Comparative behaviour of goat β and α s1-caseins at the air–water interface and in solution

Here we present a comparative study of caprine β- and α s1-caseins behaviours at the air–water interface and in solution. Both caseins were purified from the milk of a single goat homozygous at the α s1- and β-Cn loci, with a high degree of purity (98%). Physical measurements (ellipsometry, surface pressure and surface rheology) were performed at the air–water interface, whereas SAXS measurements were performed on casein solutions. Our results clearly show that self-organizations, both at the air–water interface and in solution are different for β- and α s1-caseins. β-casein is unfolded in solution and forms a network at the interface, while α s1-casein forms compact objects in solution and is organised in fluid domains at the interface. We also show that the presence of Ca 2+ in the subphase strongly disturbs the interfacial layer formed by the caseins. It is elsewhere worth noting that in solution, the aggregation of α s1-casein induced by calcium ions is associated with a pronounced change in the molecular structural organisation of the protein, which seems to adopt, in these conditions, an unfolded structure.