The secondary structure of protein adsorbed at the emulsion interface has been studied in refractive index matched emulsions using the techniques of circular dichroism (CD) and Fourier transform infrared spectroscopy. Bovine serum albumin (BSA) and bovine beta-lactoglobulin (betalg) stabilized emulsions were studied, and the refractive index was altered by the addition of glycerol or polyethylene glycol. The effect of additive on the solution and adsorbed protein structure in addition to the effect of adsorption time was considered. Both adsorption and glycerol addition alter protein secondary structure; however, the majority of secondary structure remains. Small changes are observed in the secondary structure of adsorbed protein with time. Near-ultraviolet CD studies showed the effect of glycerol and adsorption on the aromatic groups. BSA showed small changes both upon the addition of glycerol to protein in solution and upon adsorption. betalg showed slightly larger changes upon the addition of glycerol to protein in solution and a larger change upon adsorption.

Lipase is activated by binding to an insoluble emulsified or aggregated substrate. The extent of binding is related to the physicochemical as well as the compositional structure of the interface, the quality of the interface. 'Quality' is as yet undefined but thought to contain contributions from electrostatic interactions, orientation of substrate, and hydration forces. To investigate the electrostatic and compositional factors we have used olive oil-in-water emulsions prepared with phosphatidylcholine and four bile salts of varying hydrophobicities. By measurement of the droplet zeta potential we have monitored semi-quantitatively the incorporation of bile salts within the interface. No correlation was found between droplet surface charge as monitored by the zeta potential and lag phase. The duration of the observed lag phase was found to be inversely related to the degree of incorporation of the bile salts. Simultaneously there was evidence of lipase binding to monomeric bile salts, reducing its availability for adsorption. Calcium ions reduced the surface charge but there was no correlation with lag phase duration. The evidence presented here agrees with a more specific role for calcium ions, i.e., the formation of a new catalytically active enzyme complex, (enzyme)-(mixed micelle)-(calcium ion).

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