Aggregation of some food proteins in aqueous dispersions: effects of concentration, pH and ionic strength

Commercial casein, whey protein and laboratory prepared soybean protein of various pH were dispersed in distilled water at the initial concentration of 4.0% (w/v) and then diluted with NaCl solutions of known ionic strength. Particle diameter was determined by photon correlation spectroscopy and the data were converted into degree of aggregation using voluminosity and weight concentration data. Results showed that aggregation of protein particles increased in a log relationship with decreases of ionic strength. Aggregation of casein and soybean protein increased with increases of pH up to a maximum. Disaggregation of the protein particles as the result of the decrease in the protein concentration at fixed pH and ionic strength takes place by thermal agitation and Brownian movement. It is concluded that above a transition concentration of 0.04–0.07 ml/ml, depending on the nature of the protein, growth of the particles takes place by means of local surface electrostatic charges attraction.