Variability in starch extracted from taro

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S.N Moorthy, P.K. Thankamma Pillai & M. Unnikrishnan

Central Tubers Crops Research Institute, Sreekariyam, Tiruvandrum-695 017, India

(Received 5 may 1992; accepted 27 July 1992)

Starch was extracted from ten cultivars of taro and various physiochemical properties were determined. The granule size was found to vary considerably among the different accessions, C-9 starch having the largest average granule size (5.19 µm) while the lowest was recorded for C-46 (2.96 µm). the total amylose content varied between 14 and 19% C-9 starch having the highest value. Soluble amylose content ranged from 4 to 11%. Although there was not much difference between varieties in the 2% viscosity determined using a Redwood viscometer, the Brabender viscosity patterns showed considerable variation, and C-9 starch had the highest peak viscosity, almost twice as much as the others. The swelling volumes ranged from 25.0 to 60.0 ml g-1 with C-266 starch having the highest swelling volume.

Introdution

TARO (Colocasia esulenta (L) Schott) belonging to the family Aracea is cultivated in large areas of Asia, Africa the West Indies and South America. It is quantitatively the most important crop in the countries surrounding the pacific (Plunknett, 1970). Taro starch, in view of its small granule size, has been considered to be easily digestible; hence it is widely used in baby foods and the diets of people allergic to cereals and children sensitive to milk (Wang, 1983). In addition to food use, taro has found some industrial applications. The very small size of taro starch granules makes from ideal in cosmetic formulations like face powder and in dusting preparations which use aerosol dispensing systems (Griffin & Wang, 1983). Higahihara et al. (1975) have found that taro starch is suitable filler in biodegradable plastics.

In spite of the above uses, the large-scale extraction and utilization of this starch is not practiced anywhere, probably due to the difficulty in extracting the starch from fresh tubers, which contain a lot of mucilaginous material. Recently, however, it was found that the yield of starch could be considerably enhanced by using 0.03 M ammonia solution for extraction (Moorthy, 1991).

Although a large amount of variability has been reported in the morphological and tuber characteristics of the crop, little information is available on the variation in taro starch properties in relation to varietal differences. Hence a study was undertaken to explore the variability of starch extracted from different accessions of the crop and the results are presented in this paper.

Experimental

Ten cultivars of Colocasia including two released varieties (C-149 and C-266) were grown at the CTCRI farm following standard practices and harvested at the 7th month stage. The cormels were separated, washed and peeled. They were cut into small pieces and disintegrated in a blender at low speed using 0.03 M ammonia solution. The suspension was passed through 260 mesh screen twice and allowed to settle overnight. The supernatant was decanted off and the starch subjected to a second washing and settling. The starch cake formed was removed and powdered and dried at 45-50oc for 24 h. the resultant starch was used for various studies.

Granule size was determined using an ocular micrometer at a magnification of 15 × 40×. For each sample, five slides were prepared; 50 readings were randomly obtained from each slide and the average granule size was calculated.

The size of the starch granules was analysed with a Coulter Counter® Model TAII (Coulter Electronics Ltd. Luton, UK) after dispersing the defatted starch granules into Isoton II® (filters electrolyte solution, Coulter Electronics Ltd.). The equipment was calibrated by PDVB Latex® (UK). Each sample was run five times and five replications for each sample were used for size determination.

The total and soluble amylose content was determined by standard iodimetry (Sowbhagya & Bhattacharya, 1971; Shanthy et al., 1980). The viscosity of 2% solution of the starch in water was determined using a Redwood viscometer (India). The paste viscosity pattern was monitored using a Brabender Viscograph (Germany). Model 80102 using a 350 mcg cartridge. The concentrations of starch used in viscosity studies were 5, 6 and 7% and the pasting temperatures were directly read out from the viscograms. The swelling volumes were determined using a standard procedure (Schoch, 1964). The clarity was obtained by comparing the absorbance of the 2% starch solution with that of water as ’0′ at 500 nm. The sol stability was the time taken for the starch gel to start settling. Phosphorous content was determined by the Vanadomolybdate method (Smith & Caruso, 1964).