Radley, 1976). In the case of other tuber crops, especially Colocasia, presence of mucilaginous material is a major hurdle in starch extraction. Settling takes very long time, which can result in microbial contamination and hence a reduction in starch quality.

Dioscorea and Colocasia starches have special properties like high gel strength and low granule size which make them suitable for specific applications (Coursey, 1967; Griffin & Wang, 1983). Hence an easy and convenient method is desirable for extraction of these starches to produce good quality starch in good yield. An attempt has been made to use an ammonia solution instead of water for extraction of different starches and the results are given in this paper.

Experimental

Tubers of cassava, Colocasia, Xanthosoma, Dioscorea alata, Dioscorea esculenta, Dioscorea rotundata and sweet potato were obtained from the CTCRI farm. The accessions/ cultivars of the different tubers from which the starch was extracted and the stage of harvest of the tubers are given in Table 1.

Starch extraction was carried out using the standard procedure (Badenhuizen, 1964) using tap water or ammonia solution (0.03 M). Freshly harvested tubers were washed and peeled. For cassava, the rind was also removed. The tubers were cut into small pieces of approx. 50×10×10 mm and washed again. A 1 kg sample of the pieces was washed and steeped in water or ammonia solution for 2-3 min. The pieces were disintegrated in a Remi Mixie for 1 min at low speed in the presence of sufficient water or dilute ammonia solution to cover the pieces. The pulp was allowed to remain in solution for approximately 2 h before filtration. Filtration was carried out successively through 80 mesh and 260 mesh sieves. The filtrate was allowed to settle for 24 h for cassava and sweet potato, and for 48 h for the other starches which contain larger quantities of mucilaginous material. The liquid proportion was decanted to leave the starch as a cake. It was collected, powdered and dried in an oven at 40-60⁰C . The starch, after drying, was collected and stored in polythene bags and the properties studied by withdrawing samples from these bags. For calculating the yield of starch, three extractions were carried out for each species.

Table 1 : Accessions / Cultivars of Different Species Used for Starch Extraction and their Stage of

The blue values corresponding to total and soluble amyloses were determined according to standard procedures (Showbhagya & Bhattacharya, 1971; Shanthi et al., 1980) using six replicates for each sample. The viscosity of the starch was monitored using Brabender visocamylograph fitted with a 350 cmg cartridge for all the studies. The concentration of the starch used was 5% for cassava, D. alata and D. routundata starches, and 6% for the other starches in order to maintain viscosity levels in the same range. Distilled water was used for making the starch suspension and one run was carried out for each sample. The pasting temperature was obtained from the viscosity curves. The swelling volumes were obtained by the procedure of Schoch (1964), based on three replications for each sample and phosphorous content by the vanadomolybdate method (Smith & Caruso, 1964).

Results and Discussion

In earlier experiments in our laboratory, various chemicals have been tried to improve the extractability of starch, especially from the tubers of Colocasia and Dioscorea spp. (CTCRI, 1987). These include 10% ethyl alcohol, 1% calcium hydroxide solution, 1% cetyl trimethylammonium bromide solution and 1% acetic acid. Among these, only calcium hydroxide solution was found to improve the settling of Colocasia starch, but the starch obtained assumed a brownish colour, which could not be removed even by repeated washings. Hence a mixture of alcohol and calcium hydroxide solution was tried, but was found to be ineffective. Cetyl trimethylammonium bromide was not only ineffective, but also led to changes in starch properties. Use of water at 50⁰C did not lead to any improvement in yield of Colocasia starch. However, dilute ammonia solution was found to improve the settling of starch from Colocasia and Dioscorea sp. Preliminary experiments also showed that the desirable concentration of the ammonia solution is 0.03M. Table 2 gives the yield of dry starch from different tuber crops on using water and dilute ammonia solution. The data indicate that in the case of Colocasia, the yield of starch was considerably improved from 6.2 to 16.6% under identical conditions, when 0.03 M ammonia solution is used instead of water. The other tuber crops except sweet potato also showed marginal improvement in yield but with sweet potato also showed marginal improvement in yield but with sweet potato, the yield was slightly depressed.

Colocasia contains the largest quantity of mucilage and hence it is most difficult to extract starch from these tubers. The settling of starch is too slow leading to a reduction in yield and also the chance of microbial degradation while settling for 1-2 days. Therefore, the noticeable helps in preventing possible microbial damage leading to deterioration in quality especially during settling. This is very significant for Colocasia where normal settling takes a long time and makes it highly susceptible to microbial damage resulting in the noticeably lower viscosity for water extracted starch. Sweet potato behaves totally differently, the reason for this is unknown. The breakdown in viscosity, viscosity on cooling to room temperature, follows the usual pattern and no difference could be noticed between ammonia-extracted and water-extracted starches (Table 3).

Table 2 : Yield, Total and Soluble Amylose Contents (Blue Values) for Starches Extracted with Ammonia Solution and Water

Table 3 : Viscocity of Starches Extracted by Water and Ammonia

^aPV – peak viscosity.
^bV₉₇-viscosity at 97⁰C.
^cV_H-viscosity at 97⁰C for 30 min.
^dV_C-viscosity after cooling to room temperature.

Swelling volume of starches is also affected by various chemicals (Krog, 1973; Moorthy, 1985) and hence the swelling volume of starches extracted with ammonia solution was compared with that of water extracted starches. The swelling volume of Colocasia starch was increased on using ammonia solution for extraction, while it fell for sweet potato starch and did not show any noticeable change for other starches (Table 4). The high viscosity increase observed in the case of ammonia extracted D. esculenta starch was not reflected in its swelling increase in yield on the use of ammonia solution can be used for large scale extraction of starch followed by a conventional settling process. The starch of Colocasia ha special properties in particular a very small granule size (1-10 µm) and easy digestibility (Potgeiter, 1940 ; Wang, 1983). Increased utilisation in food and industry could stimulate a higher production.

There is no sizeable increase in the yield of starch from other tuber crops, and this could be the fact that the mucilage content is lower, especially cassava, which is practically devoid of it. However, the reason for the yield reduction in the case of sweet potato starch is not clear. Though the yield may not be increased, ammonia extraction medium has the added advantage that a longer settling time can be allowed without causing microbial contamination. The pH of the extraction medium is 9.0-10.0, and the normal mould, yeast and bacteria are generally unable to grow under these conditions (Frazier, 1967).

To find out whether the use of ammonia during sedimentation affects the starch quality, various properties of the starch obtained by this procedure were compared to those of starch extracted by normal water extraction. The ‘Blue Values’ corresponding to total and soluble amylose are presented in Table 2. It is seen that there is no noticeable difference in total amylose content on ammonia extraction. There is no clear idea about the structure of ‘soluble amylose’, but our earlier results have indicated this material may possess a net negative charge. Ammonia may be complexing with this fraction and hence the lower value for soluble amylose which is an agreement with the results obtained on treatment of the starch with the surfactant, cetyl trimethylammonium bromide.

The Brabender viscosity profile of ammonia-extracted starch was compared with starch obtained by water extraction. The results, given in Table 3, show that peak viscosity is enhanced for all starches except sweet potato. In the case of Colocasia and D. esculenta starches, the enhancement is even more pronounced. Viscosity can be considered as a enhancement is even more pronounced. Viscosity can be considered as a measure of the strength of starch granules, since a higher viscosity indicates that starch granules are intact, while the starch which has undergone chemical and microbial damage, loses viscosity. Hence ammonia treatment not only has no chemical effect on starch but also volume. Higher swelling indicates a lowering of associative forces between the starch granules, and hence Colocasia starch appears to undergo some reduction in associative forces on extraction using ammonia solution, but the associative force is not weakened enough to lead to granule breakdown, as indicated by the viscosity data. Bit in case of sweet potato starch, considerable disruption of associative forces seems to take place on extraction with ammonia solution.

