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Because water solubility is the main hydration property of proteins, solubility values of commercial and laboratory soy protein isolates, prepared under different conditions, were comparatively analyzed. In contrast, the surface hydrophobicity manifested by proteins is a physicochemical property that determines, to a great extent, the tendency of protein molecules to aggregate and so to lose solubility. On these grounds, the solubility of isolates was analyzed as a function of the surface hydrophobicity of their proteins, and, as a result, three well-defined groups of laboratory isolates were identified: (A) native, (B) partially or totally denatured with high solubility and surface hydrophobicity, and (C) totally denatured with low solubility and surface hydrophobicity. Commercial isolates could not be included in any of these groups; they were grouped as (A') partially native and (C') totally denatured. Solubility values in these two groups were similar to those of group C, but the surface hydrophobicity levels were much lower. The different processes leading to the groups mentioned above are discussed, along with the way the soy proteins are influenced by the specific preparation conditions, namely, protein concentration, chemical or thermal treatments, presence of salts, drying, and phospholipid addition, among others.

Electrophoretic profiles, sulfhydryl(SH)/disulfide (SS) groups content, and surface hydrophobicity (H0) values of whey soybean proteins (WSP) and native soy isolates (NSI) were determined. WSP, composed mainly by Kunitz trypsin inhibitor (KTI), and lectin (L), has a H0 value of 24.0 ± 1.0, which is 6.8 times lower than that of NSI ones, and SH/SS groups content in the same range of NSI. The thermal behavior of WSP and NSI was studied by differential scanning calorimetry (DSC). The WSP thermogram in water, similar to NSI, showed two main peaks (Tp values: 74.0 ± 0.5 C and 90.4 ± 0.8C) attributed to thermal denaturation of KTI and L, respectively. These endotherms are slightly affected by μ, whereas those of NSI are strongly affected (Tp of 7S and 11S peaks increase 17C and 20C respectively, by increasing NaCl concentration from 0 to 1M). WSP has low Ho values not noticeably affected by ionic strength changes, whereas NSI has higher Ho, that increase in saline media and favor intermolecular hydrophobic interactions. The consequent low tendency to protein aggregation by hydrophobic interactions in WSP would explain their lower thermal stability at high μ. Control proteins of both preparations (7S and 11S enriched fractions for NSI, and purified trypsin inhibitors, urease and lectin for WSP) were use to confirm these results.

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