INTERACTIONS BETWEEN TRISTEARIN CRYSTALS AND PROTEINS AT THE OIL-WATER INTERFACE

A Couette-type torsion wire surface shear viscometer was used to measu re the apparent interfacial shear viscosity of pH 7 (I = 0.05 M) buffe red solutions of lysozyme, sodium caseinate, and Tween-40 in contact w ith either n-tetradecane or purified sunflower oil at a planar interfa ce. When proteins were present in the aqueous phase and tristearin cry stals in the oil phase, there was a synergistic increase in the interf acial shear viscosity over the sum of each component in the absence of the other. The magnitude of the increase appeared to be independent o f the type of protein but dependent on the nature of the oil phase. Th is increase in the interfacial shear viscosity was found not to be due simply to the protein reducing the interfacial tension and thus affec ting the adsorption behavior of the fat crystals. When the aqueous pha se contained a small-molecule surfactant (Tween-40) instead of protein , but at the same interfacial tension as the sodium caseinate system, a significantly smaller increase was observed in the interfacial shear viscosity than in the protein system. It therefore seems likely that when proteins are present, hydrophobic peptide residues interact with the tristearin crystals at the interface. (C) 1997 Academic Press.