Variation in Properties of Starch in Cassava Varieties in Relation to Age of the Crop
By S.N. Moorthy and T.Ramanujam,
Cassava (Manihot esculenta Crantz) popularly known as tapioca is a high energy staple food crop yielding 250×10³ Cal. ha?¹ against 176×10³ and 110×10³ Cal. ha?¹ day?¹ produced by rice and wheat respectively. The tubers form a major dietary carbohydrate for millions of people in tropics. Besides, they also form tge raw material for industries in production of sago, liquid glucose and fructose, dextrine, alcohol, cattle feed etc. in view of high starch content. Considerable work on growth physiology of cassava has been done  which suggested that the pattern of growth in cassava is indeterminate even though the crop is harvested between 10th and 12th month stages. The storage roots continue to accumulate starch as long as the plant possesses sufficient leaves to support. However, the quality of the tuber may change beyond a certain period. To substantiate this fact scientific information on starch characteristics of cassava in relation to age of the crop is quite meager when compared to the voluminous work done on potato starch [3, 4,5]. In the present work, we have examined the starch characteristics of six promising cassava varieties at different stages of crop growth starting from tuber initiation phase in crop duration of 18 months with a view to evaluating the cultivars for stability characteristics of starch under prolonged period of harvest.
Materials and Methods
Five high yielding varieties of cassava namely H 97, H 165, H 226, H 1687 and H 2304 released from CTCRI, Trivandrum, and a popular table purpose variety, M4 were chosen for the study. The setts were planted at 90 cm x 90 cm apart following the recommended package of practices. Fertilizer dose adapted was 100:100:100:kg NPK. ha?¹. The crop was uprooted at monthly intervals starting from 2nd month upto 18th month stage. At each sampling period four plants at random were uprooted and a composite sample of tubers weighing 1 kg was used for starch extraction. The peeled tubers were crushed in excess of water using a Waring blender and filtered through 260 mesh sieves. The residue was rejected while the milk (filtrate) was allowed to settle for 12th and the supernatant decanted off. The settled starch was washed further with water and the clear supernatant solution was decanted after proper settling of starch. The starch cake thus obtained was dried at 60°C for 24 h powdered and stored in polythene bags for various physicochemical investigations namely granule size, reducing value, swelling volume, swelling power and amylase content.
The granule size was determined microscopically  and the reducing values, swelling volume, swelling power and solubility were recorded using standard procedures [7, 8]. Amylase content was estimated following the method of Sowbhagya and Bhattacharya . Phosphorus content was estimated by using vanadomolybdate colour reaction .
Results and Discussion
The granule size in relation to age of different varieties are given in Table1. It is clearly seen that the size of the starch granules increased only upto a period of 5-6 months and then remained nearly uniform throughout the period of the study. This is in conformity with the observations of Williams  and Cours . Therefore increase in storage root size in cassava is mainly by increase in cell number.
The reducing values of starch among the varieties studied did not differ significantly and the age of the crop also showed very little influence on reducing values (Table 2). This indicated that the molecular weight of the starch granules did not change much during the growth once the tuber bulking is initiated. Hence increase in starch content during crop growth is only by formation of new granules. The increase in granular size upto 6 months stage without corresponding increase in molecular weight may be attributed to the fact that the starch molecules attained their maximum size very fast during their synthesis before being incorporated into the parenchymatic cells.
Table 1. Granule Size (μ) in Relation to Age.
|Variety||Age in Months|
Table 2. Reducing Value (Ferricyanide Number) in Relation to Age.
|Variety||Age in Months|
The amylase content also did not show any significant variations with age of the crop (Table 3). However varietal differences were noticed which is in accordance with the earlier work . In case of potato starch, Geddes et al., , have found increased iodine affinity and amylase chain length with increase in size of the granules. In addition, it was reported that the amylopectin chain length in potato starch increased in association with increasing granular size. But in case of cassava starch no such phenomenon was found and that the amylase content of the starch was almost constant during the growth period.
