Orogenic Displacement of Protein from the Oil/Water Interface

Abstract

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.

Probing nanostructures in food with the AFM

“…The mechanism involves folding and buckling of the interfacial layer and has thus been termed an 'orogenic displacement mechanism'. The mechanism has been shown to be generic for all proteins and surfactants studied to date Gunning et al, 1999;Mackie et al, 2000a;200b;2001a;. Displacement is orogenic for water-soluble, oil-soluble, neutral and charged surfactants.…”

“…Sample preparation: In order to image the interfacial structures found in food foams and emulsions by AFM it is necessary to model these structures on a Langmuir trough and sample the interfacial structure using Langmuir-Blodgett (LB) methods Mackie et al, 1999). Samples for AFM are collected on mica substrates and imaged under 1-butanol, in order to control the applied force: for the samples described the AFM images were obtained in the dc contact mode using minimal applied force .…”

“…Mixed interfaces can be generated by spreading or co-adsorbing the molecules. Both air-water and oil-water interfaces can be investigated using this approach 2000a). Sampling the interfacial structures for spread films is comparatively simple.…”

Effects of glycerol monostearate and Tween 80 on the physical properties and stability of recombined low-fat dairy cream

“…The "orogenic" mechanism functions as surfactant molecules nucleate at defects in the protein interface during the first stage of displacement (Mackie et al 1999(Mackie et al , 2000. Surfactants adsorbed into the surface protein demonstrated the effects of shielded ζ-potential.…”

Improving Product Quality with Entrapped Stable Emulsions: From Theory to Industrial Application

“…Low molecular weight surfactants are much smaller in size than biopolymers; thus they can reduce interfacial tension more efficiently and quickly by adsorbing a large number of molecules within the same surface area. If both are present at high enough bulk concentrations they are likely to dominate at the interface after equilibration [69][70][71][72].…”