Glass transition temperature of starches with different amylose/amylopectin ratios

The glass transition temperatures (Tg) of starch with different amylose/amylopectin ratios were systematically studied by a high-speed DSC. The cornstarches with different amylose contents (waxy 0; maize 23, G50 50 and G80 80) were used as model materials. The high heating speed (up to 300 °C/min) allows the weak Tg of starch to be visible and the true Tg was calculated by applying linear regression to the results from different heating rates. It is confirmed for the first time, that the higher the amylose content is, the higher the Tg is for the same kind of starch. The sequence of true Tg of cornstarch is G80 > G50 > maize > waxy when samples contain the same moisture content, which corresponds to their amylose/amylopectin ratio. It was found that Tg was increased from about 52 to 60 °C with increasing amylose content from 0 to 80 for the samples containing about 13% moisture. The microstructure and phase transition were used to explain this phenomenon, in particular the multiphase transitions that occur in high-amylose starches at higher temperatures, and the gel-ball structure of gelatinized amylopectin.