Effects of Na2CO3 and NaOH on DSC thermal profiles of selected native cereal starches

The effects of Na2CO3 and NaOH (at a concentration of 1 g alkalising agent/100 g dry starch) on the gelatinisation properties of 1:4 and 2:3 starch/water mixtures were studied using differential scanning calorimetry (DSC). The starches studied included wheat, rice, waxy rice, corn and waxy corn. The addition of Na2CO3 or NaOH resulted in significant increases in transition temperatures (onset, peak and conclusion), associated with starch gelatinisation, suggesting a stabilisation of the starch granules, probably through electrostatic interactions between Na+ ions and starch hydroxyl groups. The presence of Na2CO3 generally caused a significant reduction in gelatinisation enthalpy (ΔHG) of the higher moisture (1:4) systems, but had no effect in the less hydrated (2:3) systems. NaOH generally had no effect on ΔHG of 1:4 systems, but gave rise to inconsistent effects in the 2:3 systems. Heating of alkali-containing samples above the gelatinisation temperature range gave rise to an intriguing exothermic peak, the origin of which was attributed to repolymerisation of the products of alkaline hydrolysis of starch. A separate experiment, conducted to investigate the influence of increasing Na2CO3 concentration (up to 3 g/100 g starch) on thermal properties of 1:4 waxy rice starch/water systems, showed a gradual increase in gelatinisation transition temperatures, a general decline in ΔHG, and a marginal decrease in peak temperature of the repolymerisation exotherm.