X-ray reflectivity studies of liquid films stabilized by mixed β-lactoglobulin—Acacia gum systems

Abstract

Acacia gum is a complex mixture of three main components, the arabinogalactan (AG), the arabinogalactan-protein complex (AGP) and the glycoprotein (GP). In the present study, the influence of complex formation between the protein β-lactoglobulin and the Acacia gum and its two major components, the AG and AGP, on the formation of vertical macroscopic thin liquid films were investigated by means of X-ray reflectivity measurements. This method allows the determination of the thickness of the formed film and its nano-structural features. The experimental results revealed that the main effect of adding Acacia gum to a 1 g/l β-lg solution at a pH of 5.3, i.e., near the protein's pI value, is related to the formation of soluble and insoluble β-lg – arabinogalactan (AG) complexes. Addition of Acacia gum to the protein solution significantly decreased the film thickness of the formed film before it ruptured. Film experiments with fractionated Acacia gum mixtures (an AG-rich and an AG-poor, i.e., AGP-rich, fraction) or with a dialyzed Acacia gum preparation allowed to conclude that the observed influence of the Acacia gum on the thickness of the β-lg stabilized films and film stability is primarily due to the extent of complex formation between the negatively charged AG component and the β-lg. This complex formation reduces the ‘free’ β-lg concentration in the bulk which is stabilizing the two water-air surfaces in the film. The degree of complex formation considerably depends on the ionic strength in the solution. At low ionic strength, the negative charges on the AG are not screened by the salt ions leading to extensive electrostatic complex formation (formation of soluble and/or insoluble complexes) with the β-lg, even at the isoelectric point of the protein. However, if the ionic strength in the β-lg – Acacia gum mixture is in the sub-millimolar range, significant charge screening effects are occurring keeping the arabinogalactan component away from complexation with the β-lg molecules and allowing the formation of stable thin liquid films up to a β-lg – Acacia gum weight ratio of 2:1. Addition of more Acacia gum under these ionic strength conditions resulted again in the formation of unstable thin films.