Table 4 : Swelling Volume and Phosphorous Content of Starch Extracted with Ammonia and water.

Colocasia starch was found to lose its ‘soluble amylose’ on extraction with ammonia, which may be due to presence of anionic groups in this fraction, the phosphorous content of starch obtained by water extraction and ammonia extraction was compared. Though a variation between different starches was observed, there was no difference between ammonia- and water-extracted starches (Table 4) indicating that the phosphate linkages may not be responsible for the observed differences in case of Colocasia starch.

The results point out that ammonia extraction will be useful for obtaining starch from different tuber crops, except sweet potato, without affecting the properties but at the same time offering good yields. In the case of Colocasia starch, which is most difficult to extract, the treatment had definite advantages in increasing yield and also quality of starch. In view of the special properties of Colocasia starch, the large scale extraction on industrial scale may also be carried out using a low concentration ammonia solution.

The effect of anionic, neutral, and cationic surfactants in three different concentrations on cassava starch properties was studied. The iodine affinity of total amylose was reduced by 20 -40% by all surfactants, with highest reduction being observed for cetyltrimethylammonium bromide. The iodine affinity of soluble amylase was suppressed by all reagents except cetyltrimethylammonium bromide, which lowered the value to almost zero. There was no significant difference between the concentrations 0.04 and 0.06 mol of surfactant per 100 g of starch. Viscosity was stabilized by potassium stearate and potassium palmitate without greatly affecting the peak viscosity of 660 BU of pure starch, but sodium lauryl sulphate and cetyltrimethylammonium bromide increased the peak of viscosity to 900 and 780 BU respectively at 0.06-mol concentration and did not show stable viscocity during the holding period. Defatted and raw starch showed similar viscosity patterns on incorporation of surfactants. Pasting temperature as increased to over90⁰C by potassium sateratte, palmitate and glyceryl monosterate, while the increase was only by 3.15⁰C over control (65⁰C) by the other two reagents. The swelling volume of starch was reduced to nearly half the original value by potassium palmitate and stearate, glyceryl monostrate did not change it noticeably. Sodium lauryl sulfate and cetyltrimethylammonium bromide increased the value by nearly 50%. Sol stability was improved considerably by all the reagents. The results are discussed in relation to structure of the surfactants.

Introduction

Surfactants have been in use in food industry since 1920s mainly as dough conditioners, as crumb softeners in breads buns and rolls, and as amylase complexing agents in starch based foods. The crumb-softening effect of the surfactants is based on the formation surfactants amylase complexing index of various surfactants and their crumb softening effect has been found (Krog and Nyobojenson, 1970; Lagendijk and pennings, 1970; Krog, 1981.

Similarly, use of monoglycerides in the production of dehydrated mashed potato is aimed at binding free amylase to control the stickness or gluiness of the product. Here also the role of the surfactant as amylase complexing agent has been established (Hoover and Hadziyev, 1981 a,1981 b, 1982.)

The structure of the amylose-surfacant complex has been investigated in detail. It has been observed early that iodine affinity of starch is reduced drastically by addition of fats and surfactants. Butanol precipitated corn amilose showed reduction of iodine affinity from 18.7% to 0 % by addition of 10% palmitic acid (Schoch and williams1944.) osman et al(1961), however found that though the iodine of affinity of corn starch amylase was reduced by addition of surfacatants, it never reached zero value. On the basis of iodine affinity studies, it has been proposed that amylase forms a helical structure that is stabilized by the hydro carbon part of the surfacatant, which full fills the hydro phobic salvation requirements of the helix (Krog,1981.) Carlson et al.(1979) have used X-ray and Raman spectroscopies to study the complexes, and their result also confirm a helical inclusion complex.

The effect of surfacatants on the siscosity of starch pastes has been studied by Osman and Dix (1960) and Krog(1973). The former group observed that the pasting temperature of corn starch was increased in the case of nonionic surfacatants. The effects was related to length of hydro carbon chain in the surfacatants. Krog used monoglycerides, steroyl 2-lactylates, and diacetylated tartaric acid easters of monoglycerides (DATE) at 0.5% concentrations in his study of their effects on various starches including cassava starch. The peak viscosity of cassava was slightly reduced by all the emusifiers. The viscosity during the holding period was stabilized by glyserylmonosterate (GMS) while it was destabilized by DATE. The pasting temperature was increased by GMS and sodium steroylactylate while calcium steroylactylate and DATE had no effect.

Hoover and Hadziyev ( 1981 a,1981b,1982) have studies in detail the poto amylosic monoglyceride complex and the effect of this complexing on starch properties like swelling power, water binding capacity, solubility and blue value index. They found that swelling power and solubility were reduced by both monoglycerides and the potassium salts of fatty acids and the reduction was dependent on the chain length of the fatty acid portion of the surfactants. Both water binding capacity and blue value index were suppressed.

Apart from the study on the bra bender viscosity pattern and pasting behavior of cassava starch in the presents of various surfactants by Krog (1973), there has been no systematic investigation on the effects of different surfactants on the properties of cassava starch. Hence the present study was undertaken using cationic,anionic, and neural surfactants in three concentrations each (0.02,0.04,and 0.06 mol per 100 g of starch). In addition to the total and soluble amylase binding capacity, the 2% solution viscosity and 6% paste viscosity of cassava starch in the presents of surfactants, the effect of swelling power and sol stability have been examined, since cassava starch has a high swelling power and low retrogradation tendency compared to cereal starches. Cassava starch find extensive use in food and industry, and the result can prove useful to find out ways to improve its undesirable properties like unstable viscosity and long cohesive texture of its paste.

Materials and Methods

Commercial grade cassava starch obtained from Lakshmi Starch Factory, Kundara, kerala,India, was used as such or after defatting by hot extraction with petroleum ether (60-80⁰C). The five surfactants used for the study include three anionicand one each of neutral and cationic surfactants. The anionic ones were potassium stearate and potassium palmitate with 16 and 18 carbon chains in the hydrophobic portion and sodium lauryl sulfate with a 12-carbon chain. The neutral surfactant was glyceryl monostearate (18 carbon chain) while the cationic one was cetyltrimethylammonium bromide (with chain length of 16 carbons in the hydrophobic portion). Analar grade reagents were used for the preparation of the complex with starch. Three concentrations of each of the surfactants were tried, viz 0.02,0.04,and 0.06mol per 100 g of starch. The concentration factor was fixed as moles/100 g of starch to eliminate the effect of the large difference in molecular weights between the surfactants.

The starch-surfactant complex was prepared by the procedure described by Hoover and Hadziyev (1981 a). the surfactant was dispersed in 50 ml of distilled water pre-heated to 65⁰C and stirred for 30 min. The temperature was brought down to 45⁰C,50 g of dry starch in 100 ml of water was added, and the slurry was heated at 45⁰C for 6hwith continuous slow stirring. The suspension was filtered, washed thoroughly, and dried at room temperature.

Blue values for total amylase and soluble amylase were determined by the colorimetric procedure described Sowbhagya and Bhattacharya(1971) and Shanty et al (1980), respectively using pure amylase (SIGMA) as standard. Three replications were used for determination of blue values.

Viscosity of the starch and starch and starch surfactant, complex solutions (2%)was determined by the ISI procedure using Redwood viscometer no.1,(ISI, 1970). A 4-gsample of material was dissolved in 200g of hot distilled water, heated at 100 ⁰C for 30 min., filtered through cheese cloth, and cooled to 75 ⁰C, and viscosity was taken as the time taken in seconds for 50ml. of the solutions to pass through the office of the viscometer. Three readings were taken for each sample.

Paste viscocity of a 6% paste of starch and its complexes were monitored on a brabender viscomylograph (Model 801020) provided with automatic heating and stirring systems ( 75 rpm, 1.5 ⁰C /min).27 g of the material (dry weight basis) was suspended in 450 ml of distilled water in the amylograph cup and heated with stirring at 75 rpm at 1.5 ⁰C/min.At 97 ⁰C , the temperature was maintained constant for 20 min. The pasting temperature range was maintained constant for 20 min. The pasting temperature range was obtained as the range between the temperature at the start of increase of viscosity and that at which it remains constant.

