An X-ray scattering study of water-conditioned injection-molded starch during isothermal heating

The in situ structure variation of injection-molded starch (as processed and after water conditioning) during heat treatment was investigated by means of wide-angle X-ray scattering using synchrotron radiation. Results confirm that the crystal structure of potato starch is destroyed after injection molding, while as-processed corn starch preserves some degree of crystallinity. This residual crystallinity in corn starch is related to the crystalline Vh-form, made of complexes of amylose with lipids. Furthermore, it is shown that both starch types can develop crystallinity by water conditioning: potato starch yields the crystal B-form, while corn starch yields the crystal A-form coexisting with the persistent Vh-form. Upon isothermal heating of samples under vacuum, a rapid decrease of crystallinity, which is a function of both time and treatment temperature, is detected. Crystallinity variations are discussed in terms of water evaporation, the leveling-off values of crystallinity being dependent on the temperature of the isothermal treatment. (C) 2003 Wiley Periodicals, Inc.