Effect of sugars on the phase behaviour, flow and interfacial properties of protein/polysaccharide aqueous two-phase systems

摘要

The paper reviews the phase behaviour, microstructure, flow behaviour and interfacial properties of four sodium caseinate (NaCAS)/galactomannan (GM) model systems with sugar (i.e. glucose, fructose, sucrose and trehalose) dissolved in citrate buffer (pH 5.8) at 20°C. The established phase diagrams showed that sugar type and sugar concentration affect the phase behaviour of all systems. Below 15 wt% sugar, the miscibility of biopolymers improved and the increment of the sugar concentration decreased the miscibility. Glucose, fructose and sucrose improved the compatibility of the biopolymers, whereas trehalose had the opposite effect. Galactomannans with a mannose-to-galactose ratio < 2 (i.e. guar gum (GG) and fenugreek gum (FG)) were more miscible than galactomannans with a mannoseto- galactose ratio > 2 (i.e. locust bean gum (LBG) and tara gum (TG)). The effect was pronounced only in systems with the sugar present. Quiescent microstructure and flow were largely unaffected by sugar type, but changed with its concentration. Sugar increased the overall viscosity and enhanced the non-Newtonian behaviour of the model systems. Phase sense (for blends of < 20 wt% sugar) depended on the volume fraction of each phase and the applied shear rate. Under quiescent conditions, phase-inversion took place for equal phase volumes. However, at higher shear regimes phase-inversion was observed for the initially polysaccharide-continuous blends. In the presence of > 20 wt% sugar, phase sense depended on the volume fraction only; no shear-induced phaseinversion was observed. Similarly, the interfacial properties of the systems were also affected by sugar type and concentration and correlated well with their position in the phase diagram. The closer the systems were to the critical point, the shorter the tile-line length they were positioned on, and the lower their interfacial tension was. The lowest interfacial tension was measured for the systems with 30 wt% sugar (disaccharides especially).