Denaturation soy proteins

IIS and 7S proteins dissociate into subunits at ionic strength below O.IM and 0.5M, respectively (11, 12). This is, therefore, a plausible explanation oT" tKe observed solubility behavior. The sharp minimum at O.IM salt concentration in the case of the NPI might correspond to the cooperative association reactions involving IIS subunits. Even though there has been no experimental demonstration, it would not be unreasonable to postulate that random association reactions also occur for denatured soy proteins. The broader minimum between 0.1 and 0.2M salt concentration might be indicative of less cooperative association reactions. 

The intrinsic viscosity of native and denatured soy proteins have been measured (13). These values, which reflect the hydrodynamic properties oT the protein molecules at infinite dilution, are of little interest to us as far as functionality is concerned. What is of interest is the apparent viscosity of concentrated slurries. In these slurries, the intermolecular protein-protein interactions dominate and are primarily responsible for the observed viscosity behavior. 

Aral et al. (9) obtained a binding constant (K) for the interaction of 1-hexanal with the partially denatured soy protein (Experiment II above) from gel filtration data analyzed by using Beidler s equation ... 

Ohtsuru et al. (25) have recently investigated the behavior of phosphatidylcholine in a model system that simulated soy milk. They used spin-labelled phosphatidylcholine (PC ) synthesized from egg lysolecithin and 12-nitroxide stearic acid anhydride. The ESR spectrum of a mixture of PC (250 yg) and native soy protein (20 mg) homogenized in water by sonication resembled that observed for PC alone before sonication. However, when PC (250 yg) was sonicated in the presence of heat-denatured soy protein (20 mg), splitting of the ESR signal occurred. On this basis, they postulated the existence of two phases PC making up a fluid lamella phase and PC immobilized probably due to the hydrophobic interaction with the denatured protein. In a study of a soy-milk model, Ohtsuru et al. (25) reported that a ternary protein-oil-PC complex occurred when the three materials were subjected to sonication under the proper condition. Based on data from the ESR study, a schematic model has been proposed for the reversible formation-deformation of the ternary complex in soy milk (Figure 2). 

As exemplified by soy protein, interaction of protein with 1-hexanal is promoted by heat denaturation. The interaction is decreased by treating a heat-denatured soy protein with protease the aldehyde is liberated to an appreciable extent. 

Wang, H. T. Wang L.A Johnson. Mechanism for refunctionalizing heat-denatured soy protein by... 

Two types of enzymes were used to introduce cross-links in protein films peroxidase and transglutaminase. A treatment by horseradish peroxidase in the preparation of films of thermally denaturated soy proteins reduced the elongation at elastic limit and increased the tensile strength. ... 

Protein, Protein structure. Physicochemical properties. Industrial application, Denaturation, Soy protein, Zein, Wheat protein. Casein, Films, Adhesives, Plastics, Blends, Composites, Green materials. Biodegradability... 

Dahl, S.R Villota, R. Twin-screw extrusion texturization of acid and alkali denatured soy proteins. J. Food Sci. 1991,56, 1002-1007. 

There is much indirect evidence that the above hypothetical mechanism fits the behavior of the soy proteins. Wolf and Tamura (2), while studying the heat denaturation of native IIS soy protein, found that soluble aggregates are formed prior to the formation of insoluble precipitates. They proposed the following mechanism to explain their results. 

The plastein reaction usually involves two steps hydrolysis of protein and resynthesis of peptide links. Yamashita et al. (20) found similar BV, digestibility, and weight gain for denatured soy meal and soy plastein. This same group (49,50) had described a one-step process by which amino acids may 5 enzymatically incorporated in intact protein to improve protein quality. With soy protein they applied a racemic mixture of D,L-methionine ethyl ester and were able to enzymatically incorporate L-methionine. As Schwimmer (51) has pointed out, one expects the methionine so incorporated to be highly available due to its location at the end of the polypeptide chains. 

Strong interaction of volatile aldehydes occur naturally in soy protein products (9). Arai et al. (10) found that 1-hexanal is one of the major odorants of soybean and that this aldehyde interacts readily with soy protein. In order to determine whether the interaction is enhanced by denaturation of protein, Arai et al. (9) did three experiments under different conditions. In the first experiment (I), an acid-precipitated fraction of... 

Figure 2. Difference spectrum for 7M urea-denatured soy 7S protein. Five minutes in (---------------) 20% ethanol and (----) in 35% ethanol.

Figure 3. Difference spectra for alcohol-denatured soy 7S protein. Fifteen minutes at room temperature.

From both the DSC-thermograms of whey and soy proteins it can be concluded that the native structure is most stable against denaturation in the isoelectric region where the net charge is low. 

This can be obtained if the second step is slower relative the first. The denatured molecules can then orient themselves to a certain degree of order before aggregation. If the second step is reversible, which is the case for soy proteins, the intermediate state is defined as a progel state. 

Solubility. The results from the solubility experiments are given in Figures 4 and 5. The broken curve is the solubility curve for the original soy protein isolate (Purina 500 E) the nitrogen solubility of this product at neutral pH is below 40%, which indicates that the isolate has been partly denatured during its processing. The sample denoted as DH = 1.0% is the control and it is clearly demonstrated that the acid treatment has caused further aggregation and denaturation. The definitely positive DH-value... 

This study stems from an attempt to find alternatives to the present method of obtaining protein from heated, solvent extracted soy flakes. As the primary process can cause protein denaturation, insolubilization, impairment of flavour, functionality and color and may possibly affect its nutritive value, it would seem that a better approach might be the direct isolation of soy protein using less harsh conditions. 

Glycerol (plasticizer) concentration used in the hterature varied from 35 to 60g/100-g soy protein (35-39). A lower amount of plasticizer will cause drying out and cracking of the film. The intermolecular disulfide bonds resulting from the denaturing of protein and intermolecular interactions between the protein chain... 

Most commercial soy protein products have been developed from the acid-precipitated fraction [85]. However, local extremes in pH can cause irreversible denaturation of the proteins [85]. The disadvantages of this method include denaturation of protein on exposure to alkali and acid treatment, high-ash content, and alteration of protein solubility after rehydration [86]. 

It was proven over 20 years ago that both native and denatured proteins bind volatile flavouring substances [7,9,12,32-34], Studies at the beginning of the 1980 s revealed that aldehydes (such as nonanal) and ketones (such as 2-heptanone, 2-octanone, 2-nonanone) formed exceptionally strong bonds to native proteins (soy protein, bovine serum albumin, fish actomyosin) in aqueous systems [35-39],... 

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