Neutron reflectivity of adsorbed β-casein and β-lactoglobulin at the air/water interface

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Abstract
Adsorption of the pure milk proteins, β-casein and β-lactoglobulin, at the air/water interface has been studied using the technique of specular neutron reflectance. By fitting appropriate models of the protein films to the reflectivity data the detailed structures of the interfaces have been determined. The effects of protein concentration, substrate pH, film ageing and the presence of calcium ions on the film structures have been investigated. At neutral pH both β-casein and β-lactoglobulin form a monolayer at the air/water boundary that can be divided into a protein-rich, hydrophobic, inner layer closest to the interface, and a more diffuse, hydrophilic, outer layer extending into the bulk aqueous phase. The adsorbed amount of protein follows the adsorption isotherms determined by other methods. Ageing has little effect on β-casein layers, but with β-lactoglobulin there is an increases in the adsorbed amount of protein with time and the two-layer model of the film configuration tends to become less distinct such that a one-layer model form is equally appropriate. The rate and extent of 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 proteins increases as the pH is lowered towards their respective isoelectric points. Models are proposed to account for the changes in layer structure occurring during these processes. The presence of calcium ions modifies the structure of β-casein layers, reducing the hydrophilic layer thickness and the adsorbed amount of protein.