Subzero Glass Transition of Waxy Maize Starch Studied by Differential Scanning Calorimetry

A mixture of waxy maize starch and water (1:2, w/w) was heated in a differential scanning calorimeter (DSC) pan to different temperatures to obtain different degrees of gelatinization. Each pan was then quenched to -30 degrees C and rescanned, and the subzero glass transition temperature (T-g') of the content was determined. A three-phase model of a starch granule-a mobile amorphous phase, a rigid amorphous phase, and a crystalline phase-was used to interpret results and explain the glass transitions in starch. Waxy maize starch had an onset gelatinization temperature (T-o) of 61.5 degrees C, peak temperature (T-p) of 70.3 degrees C, and completion temperature (T-c) of 81.7 degrees C. The T-g' was clearly noted after the starch and water mixture was heated to T-p and T-c, but was small and barely observable when the mixture was heated up to T-o and immediately cooled to -30 degrees C. When the starch and water mixture was heated to 55 degrees C, which was 6 degrees C below the T-o, and held for 2 h, a T-g' was observed. Moreover, T-g' began to appear and was observable if the starch and water mixture was heated to 10 degrees C below gelatinization onset temperature (51 degrees C) and annealed for 2 h without any gelatinization. Further holding at -7 degrees C showed a clear subzero glass transition of annealed native starch granules.