Phase Behaviour Of Oat β-Glucan/Sodium Caseinate Mixtures

Oat β-glucan is a water soluble polysaccharide which has been approved as a functional bioactive ingredient. In this thesis, β-glucan was successfully isolated from oat flour and samples of different molecular weights were produced. The structural features and molecular weights(Mw) were characterized by 13C–NMR spectroscopy and high performance size-exclusion chromatography, respectively. The rheological properties and microstructure of aqueous oat β-glucan solutions were investigated by rheometry and atomic force microscopy (AFM),respectively. The samples with β-glucan content between 78-86 % on a dry weight basis had Mw, intrinsic viscosity ([η]) and critical coil overlap concentration (c*) in the range of 142 - 2800 x 103 g/mol, 1.7 - 7.2 dL/g and 0.25 - 1.10 g/dL, respectively. The flow and viscoelasticvbehaviour was highly dependent on Mw and on the concentration of the β-glucan solutions. AFM images revealed the formation of cluster or aggregates linked via individual polymer chains scattered heterogeneously throughout the system. The aggregate size was also dependent on molecular weight of the samples and influences the rheological behaviour of β-glucan solutions. The isothermal phase behaviour at 5 oC of β-glucan/sodium caseinate mixtures were
investigated by means of phase diagram construction, rheometry, electrophoresis and fluorescence microscopy. Phase diagrams indicated that the compatibility of the β-glucan/sodium caseinate system increases as β-glucan Mw decreases. Images of the mixtures taken at various
biopolymer concentrations revealed phase separation with the presence of β-glucan aggregates,whose size depends on Mw and concentration. At the same protein concentration in the mixtures, the viscosity increases with increasing Mw and concentration of β-glucan. However, the results also revealed that in the state of thermodynamic equilibrium with comparable polymer concentrations in mixture, the lower Mw samples yielded similar or higher viscosity. At equivalent hydrodynamic volume of β-glucan component in the mixture, all the samples
exhibited similar viscosity/flow behaviour. A deviation dependent on the protein concentration
was observed for the high Mw sample in the concentrated regime due to the size of the β-glucan aggregates formed. Results demonstrate that by controlling the structural features of β-glucan in mixture with sodium caseinate, informed manipulation of rheological properties in these systems can be achieved.