Abstract:

Douchi, a soybean product originated in China, has been consumed since ancient times as a food seasoning. Douchi displays greatly improved nutrition as compared with raw material bean. Studying the changes in the nutritional components of Douchi during its processing is necessary for us to improve product quality. The influence of fermentation on the basal components of naturally fermented Yongchuan Douchi was investigated by measuring the changes in water content, total acidity, amino acid nitrogen and protease activity during the processing of Douchi. The results showed that water content was decreased from 52.30% to 32.91% during qu-making stage, and kept at around 50 % during post-fermentation stage. The content of total acids was increased rapidly during the early stage of post-fermentation, followed by a slow decrease during the later stage and reached 2.12 g/100 g (calculated as lactic acid) at the end of ripening. The content of total amino acid nitrogen increased generally during post-fermentation stage, from 0.18 g/100 g to a maximum of 0.98 g/100 g. We found that acid protease activity ascended rapidly during the first two days of qu-making stage, while neutral protease activity and alkaline protease activity also exhibited a sharp increase from days 7 to 10 with the reproduction of Mucor. Addition of salt during follow-up fermentation stage resulted in a rapid descent in the activities of all the three enzymes.

Key words: Yongchuan Douchi, total acidity, amino acid nitrogen, protease activity