Dynamic surface pressure and dilatational viscoelasticity of sodium caseinate/xanthan gum mixtures at the oil-water interface

The effect of xanthan gum (XG) on the surface dynamic properties (dynamic surface pressure and surface dilatational properties) of the sodium caseinate (CN) adsorbed film at the oil-water interface has been studied in this paper. The measurements were performed as a function of bulk protein concentration in the range of 0.001-1.0 wt% using an automatic pendant drop tensiometer. The XG concentration (0.05 wt%), temperature (25 degrees C), pH (7.0) were maintained constants. The results revealed a significant effect of XG on dynamic characteristics of CN adsorbed films. At short adsorption time, the diffusion (k(diff)) of CN to the interface decreased due to the interactions with XG in the aqueous phase which could limit protein availability for the adsorption. The presence of XG led to a decrease in the rates of penetration (k(P)) and rearrangement (k(R)) of CN at the interface at long adsorption times. High concentrations of CN in the bulk solutions showed significant impact on the dilatational viscosity close to the interface, due either to diffusional relaxation or to rearrangement of the adsorbed primary layer and multilayers of protein molecules. Moreover, CN/XG mixtures presented higher dilatational elasticity at the bulk concentration of CN beyond 0.001 wt%. (C) 2010 Elsevier Ltd. All rights reserved.