Thermodynamic interactions of micellar casein and oat β-glucan in a model food system

Abstract

Oat β-glucan is a soluble dietary fiber and its consumption is associated with the reduction of coronary heart disease risk (21 CFR 101.81). Dairy and food formulations have been used to deliver β-glucan's health benefits to consumers, but the addition of β-glucan in dairy beverages can induce phase separation because of its thermodynamic incompatibility with micellar casein. Ten binary mixtures were developed with concentrations at 0.5–2% β-glucan and 0.75–6% casein to investigate the micellar casein–β-glucan interaction mechanism and evaluate the effect of biopolymer concentrations, thermal treatment, and ionic strength on these interactions. Pasteurization (73 °C/15 s) and retort sterilization (121 °C/5 min) were the two applied thermal treatments. Three dispersion media were used in the preparation of the binary mixtures: 5 mM potassium phosphate buffer, 171 mM NaCl in phosphate buffer, and milk permeate. Upon preparation, the samples were stored for 5 days at 4 °C and then centrifuged at 4000 rpm for 2 h. The phase diagrams showed that both thermal treatments increased the biopolymer compatibility and suspension stability. The thermally treated formulations exhibited smaller particle sizes, which explains the higher stability. Increasing ionic strength decreased the biopolymer compatibility and suspension stability and increased the particle sizes. Isothermal titration calorimetry (ITC) showed that the heat of binding was small indicating entropic biopolymer interactions. Confocal microscopy images illustrated the immiscibility of the two biopolymers showing the formation of two phases.