SUB-T-G ANNEALING OF GRANULAR RICE STARCH - EFFECTS ON ENTHALPY RELAXATION AND STARCH SUCROSE INTERACTIONS

Annealing of native rice starch at a temperature somewhat below its ef fective glass transition temperature (T-g) increased both the gelatini zation onset temperature (T-o) and enthalpy (Delta H-G) to their respe ctive quasi-equilibrium values, due probably to free volume and enthal py relaxation, but had little or no effect on crystallite melting temp erature (T-m). ''Maximum'' Delta H-G was attained well before ''maximu m'' T-o, suggesting that excess volume can continue to be dissipated o r redistributed without any substantial change in internal energy. The effect of sucrose on Delta H-G appeared to be dependent on the physic al condition of the glassy regions of the starch granules, a significa nt increase in Delta H-G being observed only when sucrose was added to unannealed granular starch with apparently less stable amorphous regi ons. Addition of sucrose to annealed starch, which possessed lower ene rgy glassy states, yielded no significant changes in Delta H-G. On the other hand, elevation of both T-o and T-m was obtained on addition of sucrose to either physically aged or unannealed starch.