Effect of Crosslinking on Thermal and Microscopic Transitions of Rice Starch

Nonwaxy rice starch was crosslinked using sodium trimetaphosphate and sodium tripolyphosphate to obtain different degrees of crosslinking (9.2, 26.2 and 29.2%). Rice starch suspensions were heated from -40 to 100 °C at 2 °C/min 1) in a Differential Scanning Calorimeter and 2) under a light and polarized light microscope, in order to analyze for glass and/or melting transitions. The size of the starch granules increased with increasing temperature; the onsets for dramatic increase in size were approximately 55 °C for native and control samples and approximately 60 °C for crosslinked samples. A twostep swelling pattern was observed above 55 °C corresponding with the biphasic endothermic peaks as well as the discontinued (biphasic) loss in birefringence (due to heterogeneity in transition temperature). No disruption of granules was observed in any crosslinked samples. The low temperature transition (55-70 °C) involved both a loss in molecular order (possibly also melting) and swelling which were partly inhibited by crosslinking. Crosslinking delayed the gelatinization process (without decreasing total enthalpy) by restricting swelling and reducing the hydration of starch. No evidence of glass transition was observed in any samples studied.