Sodium caseinate/xanthan gum interactions in aqueous solution: Effect on protein adsorption at the oil–water interface

In this work, the phase behaviour of 1.0 wt% sodium caseinate (CN) in admixture with different amounts of xanthan gum (XG) in aqueous solution was investigated under different pH and ionic strength. XG concentration, pH and ionic strength showed a significant impact on biopolymer interactions. Considering the strong effects of pH on the net charge around CN, pH dependence of CN/XG interactions was investigated by zeta potential, viscometry techniques, and confocal laser scanning microscopy (CLSM). The results indicated that CN/XG interactions may take place in solution, both at neutral and acidic environments. Then, a close relationship was obtained between CN/XG interactions in the aqueous phase and CN adsorption at the oil–water interface. It showed that the complex formed at acidic pH revealed a significant influence on the dynamic characteristics of CN adsorbed films. At short adsorption time, the diffusion rate (kdiff) of CN to the interface was increased due to the interactions with XG in the aqueous phase which could accelerate protein availability for the adsorption. Moreover, XG could also lead to an increase in the rates of penetration (kP) and rearrangement (kR) of CN at the interface at long adsorption time, which was quite different from the adsorption behaviour of CN/XG mixtures prepared at neutral pH.

State diagram of the CN/XG system in aqueous solution, expressed in terms of the pH and NaCl concentration (1 wt% CN plus 0.375 wt% XG). Symbols: (○) clear solution, (□) cloudy/milky solution, (▴) solid precipitate and clear solution.Figure optionsDownload as PowerPoint slideHighlights
► Phase behaviour of sodium caseinate (CN) and xanthan gum (XG) mixtures was studied.
► Soluble CN/XG complex was formed above moderately high ionic strength at acidic pH.
► CN/XG complex improves the adsorption of CN at the oil–water interface at acidic pH.
► CN/XG complex shows excellent surface activity at acidic pH.