Protein stabilized elasticity

As reported in this chapter, the microscopic origin of both compressibility and elasticity of dense emulsions is rather well understood. Emulsions have elastic properties arising from either surface tension or surface elasticity and plasticity. Some protein-stabilized emulsions obey the same phenomenology as solid-stabilized emulsions they exhibit substantially higher osmotic resistances and higher shear moduli than surfactant-stabilized emulsions [38 0]. Moreover, they are strongly resistant to water evaporation. Proteins possess the ability to form... 

T. D. Dimitrova and F. Leal Calderon Bulk Elasticity of Concentrated Protein-Stabilized Emulsions. Langmuir 17, 3235 (2001). 

H. D. Hong, L. K. Tamm, Elastic coupling of integral membrane protein stability to lipid bilayer forces, Proc. Natl. Acad. Sci. USA, 2004, 101, 4065 4070. 

A number of refinements and applications are in the literature. Corrections may be made for discreteness of charge [36] or the excluded volume of the hydrated ions [19, 37]. The effects of surface roughness on the electrical double layer have been treated by several groups [38-41] by means of perturbative expansions and numerical analysis. Several geometries have been treated, including two eccentric spheres such as found in encapsulated proteins or drugs [42], and biconcave disks with elastic membranes to model red blood cells [43]. The double-layer repulsion between two spheres has been a topic of much attention due to its importance in colloidal stability. A new numeri-... 

The interest in vesicles as models for cell biomembranes has led to much work on the interactions within and between lipid layers. The primary contributions to vesicle stability and curvature include those familiar to us already, the electrostatic interactions between charged head groups (Chapter V) and the van der Waals interaction between layers (Chapter VI). An additional force due to thermal fluctuations in membranes produces a steric repulsion between membranes known as the Helfrich or undulation interaction. This force has been quantified by Sackmann and co-workers using reflection interference contrast microscopy to monitor vesicles weakly adhering to a solid substrate [78]. Membrane fluctuation forces may influence the interactions between proteins embedded in them [79]. Finally, in balance with these forces, bending elasticity helps determine shape transitions [80], interactions between inclusions [81], aggregation of membrane junctions [82], and unbinding of pinched membranes [83]. Specific interactions between membrane embedded receptors add an additional complication to biomembrane behavior. These have been stud-... 

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. 

The elastic modulus (G ) of MP, BCAS, and BLG5 rapidly rose to plateaus that corresponded to different G saturations (Gjat) (Table 2). MP and BCAS coagula showed the more important Gsat value (142 N/m ), meaning that the emulsions stabilized by skim milk proteins (mainly casein micelles) and 6-casein formed the coagula with the strongest protein network. 

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. 

Figure 7.10 Effect of the thermodynamic incompatibility of otsi/p-casein + high-methoxy pectin (pH = 7.0, / = 0.01 M) on phase diagram of the mixed solutions and elastic modulus of corresponding casein-stabilized emulsions (40 vol% oil, 2 wt% protein), (a) (O) Binodal line for p-casein + pectin solution with critical point ( ) ( ) binodal line for asi-casein + pectin solution with critical point ( ). (b) Complex shear modulus G (1 Hz) is plotted against the pectin concentration (O) p-casein ( ) o i -casein. Dotted lines indicate the range of pectin concentration for phase separation in the mixed solutions. The pectin was added to the protein solution before emulsion preparation. Data are taken front Semenova et al. (1999a).

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). 

Surimi is fish paste from deboned fish used to make simulated crab legs and other seafood. For preservation the paste is blended with cryoprotectants, such as sucrose, sorbitol and phosphates, and frozen. To make the final product, the frozen paste is thawed, blended with starch and extruded as a film onto a belt. The belt takes the film into an oven that heat-denatures the fish protein and cooks the starch. The film is then rolled to form striations, shaped, colored and cut. Depending on the required distribution, the product is frozen or refrigerated. Potato and tapioca starch were used in surimi products 400 years ago, since they provided a cohesive, elastic matrix consistent with seafood. Frozen distribution has made the use of highly-stabilized, moderately crosslinked tapioca starch popular, alone or with native tapioca starch. Modified waxy maize products are used, as is unmodified com starch, for increased cuttability. Kim188 reported that the gel strengthening ability of starch correlates with starch paste viscosity. 

A variety of rheological tests can be used to evaluate the nature and properties of different network structures in foods. The strength of bonds in a fat crystal network can be evaluated by stress relaxation and by the decrease in elastic recovery in creep tests as a function of loading time (deMan et al. 1985). Van Kleef et al. (1978) have reported on the determination of the number of crosslinks in a protein gel from its mechanical and swelling properties. Oakenfull (1984) used shear modulus measurements to estimate the size and thermodynamic stability of junction zones in noncovalently cross-linked gels. 

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