Competitive adsorption and thermodynamic incompatibility of mixing of β-casein and gum arabic at the air–water interface

Competitive adsorption of β-casein and gum arabic glycoprotein (GAGP) at the air–water interface has been investigated using a surface radiotracer method. Adsorption isotherms revealed that the binding affinity of GAGP to the air–water interface was about an order of magnitude lower than that of proteins. However, the saturated monolayer coverage for GAGP was about 250 mg m−2 compared to 1–2 mg m−2 for proteins. In competitive adsorption experiments involving simultaneous adsorption of β-casein and GAGP from a bulk mixture, β-casein dynamically displaced GAGP from the interface. At <8% β-casein to GAGP bulk concentration ratio, β-casein effectively inhibited adsorption of GAGP to the air–water interface. However, the ability of β-casein to displace GAGP was greatly reduced when the latter was present as a pre-formed film at the air–water interface. This was due to thermodynamic incompatibility of mixing of β-casein in the GAGP film. The incompatibility parameter, X12, for mixing of β-casein and GAGP at the air–water interface is 0.84; this value was much greater than that for most protein mixtures. The unfavorable free energy of mixing between GAGP and β-casein at the air–water interface was about 1.3 Kcal mol−1 at 25 °C.