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 (T-a) on the amorphous transitions of normal and waxy rice starches has been investigated using differential scanning calorimetry (DSC). Glass transition temperature (T-g) increased, but DeltaC(p) at the T-g decreased, as the difference (T-g-T-a) between T-g and T-a increased, because the molecular mobility was reduced as the aging temperature was farther from T-g. The extent of relaxation was analyzed by determining the limiting relaxation enthalpy (DeltaH(infinity)) at various aging temperatures. The DeltaH(infinity) versus T-g-T-a revealed the two different roles by kinetic and thermodynamic controls for the relaxation. When T-g-T-a was low (0 similar to 30degreesC), DeltaH(infinity) increased with T-g-T-a, whereas when T-g-T-a was high (above 30degreesC), it decreased with T-g-T-a. At a constant T-g-T-a, the residual moisture raised DeltaH(infinity) due to plasticization effect. The effect of aging temperature on the relaxation temperature range was opposite to that on DeltaH(infinity), and the peak range became narrower as the moisture content increased. The DeltaH(infinity), and peak temperature range were different between normal and waxy rice starches. The normal rice starch had a higher DeltaH(infinity) and a narrower peak temperature range than did waxy rice starch. (C) 2003 Elsevier Science Ltd. All rights reserved.