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 (∼10μm diameter) formed from gelatinized high-amylose maize starches and purified amylose at cooling rates of order of magnitude 100°C/min. They were more numerous and exhibited a higher melting point the greater the ratio of amylose to amylopectin. Rapid cooling rates (150-500°C/min) resulted in a more even distribution of smaller spherulites. Very rapid (liquid nitrogen quench) or slow (0.1-1°C/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°C follows gelatinization at 180°C. Despite the high maximum temperature of treatment (180°C) 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.
All Science Journal Classification (ASJC) codes
- Organic Chemistry
- Polymers and Plastics
- Materials Chemistry