Kinetics of formation and physicochemical characterization of thermally-induced β-Lactoglobulin aggregates

The kinetics of heat denaturation and aggregation for β-lactoglobulin dispersions (5% w/v) were studied at 3 pHs (6, 6.4, and 6.8) and at a heating temperature of 80 °C. Protein aggregates were characterized for hydrodynamic diameter, microstructure, and molecular weight by means of dynamic light scattering, transmission electron microscopy, and polyacrylamide gel electrophoresis, respectively. Concentration of native β-lactoglobulin decreased with holding time and with a decrease in the pH. Apparent rate constants were calculated for β-lactoglobulin denaturation applying the general kinetic equation solved for a reaction order of 1.5. Values of the apparent reaction rate constant k = 7.5, 6.3 and 5.6 × 10^-3 s^-1 were found for pH 6, 6.4, and 6.8, respectively. Decreasing the pH of the dispersions produced higher aggregate sizes. After a holding time of 900 s, average hydrodynamic diameters for β-lactoglobulin aggregates at pH 6, 6.4, and 6.8 were 96, 49, and 42 nm, respectively. These results were confirmed by transmission electron microscopy images, where a shift in the size and morphology of aggregates was found, from large and spherical at pH 6 to smaller and linear aggregates at pH 6.8. β-Lactoglobulin formed disulfide-linked intermediates (dimers, trimers, tetramers) and so on) which then formed high molecular weight aggregates. From the results obtained by DLS, TEM, and SDS-PAGE a mechanism for β-lactoglobulin aggregation was proposed. This study shows that heat treatment can be used to produce protein aggregates with different sizes and morphologies to be utilized as ingredients in foods.