Calorimetric Study of Soybean Protein Isolates: Effect of Calcium and Thermal Treatments

Abstract

The addition of calcium (1.23−9.73 mg/g of protein) during neutralization of isoelectric precipitate modified the thermal behavior of soybean protein isolates. In the samples that did not undergo thermal treatment, the calcium content increase caused an increase in thermal stability, especially in the 11S fraction, but no modifications were detected in the enthalpy values. Samples undergoing mild thermal treatments (5 min at 80 °C), after neutralization with different amounts of calcium, showed enthalpy values lower than those of the unheated samples. Isolates with no calcium aggregates or with values below 5 mg/g of protein and intense thermal treatment (15 min at 90 °C) showed no endotherms (denaturation enthalpies = 0). Calcium levels above 5 mg/g of protein protected the soybean proteins, partially preventing their denaturation by heating at 90 °C for 15 min. The activation energy of the thermal denaturation process of soybean protein isolates, calculated by the Ozawa method (1970), was modified in the presence of calcium. The Ea of the 11S fraction varied between 51.12 and 102.15 kcal/mol and that of the 7S between 42.69 and 58.05 kcal/mol. A minimal value for glycinin Ea was observed at a calcium concentration of 6.51 mg/g of protein. The Ea value of the fraction enriched in 11S, calculated by the Borchardt and Daniels method (1957), was higher than that obtained by the Ozawa method (1970) for a soybean isolate. This fraction would denature following second-order kinetics (n = 2). Keywords: Thermal properties; protein denaturation; interaction calcium; soybean proteins