Dynamics of Exchange between αS1-Casein and β-Casein during Adsorption at Air−Water Interface

Competitive adsorption of α S1 -casein and β-casein at the air−water interface has been investigated. In single-component systems, the adsorptivities of these caseins to the air−water interface were very similar and the binding affinity of β-casein was only slightly greater than that of α S1 -casein. However, in a 1:1 binary mixture of α S1 - and β-caseins, the ratio of surface load of these caseins was 2:1 in favor of β-casein adsorption. Kinetic studies showed that α S1 -casein arrived first at the interface, but the late arriving β-casein was able to displace the adsorbed α S1 -casein from the interface and establish a new equilibrium. Exchange and displacement experiments showed that bulk phase β-casein was able to readily displace adsorbed α S1 - and β-casein molecules; similarly, α S1 -casein was also able to displace α S1 - and β-caseins from the interface, although to a lesser extent. The mechanism of the dynamic exchange between α S1 - and β-caseins is discussed.