The swelling volume was obtained by Schoch’s method (Schoch, 1964). A 0.5 -g portion of dry material in 5o mL of distilled water was heated with stirring to 80 ⁰C in a water bath, maintained at 85 ⁰C for 15 min, cooled, and centrifuged at 2200 rpm for 15 min, and the swelling volume was expressed as the volume of gelatinous sediment per 1g of dry starch. The sol stablility was taken as the time taken by the starch gel to start settling on keeping undisturbed (with a little amount of toluene added to prevent microbial damage).

Results and Discussions

The total iodine affinity and the iodine affinity of soluble amylase are given in Table 1 in terms of the blue values. The maximum reduction in iodine affinity of total amlose is observed in the case of cetyltrimethylammonium bromide. All the other reagents show an almost equal reduction in affinity. Since reduction in iodine affinity value can be taken as a measure of the complex formation, it can be concluded that cetyltrimethylammonium bromide complexes to the maximum extent and potassium stearate and sodium lauryl sulfate the least.

It is also seen from Table 1 that the difference in blue values at concentrations of 0.04and 0.06 mol are nonsignificant. The reduction in blue value of amylase with addition of increasing amounts of fatty acids or surfactantsis reported by many others, but the present study indicates that blue value of cassava starch tend to taper off on increase in concentration above 0.04 mol of surfactant per 100 g of starch. Similar results have been obtained for potato starch by Hoover and Hadziyev(1982) who found that the blue value remain constant above a concentration of 0.3 glyceryl monosterate. Hence at higher concentrations the surfactant may not be forming an effective complex. The amylopeetin molucles may be offering resistence to the surfactant to complex with the amylase molucles, and hence all the amylose is not bound by the surfactants.

The blue values of the soluble amylase in the treated samples give an indication of the preference of the reagents to add to free amylase. The results (Table 1) indicates that all the surfactants reduce the value considerably and that the ratio of blue values of total to soluble amylase is in the range 2-2.5 for all reagents except in the case of cetyltrimethylammonium bromide which imparts a very low blue value for soluble amylase. Here also there is no significant difference between the concentrations 0.04 and 0.06 mol. Though all the treatments are significantly different the values with cetyltrimethylammonium bromide point out that this surfactant has a very high affinity for soluble amylase. At a 0.06-mol concentration, the blue value nearby reaches zero, indicating that this reagent is binding almost all the soluble amylose. Such high tendency of surfactantto complex with soluble amylase is reported for the first time.

Figure 1. Effect of potassium palmiate on the Brabender viscosity curve of cassava starch., cassava starch (raw defatted);?.., cassava starch + 0.02 mol of potassium palmitate (per 100 g of starch) cassava starch+0.04 mol;on cassava starch +0.06 mol.

Figure 2. Effect of potassium palmitate Brabender viscosity curve of cassava starch(raw defatted ) cassava starch +0.02 mol of potassium palmitate (per 100 g of starch)? cassava starch+0.04 mol -.-, cassava starch + 0.06 mol.

Figure 3. Effect of glyceryl monostearate on the Brabender viscosity curve of cassava starch(raw defatted ) cassava starch +0.02 mol of of GMS (per 100 g of starch);–.–, cassava starch+0.04 mol –.–, cassava starch + 0.06 mol.

Figure 4. effect of sodium lauryl sulfate on the Brabender viscosity curve of cassava starch(raw defatted ) cassava starch +0.02 mol of sodium lauryl sulfate (per 100 g of starch);–.–, cassava starch+0.04 mol –.–, cassava starch + 0.06 mol.

Figure 5. effect of cetyltrimethylamonium bromide on the Brabender viscosity curve of cassava starch., cassava starch (raw defatted);?.., cassava starch + 0.02 mol of cetyltrimethylamonium bromide (per 100 g of starch);–.–, cassava starch+0.04 mol –.–, cassava starch + 0.06 mol.

The velocity of 2 % solution of starch and surfactant-incorporated starch is given in table II. Treatment with surfactants at all concentrations increase the viscosity, especially with sodium lauryl surfate and cetyltrimethylammonium bromide. All the treatment and concentrations show significance difference in viscosity. However no direct relation between the increasing concentration and an increase in viscosity could be observed. In the case of potassium setearate, potassium palmitate, and sodium lauryl sulfate, there is only a slight increase in viscosity at 0.06 -mol concentrationcompared to that of 0.02 -mol concentration. Cetyltrimethylammonium bromide, however exhibits a regular increase in viscosity with increasing concentration. The absence of regular increase in viscosity with increase in concentration in the case of treatment with GMS may be due to incomplete gelatinization of starch granules, as observed by congo red staining experiments that showed that 20-25% of the granules in the solution remained ungelatinzed.

The Brabender viscosity curves for the samples are given in figures 1-5. The results indicate that different surfactants show different patterns. Potassium palmitate and setearate suppress the peak viscosity slightly, but the viscosity remains almost steady during the holding period. Thus, a slight reduction in swelling with a good strengthening of the starch granules against shear and temperature is imparted by the these reagents. The peak viscosity does not show an increase with increasing concentration of the reagent. The results point out that these regions can be useful in stabilizing the viscosity of cassava starch. The increased resistance to break down under shear and temperature can be useful to prevent the long cohesive nature of the starch paste. The stabilization is achieved even at the lowest concentration(0.02 mol), and the production of the surfactant -starch complex is easier compared to cross linking by chemical reactions (Srivastava and Patel, 1973; knight, 1974) or physical treatment (Moorthy, 1980).

Table II : Viscosity and Pasting temperature of Surfactant – Incorporated Starch

Sodium lauryl sulfate increase the peak viscosity considerably, and at a 0.06-mol concentration, it reaches 900 BU. However, during the holding period, the viscosity drops rapidly and reaches the value of pure starch. This results shows that sodium lauryl sulphate binds with starch molecules and allows them to swell considerably but under shear and temperature, the complex breaks down down to release the swollen starch granules that starts fragmenting.

Glyceryl monostearate increase peak viscosity slightly at a0.02-mol concentration, but at higher concentrations, the values fall. These results, as also the 2% viscosity values, shows that gelatinization is not complete at 97 ⁰C . the viscosity is maintained during the holding period, and hence this reagent can also be used at 0.02-mol concentration for viscosity stabilization of cassava starch. Krog (1973) in his study has also found that GMS at 0.5% concentration stabilizes cassava starch viscosity. However, he found, a slight decrease in peak viscosity with the reagent at 0.5% concentration.

Cetyltrimethylammonium bromide leads to a steady increase in peak viscosity with increasing concentration, reaching 780 BU at 0.06 mol concentration. However, the viscosity breaks down during the holding period, similar to the case of sodium lauryl sulfate incorporated starch, indicating that the complex is unstable on heating and stirring.

In order to compare the effects of surfactants on defatted and non defatted cassava starch, the Brabender viscosity pattern of surfactant incorporated starch prepared from non defeated (raw) starch was obtained. It was found that there is practically no difference in the viscosity pattern, peak viscosity, or pasting temperature, between defatted or starch treated with surfactants. This can be explained by the low fat content of cassava starch compared to cereal starches.

Taylor and Nelson (1920) reported values of 0.61% and 0.11 % fat content, respectively, for maize and cassava starches. Compared to the high lipid content, in maize starch (0.87%; Morrison,1976) or rice starch (0.4% Maninget and Juliano, 1980). The mirror amount of lipid in cassava starch may not be existing as a complex with amylase molecules to be affected by addition of surfactants. The gelatinization temperature is lowered by extraction of lipids from corn, wheat (Melvin, 1979), or rice (Ohashi et al.1980) starches, but no such effects is observed in cassava starch.

