Chapter 13 - Influence of pH on the Molecular Structure and Bubble Stabilising Properties of Bovine α-Lactalbumin

Abstract

The whey protein α-lactalbumin (ALA) is the second most abundant whey protein, and has been associated with the foaming and emulsifying properties of whey proteins at low pH. The aim of this study was to understand the specific molecular changes occurring in ALA to explain the observed changes in functionality at low pH. The effect of pH on the protein structure of ALA and its surface activity was investigated in 0.1 M phosphate buffer in the presence of Ca2+. Spectroscopic measurements (circular dichroism and fluorescence) showed a gradual unfolding of the protein structure as the pH was decreased from 7 to 2. This agreed with high resolution NMR, which was used to monitor the structural changes of ALA. The surface activity of ALA was determined using the pendant drop method. The mechanical properties of the protein-stabilised interfaces were characterised using surface dilatational and surface shear rheological techniques. The results showed that even subtle changes in protein structure could cause a significant increase of the protein surface activity and surface rheology, as the pH decreased from 7 to 4, consistent with improved stabilisation in foams and emulsions. At pH values below the protein's isoelectric point the surface activity decreased, despite continued unfolding of the protein. This suggests that the pH-dependent charge on the protein controlled adsorption and stabilisation processes through the surface packing efficiency which is largely limited by electrostatic repulsion. When the pH was adjusted back to 7, most of the protein structure changes were reversible, only a very small irreversible proportion remained causing an increased surface activity and surface rheology of the protein compared to its original state, even though the irreversible proportion as too small to be observed by NMR.