Physical aging of glassy normal and waxy rice starches: effect of aging temperature on glass transition and enthalpy relaxation

Effect of physical aging at different temperatures (Ta) on the amorphous transitions of normal and waxy rice starches has been investigated using differential scanning calorimetry (DSC). Glass transition temperature (Tg) increased, but ΔCp at the Tg decreased, as the difference (Tg−Ta) between Tg and Ta increased, because the molecular mobility was reduced as the aging temperature was farther from Tg. The extent of relaxation was analyzed by determining the limiting relaxation enthalpy (ΔH∞) at various aging temperatures. The ΔH∞ versus Tg−Ta revealed the two different roles by kinetic and thermodynamic controls for the relaxation. When Tg−Ta was low (0∼30 °C), ΔH∞ increased with Tg−Ta, whereas when Tg−Ta was high (above 30 °C), it decreased with Tg−Ta. At a constant Tg−Ta, the residual moisture raised ΔH∞ due to plasticization effect. The effect of aging temperature on the relaxation temperature range was opposite to that on ΔH∞, and the peak range became narrower as the moisture content increased. The ΔH∞ and peak temperature range were different between normal and waxy rice starches. The normal rice starch had a higher ΔH∞ and a narrower peak temperature range than did waxy rice starch.