Abstract: The changes in the degree of hydrolysis (DH), turbidity, components and molecular mass, surface hydrophobicity and structure of soybean protein isolate (SPI) were studied after being treated with an aspartic protease (AP) from Quambalaria cyanescens. The enzyme-induced aggregation behavior of SPI was determined by laser scanning confocal microscopy (LSCM) and atomic force microscopy (AFM). The results indicated that the DH and turbidity of SPI increased first and then tended to be stable with the increase of AP treatment time. The enzymatic treatment caused subunit dissociation of SPI, leading to its hydrolysis into smaller molecules. This treatment also increased the surface hydrophobicity of SPI. The circular dichroism (CD) spectrum and intrinsic fluorescence emission spectrum of SPI showed that the enzymatic treatment significantly changed its structure, causing the exposure of internal hydrophobic groups and hydrophobic amino acid residues. LSCM and AFM observation showed that the increases of enzymatic treatment time caused more extensive aggregation of SPI, resulting in the formation of bigger, irregular and discontinuous aggregates.

Key words: soybean protein isolate, enzymatic treatment, structural characterization, aggregation behavior