Most interesting results were obtained in this work with regard to swelling properties of the starch. The varieties showed significant variations in their swelling characteristics of starch (Fig 1). The starch obtained from varieties M4 and H 2304 showed almost constant swelling volumes throughout the period of study (2 to 18 months) while the starch from other varieties showed very distinct peaks with change in age of the crop. The maximum deviation from the normal swelling volume was notice in H 165, where the swelling volume fell to 12.0 ml at 14th month stage and increased to 49.3 ml at 16th month stage. The varieties H 226 and H 97 also showed large variations during this period. Another interesting feature noticed was that the granules regained their swelling property very quickly. For example the starch of H 1687 lost its swelling volume at 15 month stage to half of its original value but regained normal value at 16th month stage. Similarly swelling volume of starch from H 226 fell to 22.6 at 14th month from the original value of 44, but regained the original value at 15th month. Further drop in swelling volume at 16th month and regaining to normal level at 17th month were also noticed in H 226. The variety H 97 also behaved similarly.
It was also noted that granule size or reducing values did not change during these periods. This suggested that the granules lost their strength to swell due to reduction in associative forces and not due to breakdown of the granules. If the latter is true, it is generally expected that there would be drastic reduction in granule size and molecular weight but we did not observe such changes in our studies. Therefore it is suggested that the associative forces are decisive in determining swelling property of cassava starch.
Table 3. Amylose Content (%) in Relation to Age.
|Variety||Age in Months|
The above phenomenon is confirmed when we examined the solubility data (Fig.2). It is very obvious that wherever the granules have low swelling volume they have high solubility. This proves that during the growth period due to environment and associated physiological changes the strength of binding (associative) forces between the molecules in starch granules are reduced considerably in certain varieties leading to a very low swelling volume and high solubility. Varieties M4 and H2304 showed low solubility throughout the period of study and exhibited almost uniform swelling pattern suggesting that they are stable varieties and less susceptible to changing environments. Since starch properties, to a great extent, decide the cooking quality of a variety, the swelling values should have a definite role in cooking property. It is found that M4 tubers exhibit maximum stability in cooking quality throughout the growth period. The cooking property of the tubers of H 1687 has been found to be quite dependent on environmental conditions and this may be attributed to the instability of starch properties under changing environmental conditions.
The effect is further elucidated by the swelling power values which are presented in Figure3. Here also, M4 starch, as expected, showed a very uniform pattern with values between 40 and 55. H 2304 starch exhibited slightly higher range (30-70). However the starch from varieties H 165, H 97 showed extreme variations reaching 600 at the 14th month stage. H 97 starch exhibited another peak at 10th month stage also. H 226 starch also showed wide variation from 35 to 160. The high swelling power exhibited by these varieties may be due to low associative forces which lead to easy breakdown of the starch granules.
It has been reported that the phosphorus content determines the granular strength in potato starch by forming cross linkages between the starch molecules [14, 15]. In order to verify the role of phosphorus in the binding strength of cassava starch molecules, phosphorus contents in selected starch samples which showed high and low swelling were compared. The results showed that there was no remarkable differences in phosphorus concentration between the starch samples having different swelling powers. (Table 4). This indicated that in cassava starch, the associative forces are different from those of potato starch and that the phosphate cross linkages may not be determining the granular strength of cassava starch. The phosphorus content in cassava starch was also quite low when compared to potato starch. The phosphorus content in potato starch also increased with age of the crop in contrast to cassava.
Table 4. Phosphorus Content (% x 102) in Relation to Age.
|Variety||Age in Months|
Hence it is concluded that the associative forces between the starch molecules of cassava varieties vary considerably and the physiological age of the crop also has tremendous influence on the properties of the starch. However, varieties like M4 and to some extent H 2304 showed very little variations in starch characteristics in relation to age of the crop. Varieties which showed less variation in the starch properties with age, may have stronger associative forces in their granules. Such varieties are considered more stable and the stable varieties may fit into any cropping system.
Though cassava is a perennial crop, it is normally grown as annual under low input management in marginal lands under rainfed conditions. It is also common practice in Africa where cassava crop is retained for longer period upto 18 months. As the crop has to face varied environmental conditions in its long growing period it is most desirable to develop stable varieties which can provide good quality tubers when the harvesting is prolonged. The varieties which showed significant variations in starch properties in relation to age, have to be harvested at appropriate time as they may perform badly in quality point of view when harvest is prolonged. Therefore starch properties, particularly swelling power and solubility may be considered as reliable parameters while fixing the maturity period of cassava varieties.
Dr. S.P.Ghosh, Director, CTCRI and Dr. C. Balagopal, Head, Division of Postharvest Technology, CTCRI, for their help and encouragements.
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Address of authors: Dr.S. N. Moorthy and Dr. T. Ramanujam, Central Tuber Crops Research Institute, Sreekaryam, Trivandrum 695 017 (India).