Isothermal crystallization of concentrated amorphous starch systems measured by modulated differential scanning calorimetry

The slow isothermal crystallization of concentrated amorphous starch system s is measured by Modulated. Differential Scanning Calorimetry (MDSC). It ca n be followed continuously by the evalution (stepwise decrease) of the MDSC heat capacity signal (Cp), as confirmed with data from X-ray diffractometr y, Dynamic Mechanical Analysis, Raman spectroscopy, and conventional Differ ential Scanning Calorimetry. Isothermal MDSC measurements enable a systemat ic study of the slow crystallization process of a concentrated starch syste m, such as a pregelatinized waxy corn starch with 24 wt % water and 76 wt % starch. After isothermal crystallization, a broad melting endotherm with a bimodal distribution is observed, starting about 10 degrees C beyond the c rystallization temperature. The bulk glass transition temperature (T-g) dec reases about 15 degrees C during crystallization. The isothermal crystalliz ation rate goes through a maximum as a function of crystallization time. Th e maximum rate is characterized by the time at the local extreme in the der ivative of Cp (t(max)), or by the time to reach half the decrease in Cp (t( 1/2)). Both t(max) and t(1/2) show a bell-shaped curve as a function of cry stallization temperature. The temperature of maximum crystallization rate, for the system studied, lies as high as 75 degrees C. This is approximately 65 degrees C above the initial value of T-g. Normalized Cp curves indicate the temperature dependence of the starch crystallization mechanism. (C) 19 99 John Wiley & Sons, Inc.