Physicochemical and Functional Properties of Tropical Tuber Starches: A Review

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  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

2. Extraction Techniques

Whereas extraction of starch from cassava is simple and the isolated starch is pure white in colour and relatively free from other chemical impurities this is not so with other tuber starches. The settling of starch granules is often hindered by presence of various components like mucilage and latex, leading not only to loss of starch, but also lowering of the quality of extracted starch. The long residence time can also promote microbial growth bringing about breakdown of starch and resultant loss of starch quality. Besides, the presence of these microorganisms affects the colour of the starch limiting its use in food and textile applications. Maximum recovery of starch with good physicochemical and functional qualities coupled with economical extraction of starches from tuber crops other than cassava is thus important. Work carried out at the Central Tuber Crops Research Institute, Trivandrum, India on use of various chemicals in improving the yield of starch from various tubers [7-9] showed that ammoniacal solutions gave the best results (Tab. 1). Aqueous ammonia (0.03 M), when used for starch extraction, not only improved the yield, but also the functional characteristics such as paste viscosity and swelling. Ammonia acts by complexing with the mucilaginous material releasing the starch granules and enabling faster settling of starch in less viscous slurry.

Table 1 : Yield and Blue Values for starches extracted with ammonia solution and water

Starch Extraction medium Yield [%] Total amylose [Blue Value]
Cassava Water 21.8±0.54 0.37±0.01
Cassava NH3 22.2±0.37 0.37±0.02
Colocasia Water 6.2±1.79 0.28±0.01
Colocasia NH3 16.2±0.37 0.26±0.02
Dioscorea alata Water 17.0±1.43 0.45±0.01
Dioscorea alata NH3 18.3±1.0 0.44±0.01
Dioscorea esculenta Water 17.7±1.06 0.29±0.00
Dioscorea esculenta NH3 18.7±1.14 0.28±0.01
Dioscorea rotundata Water 18.8±0.85 0.40±0.01
Dioscorea rotundata NH3 19.5±1.16 0.40±0.01
Sweet potato Water 13.0±1.02 0.34±0.01
Sweet potato NH3 10.9±1.10 0.35±0.01
Xanthosoma Water 20.0±0.32 0.38±0.01
Xanthosoma NH3 20.5±1.76 0.36±0.02

The short residence time also prevents microbiological damage of the starch. Lactic and citric acids improve the yield and colour of starch from sweet potato tubers [10]. Kallabinksi and Balagopalan [11] developed an enzymatic method fro enhancing the recovery (26% increase) of starch from cassava tubers using pectinase and cellulose enzymes. These enzymes alter the integrity of the pectin-cellulostic matrix of cell membranes and thereby facilitate the release of the starch granules. It was found that using the same technique, starch recovery from sweet potatoes could be enhanced by 20% without affecting starch properties [10, 12]. Padmanabhan and Lonsane [13] studied the effect of conventional and conventional-and-enzyme-integrated extraction methods on cassava starch properties. Mathew et al. [14, 15] used a mixed culture inoculum to enhance the yield of starch from cassava tubers. Although the recovery of starch increased, the starch was invariably contaminated with fibrous material. The starchy flour has functional properties slightly different from the native starch, but possessed better attributes like puffing ability which makes it suitable for specific food applications [16, 17].

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