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Relationship of Gelatinization and Recrystallization of Cross-Linked Rice to Glass Transition Temperature

¹Food Science Department, University of Massachusett, Amherst, MA 01003. ²Current address: Department of Biotechnology, Mahidol University, Rama Rd., Bangkok, 10400 Thailand. ³Corresponding author. E-mail: pavinee@foodsci.umass.edu Phone: 413/545-1025. Fax: 413/545-1262.

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°C/min using differential scanning calorimetry (DSC) to follow melting transition of amylopectin. Biphasic transitions were observed at ≈60–95°C in all samples. Melting endotherms of amylopectin shifted to a higher temperature (≤5°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°C. No significant change in T_g was observed over the storage time studied.