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Orogenic displacement has been shown to be a mechanism by which protein can be removed from an interface by small surfactant molecules. This paper describes the progressive displacement of two different proteins from an oil/water interface by a nonionic surfactant. The process has been visualized by atomic force microscopy (AFM). Measurement of surface tension and AFM imaging of Langmuir-Blodgett (LB) films formed on mica are used to demonstrate the mechanism of protein desorption from the interface. This paper extends previous work which demonstrated a new orogenic mechanism of protein displacement from an air/water interface. The two proteins used in the present study were -casein, a largely random coil protein, and -lactoglobulin, a globular protein. The proteins were displaced from both spread and coadsorbed films using the water-soluble nonionic surfactant Tween 20. The AFM images also provide direct evidence for the formation of a heterogeneous protein layer at the interface. The heterogeneity of the protein film is important in allowing the initial adsorption of the surfactant onto the interface. These nucleated surfactant sites then expand, compressing the protein network, which initially increases in density without increasing in thickness. Once a certain critical density is reached, further compression of the protein layer results in the thickness increasing in order that protein film volume is maintained constant as the surfactant domains expand. At sufficiently high surface pressures, the network fails, releasing proteins which then desorb from the interface.

The competitive displacement of a model protein (beta-lactoglobulin) by bile salts from air-water and oil-water interfaces is investigated in vitro under model duodenal digestion conditions. The aim is to understand this process so that interfaces can be designed to control lipid digestion thus improving the nutritional impact of foods. Duodenal digestion has been simulated using a simplified biological system and the protein displacement process monitored by interfacial measurements and atomic force microscopy (AFM). First, the properties of beta-lactoglobulin adsorbed layers at the air-water and the olive oil-water interfaces were analyzed by interfacial tension techniques under physiological conditions (pH 7, 0.15 M NaCl, 10 mM CaCl2, 37 degrees C). The protein film had a lower dilatational modulus (hence formed a weaker network) at the olive oil-water interface compared to the air-water interface. Addition of bile salt (BS) severely decreased the dilatational modulus of the adsorbed beta-lactoglobulin film at both the air-water and olive oil-water interfaces. The data suggest that the bile salts penetrate into, weaken, and break up the interfacial beta-lactoglobulin networks. AFM images of the displacement of spread beta-lactoglobulin at the air-water and the olive oil-water interfaces suggest that displacement occurs via an orogenic mechanism and that the bile salts can almost completely displace the intact protein network under duodenal conditions. Although the bile salts are ionic, the ionic strength is sufficiently high to screen the charge allowing surfactant domain nucleation and growth to occur resulting in displacement. The morphology of the protein networks during displacement is different from those found when conventional surfactants were used, suggesting that the molecular structure of the surfactant is important for the displacement process. The studies also suggest that the nature of the oil phase is important in controlling protein unfolding and interaction at the interface. This in turn affects the strength of the protein network and the ability to resist displacement by surfactants.

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