The pasting temperature as observed from the Brabender viscosity curves are given in table II. Invariably all the reagents increase the pasting temperature, but to different extents. In the case of potassium palmitate and stearate, the pasting temperature initiation is increased by around 30 ⁰C and the pasting temperature range is narrowed to 2 ⁰C . At higher concentrations, the rise in visocosity appears only at 97 ⁰C . It was also found by congo red staining that all the granules do not gelatinize at 97 ⁰C , but only during the holding period. The high pasting temperature indicates the resistence offered by the surfactants that fit into the amylase helix to entry of water molecules. With GMS also, the pasting temperature is enhanced to over 90 ⁰C , but the range is comparatively more than with potassium palmitate or stearate. At higher concentrations, incomplete gelatinization becomes prominent.

Sodium lauryl sulfate and cetyltrimethylammonium bromide increase the pasting temperature to a lower extent. At higher concentration of the reagents, there is even a drop in the pasting temperature. The results pasting temperature show that reagents with small hydrophilic groups impart a higher increase in pasting temperature. This may be due to the closer paking these reagents can achieve with the starch molucles. As carbon has pointed out (1979), part of the hydro carbon chain lies outside the amylase helix and this length may be dependent on the bulk of the hydrophilic group. Sodium lauryl sulfate and cetyltrimethylammonium bromide having big hydrophilic groups do not fit so closely as the other reagents into the starch molecules, when compared to the surfactants with smaller hydrophilic groups. The reduction in pasting temperature with a higher concentration of the reagents is also indicative of mutual crowding of the surfactant molecules.

The effect of the size of the hydrophilic portion of the surfactant in comparison to the hydrophobic chain has been discussed by Osman and Dix (1960) as well as Krog (1973). The results obtained also confirm the importance of close packing of the hydrophobic group into the amylose helix to render it stable.

Table III : Swelling volume and Sol Stability of surfactants Incorporated Starch

The very slight increase observed in the pasting temperatures in the case of cetyltrimethylammonium bromide in contrast to the notable decrease in bluevalue with the same reagent indicates that this reagent is able to block the entry of bulky 1­3 ion and not the much smaller water molecule.

Swelling volume is important characteristic of starch, especially for cassava starch, which exhibits high swelling property. A large swelling can lead to reduction in associative forces, resulting in breakdown of granules and simultaneous cohesive texture. The swelling volume of treated samples are given in Table III. The treatments and concentrations show significant variations. It is seen that potassium stearate and palmitate reduce the swelling volume to almost half its original value, even at lowest concentrations. GMS also reduce s the swelling volume, but only to a very small extent. Sodium lauryl sulphate on the other hand increase the value considerably. A similar result is also obtained with cetyltrimethylammonium bromide.

GMS has been reported to reduce the swelling power of potato starch by 10%, but the value tapers off above 0.3% concentration of GMS (Iloover and Hadziyey, 1982). Such, high reduction in swelling volume in the case of potassium stearate and palmitate and increase by over 50% by sodium lauryl sulphate and cetyltrimethylammonium bromide have not been reported so far.

The swelling of starch is determined by the strength of associative forces between molecules and also contributes to the viscosity of the paste. The high paste viscosity obtained for starch incorporated with cetyltrimethylammonium bromide and sodium lauryl sulfate and the swelling volume of these starch complexes. GMS exhibits almost the same swelling volume and viscosity as pure starch, but potassium stearate and palmitate reduce the swelling volume without reducing the viscosityof starch. Associative forces between starch molecules in the starch granules may be playing a role in determining these properties.

The sol stability of starch paste was found to be increased by all the reagents (Table III). Among the reagents, sodium lauryl sulfate exhibited highest stability (10 days) while there was not much difference between the reagents. The reagents act by inhibiting parallel association of linear amylase chains or the outer chains of amylopection molecules that would otherwise lead to settling of the starch gel.

Thus, study points out that different types of surfactants modify the properties of cassava starch differently. Cetyltrimethylammonium bromide shows high affinity for soluble amylase compared to other surfactants. Potassium stearate and potassium palmitate stabilize the past viscosity of cassava starch without affecting the peak viscosity but do not stabilize the viscosity. The instability of the complexes of amylase with these reagents may be explained as due to the relatively bulky hydrophilic group in them as suggested by Krog (1973). The lack of difference between viscosity properties of defatted and non defatted starch non surfactant incorporation may be due to relatively low lipid content of cassava starch. The wide variations in swelling volume of starch imparted by different types of surfactants and the improvement in sol stability are reported for the first time. The study also shows that concentrations above0.04mol/100g of starch do not have much effecton most of the properties.

Acknowledgment is due to Dr. S.P.Ghosh, Director of CTCRI, and Dr. C.Balagopalan, Head of Division of Technology, CTCRI, for facilities provided and encouragement.

_{Registery No.GMS, 31566-31-1; starch,9005-25-8 amylose, 9005-82-7;seaterate, 57-11-4 potassium palmitate, 2624-31-9; sodium lauryl sulfate, 151-21-3; CH 3 (CH2)N (ME)3Br.57-09-0}

Literature Cited

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25. Received for review October 26, 1984. Revised manuscript received August 5, 1985. Accepted September 3, 1985.

S.N.Moorthy, B. Vimala and Archana Mukherjee.
Central Tuber Crops Research Institute, Sreekariyam, Trivandrum, 695 017, INDIA

Abstract

Key words: Canna edulis, starch, rheology, swelling, arrowroot.

Introduction

Apart from the major root crops like cassava, sweet potato, aroids and yams, many types of rhizomatous and tuberous root crops are grown in different countries including India. Queensland arrowroot (Canna edulis) is a perennial herb grown in many countries for its edible rhizome (Joseph and Peter, 1985) . According to Hermann (1994), the bakery products prepared from canna starch are much lighter, and crisper than those from wheat. This Institute has three accessions of Canna edulis classified from the leaf colour as green, purple and dark purple. Only the tubers of dark purple accession are consumed since the others are fibrous and contain large quantity of phenols. This paper describes the physicochemical and functional properties of starch isolated from these three accessions.

Materials and methods

The rhizomes were harvested at ten months stage, washed, peeled and pulped in a Remi blender with 10 vol. of water. The pulp was mixed with 5 vol. water , strained through a 150 mesh sieve and allowed to settle. Resuspension and resettling were carried out several times and the deposited cake was dried in sunlight and stored in moisture proof containers.

Starch granule size was measured microscopically using ocular and stage micrometer at X120 . Dry matter and starch contents were determined by AOAC (1975) procedures and total and soluble amylose contents by the methods of Sowbhagya and Bhattacharya (1971) and Shanty et al. (1980) respectively, X-ray diffraction analysis on a Phillips X-ray diffractometer using CuK_α radiation. Viscosity measurements of the starch pastes at 3,4 and 5% concentrations were carried out on a Brabender Viscograph model 801020 using 350 mcg cartridge with heating at 1.5⁰C /min. Viscosity of a 1% starch solution was measured on a Redwood No 1 Viscometer. A 10% solution was studied in the Rapid Visco Analyser ( Model 4, Newport Scientific) in triplicate with heating from 50 to 95⁰C at 12⁰C/min, holding at 95⁰C for 2 min, cooling back to 50⁰C at 12⁰C/min. Swelling volume was determined as described by Moorthy (1994). The clarity and paste stability were measured on 1% solutions. Phosphorus was determined by the vanadomolybdate method (Jackson, 1967)

Results and Discussion

There was a large variation in the dry matter content of three accessions (Table 1) from 20-35%. Kay (1987 ) reported 28-33% for Canna edulis tubers. The starch content also varied, but the value for dark purple accession was close to that of cassava and sweet potato.

