Proteins films, viscoelastic properties

Figure 8.12 illustrates the effect of complex formation between protein and polysaccharide on the time-dependent surface shear viscosity at the oil-water interface for the system BSA + dextran sulfate (DS) at pH = 7 and ionic strength = 50 mM. The film adsorbed from the 10 wt % solution of pure protein has a surface viscosity of t]s > 200 mPa s after 24 h. As the polysaccharide is not itself surface-active, it exhibited no measurable surface viscosity (t]s < 1 niPa s). But, when 10 wt% DS was introduced into the aqueous phase below the 24-hour-old BSA film, the surface viscosity showed an increase (after a further 24 h) to a value around twice that for the original protein film. Hence, in this case, the new protein-polysaccharide interactions induced at the oil-water interface were sufficiently strong to influence considerably the viscoelastic properties of the adsorbed biopolymer layer. 

In contrast, the adsorbed layer in protein-stabilized thin films is much stiffer and often has viscoelastic properties [3]. These derive from the protein-protein interactions that form in the adsorbed layer (Figure 1(b)). These interactions result in die formation of a gel-like adsorbed layer in which lateral diffusion of molecules in the adsorbed layer is inhibited. Multilayer formation can also occur. This serves to further mechanically strengthen the adsorbed layer. 

Hook, F., Kasemo, B., Nylander, T., Fant, C., Sott, K., and Elwing, H. (2001). Variations in coupled water, viscoelastic properties, and film thickness of a Mefp-1 protein film during adsorption and cross-linking A quartz crystal microbalance with dissipation monitoring, ellipsometry, and surface plasmon resonance study.y4na/. Chem., 13, 5796-5804. 

Cuq et al.20 did not find influence of thickness on viscoelasticity properties of films made of myofibrillar proteins from North Atlantic sardines (0.010-0.060 mm). 

Because protein-ba sed foams depend upon the intrinsic molecular properties (extent and nature of protein-protein interactions) of the protein, foaming properties (formation and stabilization) can vary immensely between different proteins. The intrinsic properties of the protein together with extrinsic factors (temperature, pH, salts, and viscosity of the continuous phase) determine the physical stability of the film. Films with enhanced mechanical strength (greater protein-protein interactions), and better rheological and viscoelastic properties (flexible residual tertiary structure) are more stable (12,15), and this is reflected in more stable foams/emulsions (14,33). Such films have better viscoelastic properties (dilatational modulus) ( ) and can adapt to physical perturbations without rupture. This is illustrated by -lactoglobulin which forms strong viscous films while casein films show limited viscosity due to diminished protein-protein (electrostatic) interactions and lack of bulky structure (steric effects) which apparently improves interactions at the interface (7,13 19). 

The low water solubility and viscoelasticity of wheat protein provide various interesting physico-chemical characteristics, such as gel- and film-forming properties. ... 

A wheat protein derivative with molecular weight of 250-500 kDa has been studied in aqueous solution and in an 0/W emulsion for its effects on the viscoelastic properties of the skin. A slight increase in skin extensibility and elasticity was found and related to its moisturizing and film-forming effects. This derivative is proposed in skin care preparation for transient skin Arming-tightening effects (54). 

Adsorbed protein molecules interact at the interfaces to form viscoelastic films. The viscoelastic properties of protein films adsorbed at fluid interfaces in food emulsions and foams are important in relation to the stability of such systems with respect to film rupture and coalescence. Interfacial rheology techniques are very sensitive methods to measure the viscoelastic properties of proteins, thereby evaluating the protein-protein or protein-surfactant interactions at the interfaces. There was an excellent review about the principal and methods of interfacial rheology [17]. 

Biswas and Haydon [20] found some correlation between the viscoelastic properties of protein (albumin or arabinic acid) films at the 0/W interface and the stability of emulsion drops against coalescence. Viscoelastic measurements were carried out using creep and stress relaxation measurements (using a specially designed interfacial rheometer). A constant torque or stress a (mN m ) was applied and the deformation y was measured as a function of time for 30 minutes. After this period the torque was removed and y (which changes sign) was measured as a function of time to obtain the recovery curve. The results are illustrated in Fig. 5.18. From the creep curves one... 

W Kiosseoglou. Minor surface-active lipids of olive oil and viscoelasticity of protein films at the olive oil-water interface. J Disp Sci Technol 13 135-144, 1992. J Benjamins, A Cagna, EH Lucassen-Reynders. Viscoelastic properties of triacylglycerol/water interfaces covered by proteins. CoUoids Surf A 114 245—254,... 

Caseinate is a mixture of fairly flexible polymers. Most proteins are of globular conformation, and their surface properties are not easy to interpret. The values of t)ls are much higher and tend to increase with the age of the film. It may take a day to obtain a more or less constant value, which is typically 0.1-0.5 N s m 1. However, the surface layer is clearly viscoelastic, and the apparent viscosity obtained will strongly depend on measurement conditions, especially the shear rate. Actually, it cannot always be ruled out that the proteinaceous surface layer is subject to yielding or fracture upon large deformation this would imply that slip occurs in the rheometer, leading to a greatly underestimated viscosity. 

As with emulsion formation, the maximal absorption of proteins has been reported to occur near the isoelectric point [6,18,47], The effects of pH on the properties of proteins at emulsion surfaces, noted previously, probably also apply to the behavior of proteins in films. While foams formed near the isoelectric point of proteins tend to be more stable than those formed at other pH values, it is often possible to obtain higher overruns away from the isoelectric point [48]. The largest effect of pH on foam stability can probably be explained by the more viscoelastic nature of the films formed in this pH region... 

Based on previous studies, we can assume that the MR has antioxidant properties [10]. In this case MR behavior will allows a stabilization mechanism that is based on interactions with protein molecules which lead the formation of a stiff viscoelastic surface film that can also work as barrier... 

Many physical and chemical properties of proteins can be modified to enhance their surface activity. Some of the most important characteristics, such as surface film strength, viscoelasticity, and colloid stability, can be changed by extrinsic changes (pH, ionic strength, temperature, etc.), and others such as hydropho-bicity, flexibility of the polymer, and net charge can be altered by affecting the intrinsic properties of the protein (chemical or enzymatic modifications). The chemical modification of proteins will affect both protein-protein interactions and the interactions of protein with other surfactants. 

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