Tropical Tuber Crops

Prev Page | Next Page

2. Extraction of starch

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

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

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

   Table 7 : Yield and total amylose content for starches extracted with ammonia solution and water

   Species	Extraction medium	Yield (%)	Total amylose  ( blue value)
   Cassava	Water	21.8 ± 0.54	0.37 ± 0.012
   Cassava	NH3	22.2 ± 0.37	0.37 ± 0.019
   Colocasia	Water	6.2 ± 1.79	0.28 ± 0.011
   Colocasia	NH3	16.2 ± 0.37	0.26 ± 0.017
   Dioscorea alata	Water	17.0 ± 1.43	0.45 ± 0.008
   Dioscorea alata	NH3	18.3 ± 1.0	0.44 ± 0.010
   Dioscorea esculenta	Water	17.7 ± 1.06	0.29 ± 0.004
   Dioscorea esculenta	NH3	18.7 ± 1.14	0.28 ± 0.008
   Dioscorea rotundata	Water	18.8 ± 0.85	0.40 ± 0.010
   Dioscorea rotundata	NH3	19.5 ± 1.16	0.40 ± 0.013
   Sweet potato	Water	13.0 ± 1.02	0.34 ± 0.008
   Sweet potato	NH3	10.9 ± 1.10	0.35 ± 0.013
   Xanthosoma	Water	20.0 ± 0.32	0.38 ± 0.014
   Xanthosoma	NH3	20.5 ± 1.76	0.36 ± 0.015