Relationship of gelatinization and recrystallization of cross-linked rice to glass transition temperature

Nonwaxy rice starch was cross-linked with sodium trimetaphosphate and sodium tripolyphosphate to obtain different degrees of cross-linking (9.2, 26.2, and 29.2%). The objective was to investigate the influence of cross-linking on thermal transitions of rice starch. Starch suspensions (67% moisture) were heated at 2 degrees C/min using differential scanning calorimetry (DSC) to follow melting transition of amylopectin. Biphasic transitions were observed at approximate to 60-95 degrees C in all samples. Melting endotherms of amylopectin shifted to a higher temperature (less than or equal to 5 degrees C) with an increasing degree of cross-linking, while there was no dramatic change in enthalpy. Recrystallization during aging for 0-15 days was significantly suppressed by cross-linking. The delayed gelatinization and retrogradation in crosslinked starch were evident due to restricted swelling and reduced hydration in starch granules. Glass transition temperature (T-g) measured from the derivative curve of heat flow was -3 to -4 degrees C. No significant change in T-g was observed over the storage time studied.