Dynamics of exchange between alpha(S1)casein and beta-casein during adsorption at air-water interface

Competitive adsorption of alpha(S1)-casein and beta-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 beta-casein was only slightly greater than that of alpha(S1)-casein. However, in a 1:1 binary mixture of alpha(S1)- and beta-caseins, the ratio of surface load of these caseins was 2:1 in favor of beta-casein adsorption. Kinetic studies showed that alpha(S1)-casein arrived first at the interface, but the late arriving beta-casein was able to displace the adsorbed alpha(S1)-casein from the interface and establish a new equilibrium. Exchange and displacement experiments showed that bulk phase beta-casein was able to readily displace adsorbed alpha(S1)- and beta-casein molecules; similarly, alpha(S1)-casein was also able to displace alpha(S1)- and beta-caseins from the interface, although to a lesser extent. The mechanism of the dynamic exchange between alpha(S1)- and beta-caseins is discussed.