Abstract: In this study, we evaluated the protein emulsions formed during the enzyme-assisted aqueous extraction of soybean oil under different conditions of enzyme dosage (1% and 2%) and hydrolysis time (1, 2 and 3 h). The surface properties and structural characteristics of the emulsion protein/polypeptide were characterized by scanning electron microscopy, emulsion activity index (EAI), emulsion stability index (ESI), surface hydrophobicity (S0), amino acid analysis and Fourier transform infrared spectroscopy. The results showed that as the enzymatic hydrolysis proceeded, the protein in the emulsions changed from a dense and orderly network structure to a loose porous structure, the EAI and ESI gradually decreased; in addition, the proportion of hydrophobic amino acid increased and surface hydrophobicity (S0) decreased due to aggregation of the hydrophobic residues through their mutual interaction, blocking the hydrophobic groups. Fourier transform infrared spectra showed that the proportions of α-helix and β-sheet structure were reduced while the proportion of irregular structure increased with increasing degree of hydrolysis, indicating that the changes in intermolecular force caused by the enzymatic hydrolysis led to conformational changes of protein in the emulsions. The above results can largely emulsion instability during the enzymatic hydrolysis, which will provide theoretical support for understanding the mechanism of emulsion demulsification during the enzyme-assisted aqueous extraction of soybean oil.

Key words: protein emulsion, surface hydrophobicity (S0), amino acid analysis, Fourier transform infrared spectroscopy, structure characteristics