Calorimetric studies of thermal denaturation of β-lactoglobulin in the presence of polysaccharides

Abstract

The thermal denaturation of β-lactoglobulin at pH 6 and 7 in the presence of polysaccharides was studied by differential scanning calorimetry. At neutral pH, the shape of differential scanning calorimetry curves was affected by polysaccharides and an increase of 2-3°C in the onset temperature of β-lactoglobulin denaturation was observed. This tendency was magnified up to 10°C at low water contents. At pH 7, the apparent enthalpy changes and the activation energy (Ea) were larger for β-lactoglobulin + polysaccharides mixtures than for pure β-lactoglobulin. At pH 6 the transition temperatures, denaturation enthalpy and activation energy of pure β-lactoglobulin were highly increased and slight changes were observed with further addition of polysaccharides. The rate constants of conversion of native β-lactoglobulin at pH 7 indicate a lower conversion of β-lactoglobulin when heated in the presence of polysaccharides, in agreement with electrophoretic results. Nevertheless, the formation of larger protein aggregates is promoted. A general analysis of the calorimetric and kinetic data indicates that polysaccharides enhance the thermal stability of β-lactoglobulin at neutral pH due to a limited thermodynamic incompatibility between the biopolymers. At pH 6 this effect is minimised because of the prevailing stabilizing effect of pH and a decreased incompatibility between β-lactoglobulin and polysaccharides.