Characterization and formation mechanism of proteins in the secondary precipitate of soy sauce

To reveal the characteristics and formation mechanism of proteins in the secondary precipitate of soy sauce (SPSS), proteins in the supernatant of soy sauce were used as control, and SPSS was prepared by centrifugation in combination with lyophilization. The proteins in SPSS were isolated and identified by sodium dodecyl sulfate–polyacrylamide gel electrophoresis and matrix-assisted laser desorption ionization-time of flight/time of flight mass spectrometry. Acidic polypeptide A1a of soy glycinin G1 (32–35 kDa) and basic polypeptide B3 of soy glycinin G4 (23 kDa) were verified as the predominant proteins (95.2 %) in SPSS. The amino acid composition, average hydrophobicity (HΦ avg), and the secondary structure of the proteins in SPSS were investigated by high-performance liquid chromatography and Fourier transform infrared spectroscopy. Results revealed that SPSS contained not only proteins but also free amino acids; the significantly higher average hydrophobicity and much lower contents of β-sheet and random coil were the main characteristics and reasons for the formation of proteins in SPSS.