The application of polymer crystal growth theory to the kinetics of formation of the B-amylose polymorph in a 50% wheat-starch gel

Abstract

Gelatinized wheat-starch gels containing 50% water (w/w) were prepared by extrusion and submitted to storage at varying temperatures between 2 and 37°C. The development of crystallinity was followed by X-ray diffraction over a period of three weeks by which time all perceptible postgelatinization changes were complete. The results were analysed by fitting to the Avrami equation (optimum fit occuring where the Avrami exponent n = 1). The derived rate of crystallization at the different temperatures was then analysed in terms of the Lauritzen-Hoffman theory of growth of polymer crystals. Although not all parameters had experimental values, it proved possible to evaluate a series of trial values of two parameters until the best fit to the growth rate data was obtained as judged by the correlation coefficient. All parameters so determined were deemed satisfactory in terms of comparable figures obtained with synthetic polymers. The theoretical T g for the above 50% aqueous wheat-starch gel was calculated to be approximately 200 K which accorded well with a value calculated by free-volume theory. The practical implications with regard to retrogradation of starch gels at different temperatures and different water contents are discussed.