Hydrothermal Modifications of Tropical Tuber Starches. 1. Effect of Heat-Moisture Treatment on the Physicochemical, Rheological and Gelatinization Characteristics

Cassava, sweet potato and arrowroot starches have been subjected to heat-moisture treatment (HMT) under different conditions using a response surface design of the variables. A comparative study was performed on the pasting properties, swelling behaviour and the gelatinization properties of the modified starches and also on the rheological and textural properties of their pastes. X-ray diffraction studies have shown that cassava starch exhibited a slight decrease in crystallinity, whereas sweet potato and arrowroot starches showed an increase in crystallinity after HMT at 120 degrees C for 14 h with 20% moisture. The swelling volume was reduced and the solubility was enhanced for all three starches after HMT, but both effects were more pronounced in the case of arrowroot starch. The decrease in paste clarity of the starch after HMT was higher in the case of cassava and sweet potato starches. Viscosity studies showed that the peak viscosity of all three starches decreased after HMT, but the paste stability increased as seen from the reduced breakdown ratio and setback viscosity. Studies on rheological properties have shown that storage and loss moduli were higher for the starches heat-moisture treated at higher moisture and lower temperature levels than the corresponding native starches. Storage of the gel at -20 degrees C resulted in a significant increase in storage modulus for all the three starches. All the textural parameters of the gels were altered after the treatment which depended on the nature of the starch and also the treatment condition.