The starch granules are quite large 35.5 to 43.5μm. Previous reports vary considerably. Fujimoto et al. (1990) obtained an average size of 45.8μm, but Soni et al. (1990) found 13 to 17 μm; a maximum granule size of 140μm was given by Kay (1987). Photomicrographs showed that granules are oval and polyhedral as reported by Kay (1987) and Soni et al. (1990)

The X-ray diffraction pattern is of type ‘B’, as reported by Fujimoto et al. (1990), as is yam starch. The amylose content is 24-30 %, similar to the 27% of Ishii et al. (1991), but lower than the 38% of Soni et al. (1990) . All these values are higher than most other tuber crops (Moorthy, 1994). The soluble amylose is nearly 40% of the total, as in all the other tuber starches.

The solution viscosity is 65 – 100 sec (Table 1), higher than other tropical tuber starches: cassava starch normally has a value 50-60 sec for a 2% solution (Moorthy 1985). The Brabender data are higher than those reported by Nagahama and Troung (1994 : 650 BU for 6% paste) and Soni et al. (1990: 500 BU for 5% paste) and also higher than other tuber starches The starch exhibits very low viscosity breakdown. On cooling, the paste is non-cohesive in texture and sets into a strong gel free from syneresis. Thus the starch will be valuable in food applications. For C. edulis starch, Perez et al. (1998) obtained a peak viscosity of 300BU for a 4% paste and negative breakdown and high setback. We confirmed the low breakdown, but we did not observe the high setback .

All three accessions continue to gelatinise even after 95⁰C, showing that the starch possesses strong associative linkages. A similar result was reported by Soni et al. (1990), while Nagahama and Troung (1994) found that gelatinisation occured at 71⁰C . These characteristics resemble yam starch. The RVA patterns do not coincide with the Brabender (Table 1), as they indicate a similar but noticeable breakdown for all three starches with a lower setback for the purple accession. In contrast to the Brabender data, Perez et al. (1998) obtained high viscosity breakdown and low setback for their C. edulis starch. The solution properties (Table 1) show that all the accessions have lower swelling volumes than the 23.5 ml/g at 80⁰C reported by Nagahama and Troung (1994). The clarity and solution stability are high as for yam and cassava starches. The phosphorus contents in are high as reported by Nagahama and Truong (1994), whereas Soni et al. (1990) found much lower values.

These Canna edulis starches resemble yam in most functional properties. The starch has good potential in food applications because of the high viscosity and gel strength especially in canned foods that require high paste stability (Smith, 1982).

Table 1 : Dry matter, starch content, granule size and amylose content in the starch

Table 2: Brabender Viscosity properties of Canna starch

Table 3 : RVA viscosity properties of Canna starch

* cP units, mean of three replications
** ⁰C

Table 4 : Swelling volume, clarity, stability and P content of the starch

References

1. AOAC, Official Methods of analysis, Association of Official Analytical Chemists, Washington D.C. , (1975).
2. Fujimoto S, Matsmoto K., Yamanaka O. , Sugamma T. and Nagahama, T. (1990) Starches from turu-dokudami, momijii-hirugao and yama-no-imo, J. Jap. Soc. Starch Sci. 37,7-11.
3. Hermann M., (1994) Achira and arracacha – processing and product development, International Potato Centre, Lima, Peru, ular 20, 10-12.
4. Ishii Y., Nakahama H., Hattori S., Kawabata A. and Nakamura M. (1991) Study of the molecular properties of amyloses and amylopectins from tropical starches, J. Jap. Soc, Starch Sci. 38, 333-342.
5. Jackson M.L. (1967) Soil Chemical Analysis, Ed. M.L. Jackson, Prentice Hall of India, Pvt. Ltd, New Delhi , pp 151-154.
6. Joseph S. and Peter K.V. (1985) Queens land arrowroot. Indian Farming 29, 11, 28
7. Kay D.E., (1987) TDRI Crop and Product Digest No.2, TDRI, London , 166-173.
8. Moorthy S.N., (1994) Tuber crop starches, Tech. Bull. No. 18, CTCRI, Trivandrum, pp 40.
9. Moorthy S.N. (1985) Cassava starch and its modifications, Tech. Bull No.4 CTCRI, Trivandrum, pp 32.
10. Nagahama T. and Truong V.,(1994) Physicochemical properties and utilization of starches from tropical root crops. In Post harvest Biochemistry of plant food metabolism in the tropics, Ed., I. Uritani, V.V. Garcia, and I.M.T. Mendoza, , Japanese Scientific Societies Press, Tokyo, 205-221.
11. Perez, E.E. , Breene,W.M. and Bahnassey, Y.A. (1998) Variation in gelatinisation profiles of cassava, sagu, arrowroot native starches as measured with different thermal and mechanical methods, Staerke/Starch 50, 70-72.
12. Shanthy A.P., Sowbhagya C.M. and Bhattacharya K.R.(1980) Simplified determination of water insoluble amylose content of rice. Staerke/Starch 32, 409-411.
13. Smith P.S. (1982) Starch derivatives and their use in Foods , In : Food Carbohydrates, ( Linebeck D.R. and Inglett G.E, Ed) AVI Publ. Co., Connecticut, USA , 237-269.
14. Soni P.L., Sharma H., Srivastava H.C. and Gharia M.M.(1990) Physicochemical properties of Canna edulis starch- Comparison with maize starch, Staerke/Starch 4, 460-464.
15. Sowbhagya C.M. and .Bhattacharya K.R. (1971)
16. Simplified colorimetric method for determination of amylose content in rice, Staerke/Starch, 23 , 53-56.

Table 5. Physicochemical properties of the Canna edulis starch

1. Senior Scientist, Central Tuber Crops Research Institute, Trivandrum 695 017, INDIA
2. Professor, Food Technology department, Lund University, SWEDEN.

Lipids and surfactants can form strong complexes with starch and thereby modify the properties of the starches. Unlike cereal starches which contain considerable quantities of lipids present in them, the tuber starches are devoid of the lipids. A study was conducted to compare the types of starches using DSC. The starches were moistened with equal quantity of water in aluminum pans and the DSC run using a Seiko Calorimeter using indium as standard The heating-cooling cycle used was as follows: 15 to 100⁰CC at 5⁰CC/min., cooling to 30⁰C at 30⁰CC/min., reheating from 30 to 100⁰CC at 5⁰CC/min., and final cooling. The reheating is carried out to mask starch gelatinisation peak during the second cycle. Ten tuber starches were studied. It was found that the tuber starches do not exhibit starch-lipid complex melting peak during the heating in the first cycle and further confirmed during the second heating. This shows absence of starch-lipid complex in the native tuber starches. When lipids/ surfactants were added to the starches and the DSC run, the peaks corresponding to complex did appear further corroborating the results.

Starch-lipid interaction has been a topic of much study because it has important implications in deciding the starch properties but also provides an interesting system^1-4. The changes that can be brought about have been exploited by the food industry for improving the texture and properties of various starch based foods. These include the improving the texture of potato mash by adding glyceryl monostearate⁵, reducing the staling of bread by adding some surfactants⁶, and imparting a firmer texture to pressure boiled rice flour⁷. In fact the importance of the resistant starch in nutrition has added further interest in the use of lipids to bring about resistance to starch digestion. However it has been observed that only cereal starches harbour lipids in the native state. Hence these starches possess characteristic lipid taste compared to bland taste of the root starches. The lipids associated with starch granules can be either on the surface of the granules as well as inside⁸. Those occurring on the surface include triglycerides, free fatty acids, glycolipids and phospholipids and are absorbed into the granules during starch isolation⁹. The internal lipids are monoacyl lipids lyso-phospholipids and free fatty acids. It is assumed that both the external and internal lipids are present in the free sate and bound to starch components either in the form of inclusion complexes or linked via ionic or hydrogen bonding to hydroxyl groups of the starches¹⁰.

