Soy proteins, interfacial

Figure 2. Interfacial tensions at the air-water interface as a function of time1/z for 10 2 % (w/w) dispersions of soy protein, WPC, and caseinate at different ionic strengths (4)...

The interfacial and emulsifying behavior of three food proteins, a soy protein isolate, a sodium caseinate and a whey protein concentrate (WPC) have been studied. A kinetic analysis of the interfacial tension decay of the proteins indicates the following characteristics. The soy proteins diffuse slowly to the interface compared to the other proteins, probably with the quaternary structure intact, which disintegrates when adsorbed at the interface. Both the whey proteins and the caseinates diffuse quickly to the interface, where for the caseinates the diffusion--controlled occupation of the interface is very evident, especially at an ionic strength of 0.2. 

FIGURE 11.11 Specific droplet surface area A (and average droplet size surfactant concentration c, obtained at approximately constant emulsification conditions for various surfactants PVA = poly(vinyl alcohol) also for soy protein a plateau value of A is reached, at about 20kg-m 3. Approximate plateau values for the interfacial tension y are 3, 10, and 20 mN-ur1 for the nonionic, caseinate, and PVA, respectively. 

Soy proteins are applied in a wide range of food products. In this context most attention has been paid to the ability of soy proteins to form a gel upon heating. Heat denaturation and subsequent gel formation of soy proteins in bulk solutions have been extensively studied.1-5 Besides this, studies have been performed on the interfacial tension and adsorbed amount of soy proteins and on their suitability for formation and stabilisation of emulsions and foams.6-10 A question that, to our knowledge, has not been discussed is how far these different functional properties are mutually related. 

The adsorption of soy protein at an interface is relatively slow compared to casein, and the rate is affected by ionic strength, being higher at 0.2 M than at zero NaCl where the subunits may be dissociated. Conceivably the reduction of the zeta potential and electrostatic repulsion (from 0 to 0.2 M salt) facilitates penetration and subsequent surface packing (28). The rate of penetration of additional molecules into the film indicated that the soy proteins initially adsorbed and spread easily at the surface ( ). However, this seems inconsistent with the highly stable disulfide linked tertiary structure of soy glycinin (30) and it is perhaps the conglycinin component that forms the initial interfacial film (31). 

What does the different r-Cp-dependence of the proteins, as is obvious in Figure 5, tell us about their interfacial behaviour Firstly, we compare the two extremes, i.e. the interfacial behaviour of the caseinate (0.2-7) and the soy protein (0-7). 

An overview of the interfacial pressure attained after 40 minutes as a function of the protein concentration can be seen in Figure 8, for the three proteins soy protein, WPC and sodium caseinate in (0-7) and (0.2-7) adsorbing at the A/W- and soya bean oil water (0/W) interface (10). 

Kim JT, Netravah AN (2011) Development of ahgned-hemp yam-reinforced green composites with soy protein resin effect of pH on mechanical and interfacial properties. Compos Sci Technol 71(4) 541-547... 

There are several available reports focusing on the NFR soybean protein-based composites for example, Liu et al. [46] evaluated the mechanical properties of grass-reinforced soy-based bioplastic, while Lodha and Netravah [47] evaluated the interfacial strength for ramie fiber-reinforced soy protein isolate (SPI). It must be noted, however, that the long-term performance of soy proteins is Hmited because of their high sensitivity to moisture as a result of the presence of amine, amide, carboxyl, and hydroxyl groups. To reduce their hygroscopicity, the soy proteins are cross-Hnked with aldehydes such as formaldehyde. 

Lodha, P. and Netravali, A.N. (2002) Gharacterization of interfacial and mechanical properties of green composites with soy protein isolate and ramie fiber. /. Mater. Sci., 37, 3657-3665. 

Kim, ).T. and Netravali, A.N. (2010) Effect of protein content in soy protein resins on their interfacial shear strength with ramie fibers. /. Adhes. 

It is beyond the scope of this chapter to analyze in detail the various surface interactions and forces that proteins can provide. The number of amphiphilic proteins in the world of proteins is limited, which means that the proteins in use are mainly caseins, whey proteins, P-lactoglobulins (BLGs), egg albumin, bovine semm albumin (BSA), lysozyme, and soy proteins. All other plant proteins have very limited ability to strongly adsorb onto interfaces and reduce interfacial tension to only a minor extent. However, chemical and enzymatic modifications will improve the performance of these proteins (pea, cotton, and cereal proteins), and as a result a few modified proteins can be found in the marketplace that have relatively improved performance. 

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