Sub- Tg annealing of granular rice starch: effects on enthalpy relaxation and starch-sucrose interactions

Abstract

Annealing of native rice starch at a temperature somewhat below its effective glass transition temperature ( T g) increased both the gelatinization onset temperature ( T o) and enthalpy (Δ H G) to their respective quasi-equilibrium values, due probably to free volume and enthalpy relaxation, but had little or no effect on crystallite melting temperature ( T m). “Maximum” Δ H G was attained well before “maximum” T o, suggesting that excess volume can continue to be dissipated or redistributed without any substantial change in internal energy. The effect of sucrose on Δ H G appeared to be dependent on the physical condition of the glassy regions of the starch granules, a significant increase in Δ H G being observed only when sucrose was added to unannealed granular starch with apparently less stable amorphous regions. Addition of sucrose to annealed starch, which possessed lower energy glassy states, yielded no significant changes in Δ 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.