The question whether the lipids form complexes only with amylose has been examined by various workers and lot of evidence with waxy starches has shown that the amylopectin molecules also can form complexes with the lipids by using the outer chains in them¹¹. Our interest was to see whether there is any inherent restriction of the tuber starches to form complexes with lipids and surfactants.

Differential Scanning Calorimetry (DSC) is being increasingly used to study starch gelatinisation in view of the fact that the method quite fast, requires only very small quantity of the material and is reproducible ^12-14. When starch is heated in excess water, it undergoes a series of changes which can be followed using the DSC. The peak corresponding to starch gelatinisation occurs at a range of temperature depending on the type of starch used. This process is endothermic and irreversible, There can be another peak corresponding to the melting of the starch-lipid complex which occurs at a much higher temperature (80-130⁰)¹⁵ , This process is also endothermic, but it is reversible. The latter appears as a peak during second heating cycle and hence a second heating cycle is followed for studying starch-lipid interaction.

It is well established that root and tuber starches contain only very small quantity of lipids in them. The DSC of some of these starches have been studied and they do not show the starch-lipid melting peak¹⁰,^16-17. However it was desired to find out if this is true for all tuber starches and hence ten of these tropical starches were studied and DSC was run under identical conditions using the second heating cycle also. In order to also find out if there is some factor which inhibits the formation of starch-lipid complexes in tuber crop starches, two surfactants and lipids extracted from cereals were externally added and the DSC studied and the results are presented in the paper

Experimental

Starch was extracted from freshly harvested tubers of cassava, Colocasia esculenta, Xanthosoma saggittifolium, Amorphophallus paeonifolius, Dioscorea alata, Dioscorea esculenta, Dioscorea rotundata, Arrowroot, Pacchyrrhizus, and Canna edulis by standard method ¹⁸(Moorthy, 1991),. Accurately weighed quantity of the dry starch was transferred to the aluminium pans , water was added to get starch: water ratio of 1:2 and the pans were hermetically sealed. The pans were transferred to the heating chamber of the DSC equipment. The DSC was run on a Seiko 6000 DSC equipment using indium as standard and under nitrogen atmosphere. The heating cycle used for the study was as follows;

I heating 15-150 at 5⁰/min
I cooling 150-30 at 30⁰ /min
II heating 30-130 at 5⁰/min
II cooling 130-30 at 30⁰/min

The onset of gelatinisation To, end of gelatinisation Te and enthalpy of gelatinisation H were obtained from the graphs.
Starch lipids interaction was studied by using a starch-lipid mixture such that the starch:lipid ratio was and starch:water ratio was 1:2. Similarly starch-surfactant interaction was examined by using Sodium dodecyl sulphonate and cetyl trimethylammonium bromide solutions (5%) in distilled water.

Results and discussion

The DSC graphs of the ten native tuber starches are presented in Figures. The thermograms clearly show the absence of peaks corresponding to melting of the starch-lipid complex in any of the starches whether during the first heating or second heating whereas the expected peaks for starch gelatinisation were present during the I heating. When a suspension of starch in water is heated, the starch undergoes a series of changes in their morphology and physicochemical and rheological properties. The changes in the thermal properties are clearly visible in the DSC thermograms. When a specific temperature is attained, a peak corresponding to starch gelatinisation is observed. This takes place over a range of temperature and is known as gelatinisation temperature. The granules absorb a large quantity of water and swell which leads to increase in viscosity. There will be first absorption of water by the amorphous regions which is followed by the disorganisation of the crystalline regions . These occur together under excess water whereas they can get resolved into two distinct peaks at lower levels of water, The temperature depends on the starch source and also on the presence of other ingredients which can modify the gelatinisation temperatures considerably. Salts, sugars, lipids are known to affect the gelatinisation temperatures.

If lipids are present in the starch, they can form complexes with the starch and the melting of the starch-lipid complex occurs at a much higher temperatures. The temperature range can be 80-135⁰C compared to the gelatinisation temperatures of 55 to 90⁰C. Whereas starch gelatinisation is an irreversible process, the starch-lipid melting is a reversible process and so on cooling, the complex is reformed and during second heating the starch gelatinisation peak is absent, but the peak corresponding to the melting of the starch-lipid complex appears. Hence studies on the starch lipid interaction are always carried out during the second heating. It has also been observed that the enthalpy of starch-lipid melting is higher during second heating and this is attributed to the release of amylose when the starch has gelatinised and this extra amylose complexes with the starch. In the present studies, we did not observe any peak corresponding to the starch-lipid melting even during the second heating confirming the absence of starch-lipid complex in any of the tuber starches studied. In our studies we have tried all types of tuber starches with varying properties and none of them gave any peak corresponding to starch-lipid melting. We were particularly interested in Pacchyrrhizus starch since the crop has leguminous characteristics in producing pods and hence whether the starch has some lipid similar to pulse starches. Even for this starch, there was no peak corresponding to melting of starch-lipid peak showing absence of the leguminous character in the starch. Similarly we also did not find any difference between the starches having different XRD patters. Whereas cassava, aroids, arrowroot and Pacchyrrhizus starches have A pattern, the yam starches and canna starches possess B pattern. However all the starches did not exhibit the starch-lipid melting peak. Hence the arrangement of the starch molecules in the granules do not have any bearing on the starch-lipid complex formation. It is well documented that cereal starches show the peak whereas potato starch does not exhibit the starch-lipid complex melting. So all the tuber starches are free from lipids in their native state similar to potato starch

In order to find out whether the starches have some inhibition in forming complexes with lipids or surfactants, these starches were complexed with externally added lipids and surfactants and the DSC of the complexes examined. On addition of these, the peak corresponding to melting of the starch-lipid or starch surfactant complex was observed and that too for all the starches examined. Both the surfactants used gave complex formation showing there is no inhibition in complex formation. Various workers have compared the complexing ability of different surfactants and found the optimal length for complex formation is eighteen to eighteen carbon surfactants¹⁹. The stability is very low beyond this limit. Though there was not much difference between the SLS and CTAB, the latter seems to form slightly stronger complex because of the slightly higher enthalpy obtained for them. Similarly there was not much difference among the polar and nonpolar lipids in forming complex with starch. Polar lipids were expected to form stronger complexes , but such effect was absent Thus the results clearly indicate that there is no inherent problem for the tuber starches to form complexes with lipids It is possible that t during synthesis of starch, the tuber starches do not come into contact with lipids and hence unlike the cereal starches, they do not have inherent lipids in them It also shows that the starch properties of the tuber starches can be modified by adding lipids or surfactants. Such practice is already there in cereal starches and potato starch but not for the tuber starches. Some of the tuber starches have some special characteristics and by incorporation of lipids or surfactants , the properties may be further improved. This is also important for producing resistant starch using the tuber starches and lipids or surfactants.

References

1. Hoover, R, In: Polysaccharide association in foods, Ed Walter, R,H. Marcell and Decker, New York, 1998, 227
2. Eliasson, A.C. and Gudmundsson, In: Carbohydrates in Food, Ed Eliasson, A.C.,Marcel l and Decker, New York, 1996 431.
3. Billiaderis, C.G., 1998, Structures and Phase Transitions of Starch Polymers.
4. Billiaderis, C G and Seneviratne, H D,. Carbohyd. Polym., 13, 1990,185.
5. Hoover, R. and Hadziyev, D., Starch, 33., 1981, 346.
6. Biliaderis, C.G. and Tongai, J.R., , J Agric. Food Chem., 39, 1991, 833.
7. Billiaderis, C A, Tonogai, JR, Perez, C.M., and Juliano, B.O Cereal Chem., 70, 1993. 512,
8. Vasanthan, T., and Hoover, R. Food Chem., 43, 1992, 19.
9. Morrison, W.R., Starch, 33, 1981, 408.
10. Vasanthan T. and Hoover, R., Food Chem., 45, 1992, 337.
11. Slade, L. and Levine, H., , In: Industrial Polysaccaharides, Ed. Stivala, S.S., Crescenzi, V. and Dea, I.C.M., Gordon and Breach, New York, 1987 p 387.
12. John, A. and Shastri, P.N., J. Food Sci. Tech., 35, 1998, 1
13. Donovan, J.W., Biopolymers, 18, 1979, 263.
14. Billiaderis, In;Polysaccharide association in foods, Ed Walter, R,H. Marcell and Decker, New York, 1998, 58
15. Eliasson, A.C., , Starch , 32, 1980, 270,
16. Moorthy, S.N.,Wenham, J.E. and Blanshard, J.M.V., J. Sci. Food. Agric., 72, 1996, 329
17. Morrison, W.R. and Laignelet, , J. Cereal Sci., 1, 1983, 9
18. Moorthy, S.N., Carbohyd. Polym., 1991.
19. Krog, N., Starch , 23, 1971, 206,

