Phase behaviour, rheological properties, and microstructure of oat β-glucan-milk mixtures

Incorporation of high molecular weight oat β-glucan into milk to obtain calorie-reduced and cholesterol-lowering dairy products is challenged by the thermodynamic incompatibility and phase separation of milk proteins and β-glucan. Fine tuning the concentrations of the biopolymers in the mixture may result in dairy matrices with unique structure and texture. The thermodynamic incompatibility behaviour may follow Asakura and Oosawa (1958) theory for depletion interactions between non-interacting polysaccharides and colloidal hard spheres. A phase behaviour diagram was developed to describe the separation of the mixtures into a protein-rich (lower) and a β-glucan-rich (upper) phase, but unlike previous literature, the concentration of lactose and ions in the soluble phase was kept constant. Analysis of the protein composition in the lower phase demonstrated that whey proteins did not play a role in phase separation. The experimental phase boundary agreed well with the phase boundary calculated using Vrijʹs theory (1976) for mixtures of β-glucan and casein micelles demonstrating that phase separation was driven by depletion interactions. Confocal images showed the formation of different structures (droplet-like or bi-continuous) at different points of the phase behaviour diagram. The flow behaviour of the mixtures with concentrations higher than the binodal curve was not only governed by the presence of β-glucan chains, but also by the formation of these structures.