Hydrodynamic investigation of polysaccharides and their interactions with casein
Polysaccharide systems (pectin, carrageenan, guar, locust bean gum, xanthan and xylan) have been characterised using a variety of hydrodynamic techniques including sedimentation velocity, sedimentation equilibrium, size exclusion chromatography - multi-angle laser light scattering (SEC-MALLS), and viscometry. Results suggest that the polysaccharides selected are, in general, rigid or semi-rigid molecules with a large hydrated volume, this is important in relation to polysaccharide structure - function relationships. In addition the effect of incorporating a UV absorbing substituent group was also investigated for two pectin samples and an arabinoxylan polysaccharide, the reaction conditions result in beta-elimination of the pectin chain in one case, but there was no significant effect on the other pectin or arabinoxylan. The effect of increased temperature was also investigated with respect to high and low methoxy pectins, this resulted in beta-elimination of high methoxy pectin and a mild conformational change in low methoxy pectin. Milk protein systems have also been studied using the above techniques, and it was proven that casein micelles are large, spherical hydrated molecules and sodium caseinate undergoes complex concentration dependant self-association under the conditions studied, which is significant even at low concentrations. Beta- and x-casein also undergo self-association reactions, the former of which was concentration dependent. An investigation into the effect of high temperature on beta lactoglobulin suggested that temperature induced aggregation is a two-step process (denaturation and aggregation), the first of which is thermo-reversible denaturation as indicated by capillary viscometry. The characterisation of the polysaccharide and milk protein substrates then allowed an investigation into the interactions of casein micelles with polysaccharide molecules. It was estimated that only K- and i-carrageenan interacted significantly with casein micelles in the reaction conditions studied (pH ~ 6.8, I ~ O.1M). A more detailed investigation into the k-carrageenan interaction suggests that at low concentration a sterically stabilised complex is formed, which undergoes depletion flocculation upon increasing k-carrageenan concentration.