Physical aging of glassy normal and waxy rice starches: thermal and mechanical characterization

Physical aging of glassy amorphous rice (normal and waxy) starches (14.5% moisture) was characterized in terms of thermal and mechanical properties, using a differential scanning calorimeter (DSC), dynamic thermal analyzer (DMTA), and Instron texturometer. The starch samples were prepared by compressing under heat (100 degreesC, 2000 psi), and subsequently aged at 25 degreesC for up to 29 days. Relaxation enthalpy, as measured by DSC, increased gradually with aging time, reaching a structural equilibrium. The peak temperature of the relaxation endotherm in DSC thermograin linearly increased with the logarithm of aging time. Relaxation kinetics revealed that relaxation rate was much slower for normal rice starch than for waxy rice starch. DMTA showed that storage modulus increased by aging, but the tan delta peak height reduced, indicating that the chain mobility of glassy starch was reduced by aging. Dual glass transitions were observed in both DSC and DMTA thermograms for normal-rice starch samples, whereas a single transition was observed for waxy rice starch samples. It suggests that the amorphous regions in normal starch samples are heterogeneous. Breaking strength, measured using an Instron texturometer, increased gradually with aging time, in accordance with the change in relaxation enthalpy. The relaxation kinetics of glassy starch aging could be explained by thermal transition enthalpy and by mechanical properties such as storage modulus and breaking strength. (C) 2004 Elsevier Ltd. All rights reserved.