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beta-Lactoglobulin and whey protein isolate (WPI) were heated in aqueous solutions at pH 2 and 7 at 80 degrees C, spread onto freshly cleaved mica surfaces, and visualized under butanol using atomic force microscopy. Fine-stranded aggregates were formed at pH 2, the diameter of strands being ca. 4 nm for beta-lactoglobulin and 10 nm for WPI. At pH 7, aggregates were composed of ellipsoidal particles, regardless of the concentration of added NaCl. This observation supports the previously proposed two-step aggregation model at neutral pH (Aymard, P.; Gimel, J. C.; Nicolai, T.; Durand, D. J. Chim. Phys. 1996, 93, 987-997), consisting of the formation of primary globular particles and the subsequent aggregation of those primary particles. The AFM provides the first direct evidence for the anisotropic shape of these primary particles. The heights of primary particles increased from ca. 11 to 27 nm with increasing concentrations of added NaCl from 0 to 0.3 M in the case of WPI. The rate of aggregation was also accelerated with increasing NaCl concentrations, which appeared to induce transitions in gel networks from fine-stranded toward particulate networks. The present study provides structural information essential for understanding the diverse physical properties of heat-induced whey protein gels.

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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.