ANNEALING OF GRANULAR RICE STARCHES - INTERPRETATION OF THE EFFECT ONPHASE-TRANSITIONS ASSOCIATED WITH GELATINIZATION

The effects of annealing on phase transitions associated with gelatini zation at different moisture contents of an atypical non-waxy rice sta rch exhibiting a clearer-than-usual glass transition were studied by d ifferential scanning calorimetry. Consistent with observations on synt hetic glassy polymers, annealing at temperatures below the glass trans ition for limited periods caused an elevation of T(g) which is attribu ted to the slow relaxation of the amorphous regions of the starch gran ules to their equilibrium glassy states. The crystalline regions were significantly affected only after annealing at temperatures above the glass transition, the resulting increase in crystallite melting temper atures and narrowing of the gelatinization range being ascribed to cry stal growth and/or perfection. Passage through the glass transition (w hich precedes crystallite melting) appears to give rise to an apparent endotherm which becomes even more prominent on sub-T, annealing of sa mples at low moisture contents. Annealing of an ordinary rice starch ( with no clearly discernible glass transition) at a temperature below i ts onset gelatinization temperature (T0) produced effects which parall eled those observed after sub-T(g) annealing of the atypical rice star ch. The conclusion drawn is that a glass transition is superposed on t he crystalline melt at the leading edge of the single endothermic peak observed at high moisture contents. Where biphasic endotherms at inte rmediate moisture levels are concerned, however, the present results s uggest that the first or G endotherm very likely arises from a glassy- to-rubbery state transformation while the second or M1 endotherm is as sociated with crystallite melting.