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: This article reviews and discusses the relationship between surface hydrophobicity and other surface properties of proteins and the possibility of using surface hydrophobicity as a key indicator to predict and evaluate the changes in the surface properties of a protein. Hydrophobicity is the main driving force of protein folding; it affects the structure and functions. Surface hydrophobicity and other surface properties of proteins are controlled by their spatial structures. Due to the hydrophobic interactions, most proteins fold into their globular structures, and they lack sufficient hydrophobic residues on the molecular surface; thus, they do not exhibit excellent surface properties. Surface hydrophobicity is closely related to the changes in the surface property of proteins because it directly reflects the actual distribution of the hydrophobic residues on the surface of a protein. The molecular structure of a protein can be changed or modified to remove the constraints of spatial structures and expose more hydrophobic residues on the molecular surface, which may improve the surface properties of proteins. Therefore, the changes in the surface hydrophobicity caused by changes in the molecular structure can be an ideal key indicator to predict and evaluate the changes in the surface properties of a protein.

In this study, the effects of the disulfide bond cleavage induced by peracetic acid oxidation on the surface properties and surface hydrophobicity of soy proteins isolate (SPI) were investigated. The surface hydrophobicity, foaming capacity and emulsifying capacity of oxidized-SPI increased gradually at the initial stage with the increase of peracetic acid concentration. When the concentration of peracetic acid was increased up to 0.4%, compared with that of native SPI, the surface hydrophobicity, foaming capacity, emulsifying capacity and emulsifying stability of oxidized-SPI increased by 114.0, 81.4, 65.2, 49.8%, respectively, and achieved optimal results. However, excessive oxidation led to a decrease in surface hydrophobicity, foaming capacity and emulsifying stability of SPI, but it had no obvious effect on the foaming stability and emulsifying capacity of SPI. The foaming capacity, emulsifying capacity and emulsifying stability of SPI were positively related to the changes of surface hydrophobicity which caused by disulfide bond cleavage. The results of fluorescence spectroscopy, CD spectroscopy, and particle size analysis showed that the disulfide bond cleavage did cause great changes in the molecular structure of SPI, but there was no clear correlation between the molecular structural change and the surface activity of SPI. These suggested that the improvement of foaming capacity, emulsifying capacity and emulsifying stability of SPI could be achieved by changing its surface hydrophobicity via peracetic acid oxidation.

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