Interaction forces between κ-casein adsorbed on mica

Milk is a complex colloidal suspension of proteins, inorganic materials and lipids. Of the proteins, caseins are present in the highest concentrations, and are themselves organised into a complex structure termed the casein micelle. The remarkable stability of the milk towards pasteurisation, sterilisation and dehydration is directly related to the stability of the casein micelle, which is in turn related to its surface components, notably κ-casein.

In this study, a surface force apparatus has been used to measure interactions between κ-casein surfaces, with a view to determining the forces involved in the stabilisation of the casein micelles. The observed interaction on a first compression is attractive, commencing at a protein surface separation of about 40 nm. This attraction is weak, around 150 μN m⁻¹, and may be hydrophobic in origin. The thickness of the κ-casein layer is 9.8 nm. On separation of the surfaces, no attraction is noted, only a short-range steric interaction being seen, indicating that some configurational change of the protein is occurring.