Spherulitic crystallization of gelatinized maize starch and its fractions

Binary systems of polymers often display spherulitic morphologies after cooling from the melt, but these phenomena have rarely been reported among food polymers of native-size. Here we report the observation of spherulitic and other morphologies in gelatinized maize starch. The morphology could be manipulated by choosing polymer compositions and kinetic regimes. Spherulites (similar to10 mum diameter) formed from gelatinized high-amylose maize starches and purified amylose at cooling rates of order of magnitude 100 degreesC/min. They were more numerous and exhibited a higher melting point the greater the ratio of amylose to amylopectin. Rapid cooling rates (150-500 degreesC/min) resulted in a more even distribution of smaller spherulites. Very rapid (liquid nitrogen quench) or slow (0.1-1degreesC/min) cooling rates resulted in mixed morphology, as did addition of 15 or 60% (w/w) sucrose to a 10% (w/w) dispersion of high-amylose starch (HAS). Spherulites were observed in aqueous suspensions of high-amylose maize starch between 5 and 30% (w/w). Lower starch concentrations resulted in a broader size distribution and spherulites of more distinct shape. WAXS patterns of B-type were observed. Negatively birefringent spherulites predominated, but positive spherulites were found. The spherulite melting range overlapped with that for amylose-lipid complex. Evidence indicated that micro-phase separation takes place when a holding period at 95 degreesC follows gelatinization at 180 degreesC. Despite the high maximum temperature of treatment (180 degreesC) there was evidence for a memory effect in samples of 30% HAS. Spherulite morphology closely resembled that of native starch granules in very early stages of development. (C) 2002 Elsevier Science Ltd. All rights reserved.