Physical aging of glassy normal and waxy rice starches: Effect of crystallinity on glass transition and enthalpy relaxation

Effect of crystallinity on the relaxation behavior of normal and waxy rice starches was examined by differential scanning calorimetry (DSC). Starch samples with different crystallinities were prepared by heating granular rice starch (14% moisture) in DSC pans to different temperatures (170 and 200°C), and an amorphous sample was prepared by dissolving the starch in dimethylsulfoxide and then recovering it in absolute ethanol. The heat capacity increment (ΔC_p) at the glass transition decreased with increasing sample crystallinity. The glass transition temperature (T_g) of native granular starch was higher than that of partially melted starches, but the starch heated to 200°C displayed a higher T_g, than did the starch heated to 170°C, suggesting the occurrence of additional changes besides crystal melting during heating to 200°C. Moreover, network structure formed between crystalline and amorphous regions could affect the amorphous transition. With regard to amorphous structural relaxation, the peak temperature of relaxation endotherm (T_p) increased linearly with log aging time (t_a), but the rate of the relaxation temperature increase (dT _p/d log t_a) depended on residual crystallinity. The temperature of the relaxation endotherm positively correlated with glass transition. For partially melted starches, the extent of relaxation increased proportionally with the level of amorphous fraction, whereas the rate of relaxation decreased as the amorphous fraction increased.