EFFECT OF MOISTURE-CONTENT ON THE MELTING AND SUBSEQUENT PHYSICAL AGING OF CORNSTARCH

Differential scanning calorimetry and dynamic mechanical analysis were used to study low moisture (10-50%) gelatinized cornstarch in order to understand the severe embrittlement on aging often seen in these systems. On aging at 22-degrees-C, gelatinized starch having 11-20% moisture (T(g) = 112-30-degrees-C) developed endothermic differential scanning calorimetry peaks characteristic of enthalpy changes due to structural relaxation. The increase in area (enthalpy) of these peaks with time correlated well with decreases in ultimate elongation as measured by tensile tests of extruded starch-water ribbons. The rates of increase of enthalpy and decrease in elongation with time increased with moisture content. At moisture contents of 25% and above (T(g) < 15-degrees-C). high temperature endotherms characteristic of the melting of recrystallized amylopectin and amylose appeared. These data indicate that embrittlement of gelatinized starch can occur as a result of free volume relaxation during sub-T(g) aging as well as a result of plasticizer (water) loss by evaporation.