Effects of physical aging on thermal and mechanical properties of glassy normal corn starch

Physical aging of glassy corn starch and its effect on the physical properties of compressed starch bars were investigated by using differential scanning calorimetry (DSC), dynamic mechanical thermal analyzer (DMTA), and Instron texturometer. Amorphous corn starch samples were prepared by drying a starch paste, followed by compressing the dried powders to bar-type specimens under heat (100°C, 1.38 × 10⁷ Pa). These bars were then aged at 25°C for up to 22 days, after which thermal and mechanical characteristics were determined. There were progressive increases in glass transition temperature and relaxation enthalpy during the aging, which reached limiting values as structural equilibrium of the amorphous matrix was approached. The peak temperature of the relaxation endotherm (DSC thermogram) changed with time in a trend similar to the relaxation enthalpy. The storage modulus (DMTA) increased with aging time, but the peak intensity of tan 5 decreased. The breaking strength (Instron texturometer) increased gradually with time of aging and reached a maximum. It has been demonstrated the relaxation kinetics for glassy amorphous corn starch can be characterized by both thermal and mechanical properties, such as relaxation enthalpy, storage modulus, and breaking strength.