The tropical tuber crops play a dual role in being a source of food as well as industrial raw material. However, they have not received enough importance due to various factors. The main factors are that the tubers are considered poor mans crop and there is little awareness of the variability available in the starch properties of these crops. Again the knowledge about the innumerable products, which can be produced from starch, has been scanty. So all these aspects needed to be addressed to enhance the importance of these crops and research projects covering food and industrial applications of these neglected crops were formulated and carried out. In addition, an ad-hoc project on minor tuber starches was undertaken in collaboration with RRL, Trivandrum. Similar work was carried out during my advanced training at the Food Science department of Nottingham University, and Natural Resources Institute, UK and later on at Food technology department of Lund University, Sweden during the Sabbatical leave.

For use of cassava as food, the cooking quality is of utmost importance and success of any new variety depends on the acceptability by the consumers. Previously only yield was taken into consideration, but subsequently the importance of quality was realized and hence was accepted as an essential criterion for variety release. The experience gained from analyses of a large number of samples was useful in formulating detailed studies on factors responsible for good cooking quality of cassava tubers.

The idea of starch data bank which provides exhaustive data on different starches and also starch bank having a collection pf natural starches with different characteristics has been mooted and CTCRI having the most abundant collection of root crops could best serve as the centre for this activity. Hence a detailed study of the properties of all the tropical tuber starches was carried out keeping in view effect of varietals variation and environmental factors on the properties.

Since there has been increasing stress on value addition, processes for a number of products based on the starches were developed which could in future make these crops highly sought after. In view of the increasing demand for environment -friendly products development of technologies for such products is highly relevant to the present day context.

When, where and how it was conducted ?

The work was carried out over a period of nearly 25 years and most of the work was conducted at the Crop Utilization and Biotechnology division of CTCRI, Trivandrum. Part of the work was also carried out at Food Science department, University of Nottingham, NRI, UK ( 6 months ) and at Food Technology department, Lund University, Sweden (11 months).

In view of the importance of the tuber crops in food and industry, the work on the cooking quality of cassava tubers, starch properties and product development was taken up. First of all infrastructural facilities were built up to carry out the work. Visits were made to ATIRA, Ahmedabad, CFTRI, Mysore etc. and discussions with scientists and technologists working on starches and related compounds were carried out. Based on the ideas thus generated, equipments like Brabender Viscograph, IR and UV Visible spectrophotometers were procured in addition to common laboratory equipments. Books and journals related to starch and food were added to the institute library. The cooking quality studies were carried out using the tubers from varieties available in the Institute. The studies on cassava starch were helpful in my getting deputed to UK . I carried with me a number of samples of starch and flour from different varieties and used the facilities available there to analyse them. DSC studies were carried out in detail on the starches. I also made use of this opportunity to get acquainted with Deer Viscometer and Rheogonimeter. Methodology for Gel Permeation chromatographic analysis of debranched starch samples and use of Coulter Counter for granule size measurement were the other techniques which I learned there. A lot of data on starch properties of tuber crops was generated and published in International journals. Meanwhile a project on the minor starches was undertaken in collaboration with Dr. Raja of RRL, Trivandrum financed by ICAR-Cess Fund. When I got an offer for Sabbatical work at Lund University to work with Prof Eliasson, an authority on starch, I carried with me the different tuber starch samples and studied their thermal properties using the highly advanced Modulated DSC technique and generated voluminous data on the tuber starches, especially interaction with lipids and surfactants. Similarly detailed studies on rheological properties of the starches were carried out and this is the first time that exhaustive work on rheology of starches has been carried out using a Bohlin Rheometer, which provides a wealth of data on starch rheology. Since value addition is of utmost importance, various products were developed from the tuber crops and their starches, which can have application in food and industry. These include food items, adhesives for different applications, fructose syrup and starch derivatives having varying physicochemical and functional properties. The products and technologies were demonstrated at different exhibitions, training classes and visitors to the Institute for possible adoption. Thus the results presented are based on a concerted study carried out over a long period and covers basic and applied aspects.

What were the Socio- economic, Technological and Scientific Relevance and Priority of the research project ?

The tuber crops are considered poor mans crop in spite of the fact that they provide food having high calorific and also starch useful in industry. Mostly marginal and small farmers cultivate them. Except cassava they do not have any industrial base at present. The lure of cash crops and easy availability of cereals and changing food habits are further threatening the survival of these crops. The crops can grow well under adverse conditions (as recently proved by the experience of the farmers during the Orissa Cyclone a few years back when only the tuber crops survived) and so it is necessary that these crops are retained in the cropping schedule. This is possible only by value addition to a good extent so that the farmers get good profit from these crops. The crops can be very suitable for cultivation in North Eastern belts and tribal areas of the country, where the major use will be as food and hence good cooking quality and development of new simple foods need priority. These important aspects have been duly addressed.

Except cassava other tuber crops are not used in India for the extraction of starch mainly due to the difficulty in extraction of pure starch from them. If starch can be commercially extracted from the other tuber crops also, they will have higher demand and the farmers will be eager to cultivate the crops. The knowledge of starch properties of the tuber crops is scanty and hence the awareness of their potential applications is limited. So the studies on the tuber starches are very pertinent and can throw open new vistas of their applications. In addition to providing basic knowledge, the studies can throw light on the most desired characteristics of starch, which may be achieved by use of biotechnological tools. They can also substitute the chemically modified starches, which may create health problems.

Product and process developments are becoming very important in the present day context in view of the necessity to be competitive in the world economic scene, Value added food and industrial products with domestic and export potential need to be developed and has also been given high priority in the work. The demand for environmentally friendly products and food items suited to the health-conscious public have also been given due consideration.

Abstract The physicochemical properties of starch extracted from sweet potato tubers using five different concentrations of the enzyme system cellulase-pectinase was studied. The starch content of the extract was 90-93% on a dry weight basis. The reducing values were not noticeably increased by the enzyme treatment. The peak viscosity values of the extracted starches showed an increase up to 0.05% concentration of enzyme, and thereafter registered a minor fall. The viscosity stability also showed a small reduction at enzyme concentrations above 0.05%. The swelling volumes exhibited a slight decrease and solubility was almost doubled at the highest concentration of the enzyme. The SEM photographs did not indicate any major change in the surface morphology of the extracted starch. Thus the enzyme treatment does not adversely affect the starch properties up to 0.05% concentration.

Keywords: sweet potato, starch, cellulase, pectinase, physicochemical properties.

Introduction

Sweet potato (Ipomea batatas L.) is cultivated more widely than other trophical tuber crops, and it occupies third position in terms of calories produced per square metre (Leung et al 1972). However, the extraction of starch from sweet potato tubers has been practiced in only a few countries, particularly China and Japan. The reason for this is the difficulty of getting a good yield of starch. The tubers contain 15-30% of starch, but the yield is usually less than 15%, even for the high starch varieties. In contrast, with cassava, over 80% of the starch is obtained by simple extraction. The low recovery makes the starch more expensive. The reason for the low extraction rate is probably difficulty in breaking down non-starchy constituents like cellulose, hemicellulose and pectin, which restrict the starch granules from moving into the aqueous phase during the conventional process (Balagopalan et al. 1996). With the increased availability of commercial enzymes which break down cellulose and pectin, attempts have been made to use them to improve the extractability of starch. Kallabinski and Balagopalan (1991) studied the effect of cellulolytic and pectinolytic enzymes on the extraction of starch from sweet potato tubers: the yield showed a substantial increase.

