Glass transition and enthalpy relaxation of cross-linked corn starches

Thermal transition of cross-linked corn starches was characterized by using a differential scanning calorimeter (DSC) in the presence of excess (67%, based on total weight) or limited (15%) water. Normal corn starch (∼24% amylose) was cross-linked by reacting with a mixture (99:1) of sodium trimetaphosphate (STMP) and sodium tripolyphosphate (STPP) in an aqueous alkaline slurry containing sodium sulfate. The amount of the cross-linking agent (STMP/STPP mixture) in the starch slurry (100 g starch, 140 ml water, and 10 g sodium sulfate) was varied (4–12 g), and the level of cross-linking was monitored by the level of incorporated phosphorus, swelling volume, and α-amylase resistance. When analyzed with excess water, the glass transition temperature (Tg′, −5.6 and −5.7 °C) of the cross-linked starches was higher than those of native and control starches (−6.6 and −6.5 °C, respectively), and the ice melting enthalpy was also increased by the cross-linking, indicating the increased amount of freezable water. However, in the DSC analysis with limited water, Tg and heat capacity increment (ΔCp) at Tg decreased by cross-linking. It indicates that the free volume for starch chains was increased by the bulky and ionic phosphate groups. The relaxation enthalpy increased, but the peak temperature of relaxation endotherm decreased as the cross-linking level increased. Retrogradation enthalpy after a week storage at 4 °C was decreased by cross-linking.