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

Competitive adsorption of beta-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 beta-casein and GAGP from a bulk mixture, beta-casein dynamically displaced GAGP from the interface. At < 8% beta-casein to GAGP bulk concentration ratio, beta-casein effectively inhibited adsorption of GAGP to the air-water interface. However, the ability of beta-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 beta-casein in the GAGP film. The incompatibility parameter, X-12, for mixing of beta-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 beta-casein at the air-water interface was about 1.3 Kcal mol(-1) at 25 degreesC. (C) 2002 Elsevier Science Ltd. All rights reserved.