Thermal transition characteristics of heat-moisture treated corn and potato starches

Normal corn starch containing 25 or 30% moisture and potato starch containing 20 or 25% moisture were heat-moisture treated at 120°C for 1 h and the changes in thermal transition properties of the starches were examined using a differential scanning calorimeter (DSC). Granular crystallinity monitored by the X-ray diffractogram was reduced by the heat-moisture treatment (HMT), especially for potato starch. The T_g′ of starch measured in frozen state in excess water (starch:water = 1:4) increased by 1-4°C due to the HMT. Under limited moisture conditions (15% based on total weight), however, T_g measured in granular form of starch decreased by 2-6°C. Potato starch had a T_g about 25°C higher than corn starch, regardless of the HMT. At T_g, the change in heat capacity (ΔC_p) of the treated starch was substantially higher than the corresponding native starch. Enthalpy and onset temperature of the relaxation endotherm of the glassy starch was also increased by the HMT. Crystal melting of the heat-moisture treated starches, measured at 80% moisture, appeared to be biphasic on a DSC thermogram, in which the original endotherm became smaller while a new endotherm at higher temperature was observed. However, the total melting enthalpy for starch decreased, indicating a partial loss of crystallinity. The enthalpy associated with melting of the amylose-lipid complex in corn starch was increased by HMT. The degree of retrogradation under DSC was not significantly different between the native and treated starches.