NEUTRON REFLECTIVITY OF ADSORBED BETA-CASEIN AND BETA-LACTOGLOBULIN AT THE AIR WATER INTERFACE/

Adsorption of the pure milk proteins, beta-casein and beta-lactoglobul in, at the air/water interface has been studied using the technique of specular neutron reflectance. By fitting appropriate models of the pr otein films to the reflectivity data the detailed structures of the in terfaces have been determined. The effects of protein concentration, s ubstrate pH, film ageing and the presence of calcium ions on the film structures have been investigated. At neutral pH both beta-casein and beta-lactoglobulin form a monolayer at the air/water boundary that can be divided into a protein-rich, hydrophobic, inner layer closest to t he interface, and a more diffuse, hydrophilic, outer layer extending i nto the bulk aqueous phase. The adsorbed amount of protein follows the adsorption isotherms determined by other methods. Ageing has little e ffect on beta-casein layers, but with beta-lactoglobulin there is an i ncrease in the adsorbed amount of protein with time and the two-layer model of the film configuration tends to become less distinct such tha t a one-layer model form is equally appropriate. The rate and extent o f the changes occurring as a function of the age of the surface depend on the pH of the substrate solution. The adsorbed amount of both prot eins increases as the pH is lowered towards their respective isoelectr ic points. Models are proposed to account for the changes in layer str ucture occurring during these processes. The presence of calcium ions modifies the structure of beta-casein layers, reducing the hydrophilic layer thickness and the adsorbed amount of protein.