Self-association of bovine β-casein as influenced by calcium chloride, buffer type and temperature

Abstract

The aim of this study was to investigate the aggregation behaviour of a pure β-casein (β-CNpure) and a β-casein concentrate (β-CNconc) as a function of temperature, buffer type (pH 6.8) and the presence of CaCl2. The particle size distribution and turbidity of β-casein (β-CN) dispersions were measured by dynamic light-scattering (DLS) and UV/vis spectroscopy between 4 and 55 °C. Upon heating (4–55 °C), the particle size of both β-CN samples increased, indicating self-association via hydrophobic interactions. It was shown that the self-association of β-CN increased with increasing β-CN concentration and that β-CNpure self-associated at significantly lower concentration than β-CNconc. Both turbidity and particle size measurements showed that the β-CN samples had similar aggregation behaviour in water and imidazole buffer (pH 6.8) but differed in sodium phosphate buffer (pH 6.8), especially at higher ionic calcium concentrations. Fourier Transform Infrared (FTIR) spectroscopy revealed very little change in the secondary structure of β-CN during heating (4–55 °C). The microstructure of β-CN aggregates was monitored during heating from 10 to 55 °C, followed by cooling to 10 °C, using polarised light microscopy. Spherical and heterogeneous aggregates were observed when heated at temperatures above 37 °C, which were reversible upon cooling. This study confirmed that β-CN undergoes self-association on heating that reverses upon cooling, with the aggregation process being highly dependent on the purity of β-CN, the solvent type and the presence of ionic calcium.