Elasticity protein-stabilized foams

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 gas bubbles in food foams are separated by sheets of the continuous phase, composed of two films of proteins adsorbed on the interface between a pair of gas bubbles, with a thin layer of liquid in between. The volume of the gas bubbles may make up 99% of the total foam volume. The contents of protein in foamed products are 0.1-10% and of the order of 1 mg/m2 interface. The system is stabilized by lowering the gas-liquid interfacial tension and formation of rupture-resistant, elastic protein film surrounding the bubbles, as well as by the viscosity of the liquid phase. The foams, if not fixed by heat setting of the protein network, may be destabilized by drainage of the liquid from the intersheet space, due to gravity, pressure, or evaporation, by diffusion of the gas from the smaller to the larger bubbles, or by coalescence of the bubbles resulting from rupture of the protein films. 

In a gas and liquid system, when gas is introduced into a culture medium, bubbles are formed. The bubbles rise rapidly through the medium and dispersion of the bubbles occurs at surface, forming froth. The froth collapses by coalescence, but in most cases the fermentation broth is viscous so this coalescence may be reduced to form stable froth. Any compounds in the broth, such as proteins, that reduce the surface tension may influence foam formation. The stability of preventing bubbles coalescing depends on the film elasticity, which is increased by the presence of peptides, proteins and soaps. On the other hand, the presence of alcohols and fatty acids will make the foam unstable. 

When there is a high percentage of proteins, such as in gelatin—which will form an elastic film in conjunction with corn sirup and other sugar products—stability is not too much of a problem. In freshly whipped egg or vegetable protein foams, frappes, or mazettas, in which the foam product is incorporated as part of a complete food batch, the foam is assimilated before its stability becomes a factor and further processing tends to stabilize the foam. 

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

Because of its elastic texture and tendency to harden, marshmallow was assumed to exist in a rubber state. Molecules in the rubbery state of marshmallow can be mobile and lead to sugar crystallization, loss of moisture from the matrix, cross-linking of proteins in gelatin, and collapse of foam. In order to stabilize the product, it is necessary to identify the factors limiting the shelf life of the marshmallow under a specific storage condition. 

A foam is a dispersion of gas bubbles in a relatively small volume of a liquid or solid continuous phase. Liquid foams consist of gas bubbles separated by thin liquid films. It is not possible to make a foam from pure water the bubbles disappear as soon as they are created. However, if surface active molecules, such as soap, emulsifiers or certain proteins, are present they adsorb to the gas-liquid interfaces and stabilize the bubbles. Solid foams, e.g. bread, sponge cake or lava, have solid walls between the gas bubbles. Liquid foams have unusual macroscopic properties that arise from the physical chemistry of bubble interfaces and the structure formed by the packing of the gas bubbles. For small, gentle deformations they behave like an elastic solid and, when deformed more, they can flow like a liquid. When the pressure or temperature is changed, their volume changes approximately according to the ideal gas law (PF/r= constant). Thus, foams exhibit features of all three fundamental states of matter. In ice cream, the gas phase volume is relatively low for a foam (about 50%), so the bubbles do not come into contact, and therefore are spherical. Some foams, for example bubble bath. 

The above data indicate positive relationships between protein conformation, net hydrophobicity, surface pressure, surface yield stress, film elasticity, and foam stability. 

With respect to the rheological parameters fliey come to the conclusion that surface elasticity effects are superior to surface viscosity effects. This, however, apphes to pure surfactant layers and may be different for pure protein or mixed surfactant/protein adsorption layers. It has been stressed also by Langevin (26), in her review on foams and emulsions, fliat studies on the dynamics of adsorption and dilational rheology studies for mixed systems, in particular surfactant-polymer systems, are desirable in order to understand these most common stabilizing systems. 

The interfacial rheology of protein adsorption layers has been intensively studied in relation to the properties of foams and emulsions stabilized by proteins and their mixtures with lipids or surfactants. Detailed information on the investigated systems, experimental techniques, and theoretical models can be found in Refs. [762-769]. The shear rheology of the adsorption layers of many proteins follows the viscoelastic thixotropic model [770-772], in which the surface shear elasticity and viscosity depend on the surface shear rate. The surface rheology of saponin adsorption layers has been investigated in Ref. [773]. 

As already mentioned, the surfactants are used to stabilize the liquid films in foams, in emulsions, on solid surfaces, and so forth. We will first consider the equilibrium and kinetic properties of surfactant adsorption monolayers. Various two-dimensional equations of state are discussed. The kinetics of surfactant adsorption is described in the cases of dijfusion and barrier control. Special attention is paid to the process of adsorption from ionic surfactant solutions. Theoretical models of the adsorption from micellar surfactant solutions are also presented. The rheological properties of the surfactant adsorption mono-layers, such as dilatational and shear surface viscosity and suiface elasticity, are introduced. The specificity of the proteins as high-molecular-weight surfactants is also discussed. 

In addition to the thermodynamic characteristics of the adsorption equilibrium the dynamic dilational visco-elasticity of the surfactant interfacial layers is a very important quantity [5]. This Ireqnency dependent property of a liquid interface is a significant quantity in the stabilization of foams and emulsions. Of course, in many practical situations mixtures of surfactants with particles, polymers or proteins are used, however, these rather complex systems are not the subject of this work. 

In order to look for the origin of this large difference, it is interesting to evaluate some surface properties of these proteins. Table 10.1 shows the final surface tension, the surface elasticity ( in Equation 10.3), the surface viscosity ( "7co in Equation 10.3), and the final foam film thickness of both proteins under similar conditions to the foam stability experiments. Let us evaluate in detail each of them and their implications for foam stability. 

Several experiments indicate that the life time of a foam film is correlated with the surface elasticity [738, 786, 800]. One explanation is that high surface elasticities dampen fluctuations in the film [786, 791]. Fluctuations are one possible reason for film rupture. For the same reason, surface viscosity influences the stability of Aims [792, 800]. In particular for large surface-active molecules such as proteins, this has been analyzed for emulsion films due to the importance in food science [721, 793]. The rupture of thin films has been extensively studied for liquid films on solid surfaces. Therefore, we describe it in more detail in Section 7.6.3. 

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