Effect of concentration, ionic strength and freeze-drying on the heat-induced aggregation of soy proteins

The effects of protein concentration, ionic strength, and lyophilization on heat-induced aggregation of soy proteins were analyzed by SDS-PAGE, SEC-HPLC and laser light scattering (LLS). SDS-PAGE profile suggested that the aggregates were formed via non-covalent forces and/or disulfide bonds. At ionic strength of zero, SEC-HPLC revealed that the samples were composed of three major fractions: aggregates, intermediate fractions and non-aggregated molecules. Furthermore, the relative proportion of the aggregate fraction increased as protein concentration increased. Similarly, LLS indicated that the average hydrodynamic radius (Rh) increased at higher protein concentration. In sample with an ionic strength of zero, the intermediate fraction decreased after freeze-drying with a concomitant increase of the aggregate fraction. When the sample was heated at elevated ionic strength, the SEC-HPLC and LLS profiles changed substantially, the intermediate fractions decreased, and lyophilization had effect on the fraction of aggregates. These experiments suggest novel strategies for producing soy protein aggregates with controlled properties.