Abstract: Naturally fermented soybean pastes were prepared by the same method as that for traditional fermented soybean paste in northeast China with different fermentation periods, and they were investigated for changes in protein content and amino acid composition during the fermentation. In addition to the contents of protein, amino nitrogen, amino acids and non-protein nitrogen and hydrolysis index, amino acid score (AAS), chemical score (CS) and essential amino acid index (EAAI) were also analyzed. The results showed that the content of protein, non-protein nitrogen, and proteolysis index of soybean pastes were increased at first and then decreased, but amino nitrogen content was increased until reaching a plateau. A total of 17 kinds of amino acids were measured in the naturally fermented soybean pastes. It turned out that, with the fermentation, the contents of different amino acids were extremely different. The total amount of amino acids in the ripen soybean pastes remained at around 41.00 mg/g, which was significantly higher than that of raw soybean flour (11.42 mg/g) and cooked flour (11.06 mg/g). The ratio of essential amino acids to non-essential amino acids (E/N) was changed from 0.48 to 0.77, which proved that the soybean pastes contained high quality protein. Based on analysis of AAS, CS and EAAI, the EAAI reached the highest value of 29.41 on day 20 of fermentation and then was decreased to a stable level in the range from 16.16 to 17.40 during day 50 to 75. The contents of different taste-active amino acids were in the order of sweet amino acid > bitter amino acid > umami amino acid > tasteless amino acid.

Key words: soybean paste, protein, amino nitrogen, amino acid, taste-active amino acids