Dynamics of Phase Separation in Oat β-glucan/Milk Mixtures Studied with Ultrasonic and Diffusing Wave Spectroscopy

Abstract

When high molecular weight β-glucan is added to milk, macroscopic phase separation occurs due to thermodynamic incompatibility between mixed-linkage β-glucan and milk proteins. In this research, the evolution of microphase separated domains was investigated using ultrasonic and diffusing wave spectroscopy. It was possible to follow the dynamics of phase separation, in situ, without dilution. Immediately after mixing, there was a significant decrease in the initial turbidity of the mixtures, measured using diffusing wave spectroscopy, even at concentrations below macroscopic phase separation. At low and intermediate concentrations of β-glucan (0.1–0.2 w/w) the turbidity parameter, 1/l*, changed after mixing, because of rearrangements within the protein rich domains and growth of the phase separated domains. At high concentrations of β-glucan (0.6 % w/w), after an initial change in 1/l*, no further changes in turbidity were observed, because of the high viscosity of the mobile phase and the low diffusivity of the micelles due to crowding within the phase separated domains. Similar conclusions were drawn by observing the changes in ultrasonic attenuation, when probing at larger length scales. By using these two spectroscopic techniques it was possible to obtain complementary information on the dynamics of phase separation in β-glucan milk mixtures, and determine which systems still show rearrangements within the domains after mixing.