Interactions between fat crystal networks and sodium caseinate at the sunflower oil-water interface

A Couette-type torsion wire surface shear viscometer was used to measure th e apparent interfacial sheer viscosity of pH 7 (I = 0.05 M) buffered soluti ons of sodium caseinate in contact with sunflower oil. The sunflower oil co ntained 1% fat crystals in either the beta or beta' polymorphic form, or wa s crystal free. In all cases, the fat crystals increased the interfacial sh ear viscosity synergistically, with the beta' crystals causing the biggest increase. Substituting the protein for a small-molecule surfactant (Tween-4 0) showed that this was not simply due to the protein lowering the interfac ial tension. Sedimentation studies of the different fat crystal slurries su ggested that the extent of the interfacial shear viscosity increase was rel ated to the strength of crystal-crystal interactions in the oil phase. It s eems likely that when protein is present at the interface, it fixes the ads orbed layer of fat crystals to the cross-linked protein film at the interfa ce. When this film was sheared, the strength of the crystal-crystal interac tions in the oil phase became important. However, when Tween-40 was in the aqueous phase instead of the protein, the crystal-crystal interactions were not relevant, presumably because the Tween-40 interfacial film simply flow ed around the adsorbed crystals.