Collapsed protein layer

Presumably, when fat was present it was drawn into the layer between the bubbles and disrupted the protein layers, causing the foam to collapse. 

The elasticity of the protein layer structure is supposed to act against the tendency of an emulsion or foam to collapse because it allows the stretching of the interface. This behaviour is most commonly observed for globular proteins, which adsorb, partially unfold, and then develop attractive protein-protein interactions (Dickinson, 1999a Wilde, 2000 Wilde et al., 2004). The strength of such an adsorbed layer, reflected in the value of the elastic modulus, and the stress at which the structure breaks down, can be successfully correlated with stability of protein-based emulsions and (more especially) protein-based foams (Hailing, 1981 Mitchell, 1986 Izmailova et al., 1999 Dickinson, 1999a). 

The possible development of gradients in the components of the interfacial stress tensor due to flow of an adjacent fluid implies that the momentum flux caused by the the flow of liquid at one side of the interface does not have to be completely transported across the interface to the second fluid but may (partly or completely) be compensated in the interface. The extent to which this is possible depends on the rheological properties of the interface. For small shear stresses the interface may behave elastically or viscoelastically. For an elastic interfacial layer the structure remains coherent the layer will only deform, while for a viscoelastic one it may or may not start to flow. The latter case has been observed for elastic networks (e.g. for proteins) that remciln intact, but inside the meshes of which liquid can flow leading to energy dissipation. At large stresses the structure may yield or fracture (collapse), leading to an increased flow. 

The adsorption of proteins at interfaces is a key step in the stabilization of numerous food and non-food foams and emulsions. Our goal is to improve our understanding of the relationships between the sequence of proteins and their surface properties. A theoretical approach has been developed to model the structure and properties of protein adsorption layers using the analogy between proteins and multiblock copolymers. This model seems to be particularly well suited to /5-casein. However, the exponent relating surface pressure to surface concentration is indicative of a polymer structure intermediate between that of a two-dimensional excluded volume chain and a partially collapsed chain. For the protein structure, this would correspond to attractive interactions between some amino acids (hydrogen bonds, for instance). To test this possibility, guanidine hydrochloride was added to the buffer. A transition in the structure and properties of the layer is noticed for a 1.5 molar concentration of the denaturant. Beyond the transition, the properties of the layer are those of a two-dimensional excluded volume chain, a situation expected when there are no attractive interac-... 

Fig. 7.17. Structure of the Vl and Cl domains of IgG. Layers of antiparallel 3-sheets are stacked in these domains, which have been referred to as collapsed -barrels. The antigen binds between the Vh and Vl immunoglobulin folds, and NOT in the barrel. The Cl domain is also called the immunoglobulin fold. (Top modified from Richardson JS. Adv Protein Chem. The anatomy and taxonomy of protein structure 1981 34 167 bottom reprinted in part with permission from Edmundson AB, et al. Biochemistry 1975 14 3954. 1975 American Chemical Society.)...

As an example of a membrane model, phospholipid monolayers with negative charge of different density were used. It had already been found ( ) and discussed O) that the physical and biological behavior of phospholipid monolayers at air-water interfaces and of suspensions of liposomes are comparable if the monolayer is in a condensed state. Two complementary methods of surface measurements (using radioactivity and electrochemical measurements), were used to investigate the adsorption and the dynamic properties of the adsorbed prothrombin on the phospholipid monolayers. Two different interfaces, air-water and mercury-water, were examined. In this review, the behavior of prothrombin at these interfaces, in the presence of phospholipid monolayers, is presented as compared with its behavior in the absence of phospholipids. An excess of lipid of different compositions of phos-phatidylserine (PS) and phosphatidylcholine (PC) was spread over an aqueous phase so as to form a condensed monolayer, then the proteins were inject underneath the monolayer in the presence or in the absence of Ca. The adsorption occurs in situ and under static conditions. The excess of lipid ensured a fully compressed monolayer in equilibrium with the collapsed excess lipid layers. The contribution of this excess of lipid to protein adsorption was negligible and there was no effect at all on the electrode measurements. 

Cream is obtained by collecting the fatty fraction of the milk in a fat-rich phase leaving the skimmed milk behind. Like milk, cream is still a fat-in-water emulsion. When the cream is whipped, air bubbles are incorporated into the cream. The emulsion is transformed into a foam. The bubbles are stabilized by a layer of denatured milk proteins and surrounded by fat globules. Further whipping results in collapse of the foam structure and destruction of the fat globules. Fat becomes the continuous phase in which water droplets are dispersed. The emulsion, as it was in milk and cream, has inverted butter is formed. 

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