The physicochemical and functional properties of sweet potato starch have been summarized by Tian et al. (1991): they vary with variety, climate, environment, etc. when cassava is fermented before extraction, the starch undergoes minor changes in its properties but the main structural features are unaffected (Moorthy et al. 1993). This work aimed to establish whether enzymatic separation affected the physicochemical properties of sweet potato starch, on which the food and industrial uses depend.

Experimental

Sweet potato tubers were washed and the outer skin was peeled with a knife. The extraction of starch was by the procedure of Kallabinski and Balagopalan (1991), using the enzymes Celluclase and Pectinex (Novo, Denmark) at 0.01, 0.025, 0.05, 0.1 and 0.2%. The starch content in the extracted starch was monitored titrimetrically (Moorthy et al. 1996). The reducing values were determined by the method of Schoch (1964a). Viscosity, viscosity stability and pasting temperatures were obtained from Brabender runs with 5,6 and 7% starch is distilled water and a heating rate of 1.5° C/ min. The swelling volumes, solubility and swelling power were determined by the methods of Schoch (1964b). Swelling volumes were based on the sedimentation volumes, while the solubility was calculated from the weight of residue left on drying a fixed volume of the supernatant. The scanning electron microscopic analysis of the samples was on a Joel unit after coating the starch with gold in vacuum, at three magnifications, 1500x, 2000x and 4500x.

Results and discussion

The extract from sweet potato with different concentrations of enzyme contained 90-93% of starch (Table 1). This indicates that only a small quantity of fibrous material is being extracted with the starch, even with the enzyme at 0.2%.Padmanabhan and Lonsane (1992) found that in an enzymic extraction of cassava starch, the total ash content was lower than in the conventional procedure, and this was attributed to the liberation of minerals from the root cells. When fermentation was carried out on cassava tubers using and inoculum provided culture, the resulting starch had significant fibre content, which increased with the fermentation time (Moorthy et al. 1993). The absence of large amounts of fibre in the enzymatically separated starch from sweet potato indicates that the non-starchy polysaccharides are completely broken down and do not contaminate the starch, so the enzymes appear more efficient than the cultures in breaking down the pectins and cellulosic components. The reducing values of the starch from the enzyme treatments were small (Table 1), as expected, since the enzymes are pectinolytic and cellulolytic, and should not affect the starch granules.

Table 1. Properties of enzymatically separated sweet potato starch

The viscosity data of the starches from enzymatic and conventional extraction are in Table 2. the peak viscosity varied depending on the concentration of starch used. With 5 and 6% pastes, the peak viscosity increased for the extracts obtained with increasing amounts of the enzymes, up to 0.025 or 0.05%, and with 0.2% of enzyme it dropped noticeably. With the 7% paste, there was a fairly steady drop in peak viscosity as the enzyme concentration increased. This is probably due to a weakening of the associate forces rather than to a breakdown of the starch granules. The breakdown in viscosity also increased with higher levels of the enzymes. When the concentration of the enzyme was 0.1%, the breakdown was large and it became very significant at 7% starch concentration. A reduction in the breakdown was observed with 0.2% enzyme, attributable to the correspondingly lower peak viscosity. These results also show that the strength of the associative forces is somewhat affected at higher concentrations of the enzyme. The presence of fibre reduces the breakdown of starch viscosity by protecting the starch granules against heat and shear (Moorthy et al. 1994). In the enzymic extraction of cassava starch, the peak viscosity showed a slight reduction while the breakdown increased marginally (Padmanabhan and Lonsane 1992), as in our work. The pasting temperature did not show any definite pattern, but generally there was a shift to lower temperatures with increasing concentrations of the enzyme. This can also be explained on the basis of a weakening of the associative forces rather than the presence of fibrous residues, which would have led to higher pasting temperatures, as observed with cassava fermentation (Moorthy et al. 1993), Padmanabhan and Lonsane (1992) did not notice such an effect in the enzymic extraction of cassava starch.

Table 2. Pasting temperature and viscosity of enzymatically separated sweet potato starch.

The enzymatically separated starch had a slightly lower swelling volume at 95°C than the control (Table 1), and there was no relationship between the reduction in swelling volume and the concentration of enzyme used. Padmanabhan and Lonsane (1992) found a slight reduction in the swelling volume in enzymatically extracted cassava starch and attributed this to a small reduction in Ph. The solubility of enzymatically extracted starch was less than the control at low enzyme concentrations, but a higher levels it increased substantially. This could be explained by the weakening of associative forces at higher concentrations. A similar increase in solubility was observed with cassava starch (Padmanabhan and Lonsane 1992).

Scanning electron microscopy of the starch granules showed that there was no marked change in the surface morphology upto the 0.1% enzyme level (Figure 1a-d). Above 0.1% some minor fissures appeared on the surface (Figure 1e and f). The effect appears to be restricted to the surface of the granules, since the other granular properties were affected to only a small extent. Padmanabhan and Lonsane (1992) did not observe any difference between conventionally and enzymatically extracted starch from cassava.

Conclusion

The starch separated from sweet potato using pectinase and cellulase does not undergo any major breakdown up to and enzyme concentration of 0.05%, but above that some level some weakening of the associative forces takes place.

References

1. Balagopalan C. Ray R.C. and Sheriff J.T. (1996) Enzymatic separation of food grade industrial starch from sweet potato (Ipomoea batatas (L.) Lam). In: Tropical tuber crops (Kurup G.T., Palaniswami M.S., Potty V.P., Padmaja G., Kabeeruthumma S. and Pillai S.V., eds). New Delhi: Oxford IBH, pp. 471-6.
2. Kallabinski J. and Balagopalan C. (1991) Enzymatic starch extraction from tropical root and tuber crops. In: Proceedings of the Ninth Symposium of the International Society for Tropical Root Crops (Ofori F. and Hahn S.K., eds), pp. 83-8. Wageningen, Netherlands: ISRTC.
3. Leung W.W., Butrum R.R. and Chang H. (1972) Proximate composition, mineral and mineral contents of East Asian foods. In: Food composition tables for use in East Asia. Bethesada, MD: US Dept of Health, Education and Welfare, pp.1-187.
4. Moorthy S.N., Mathew George and Padmaja G. (1993) Functional properties of the starchy flour extracted from cassava on fermentation with a mixed culture inoculum. Journal of the Science of Food and Agriculture 61, 443-7.
5. Moorthy S.N., Mathew George and Padamaja G. (1996) A rapid titrimetric method for starch determination in cassava tubers. Journal of Root Crops, in press.
6. Moorthy S.N., Rickard J.E. and Blanshard J.M.V.(1994) Influence of the gelatinization characteristics of cassava starch and flour on the textural properties of some food products. International meeting on cassava starch and flour. CIAT, Colombia.
7. Padmanabhan S. and Lonsane B.K. (1992) Comparative physicochemical and functional properties of cassava starch obtained by conventional and enzyme integrated conventional techniques. Starch/Starke 44, 328-37.
8. Schoch T.J. (1964a) Determination of reducing value. In: Methods in carbohydrate chemistry, vol. 4 (Whistler R.I., ed.), pp. 64-6. New York: Academic Press.
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Figure 1. SEM photographs of starch granules of sweet potato extracted using different enzyme concentrations. (a) 0% enzyme; (b) 0.01% enzyme; (c) 0.025% enzyme; (d) 0.05% enzyme; (e) 0.1% enzyme; (f) 0.2% enzyme.