THERMODYNAMICS OF STARCH-WATER SYSTEMS - AN ANALYSIS FROM SOLUTION-GEL MODEL ON WATER SORPTION ISOTHERMS

The water sorption of several starch samples and resistant starch (RS) samples were analyzed using an equilibrium solution-gel structure mod el and the results were compared to various sorption theories. It is f ound that the water sorption relations to water activity in starch in the high water content region could be properly described by the equil ibrium two-phase structural model. The results suggested that favorabl e starch-starch interaction determined the formation of starch gel in water. The gel structure had a high molecular modulus of 10(8) Pa and thus had limited water sorption capability. Part of the starch also ex hibited the solution properties with water due to chain ends and defec ts of the gel structure. Despite the unfavorable starch-water interact ion, starch-water solution might be formed due to the predominant cont ributions from the entropy of mixing. The solution phase was responsib le for the rapid increase of water sorption at high water activity. It was also demonstrated that the starch could maintain a maximum dynami c unfreezable water up to similar to 36%, which was consistent with th e DSC measurements. (C) 1996 John Wiley & Sons, Inc.