Changes of enzyme activity and physiochemical properties during the fermentation of Taiwanese style sufu at different temperatures

Different from that of Chinese sufu, the manufacturing process of Taiwanese sufu involves the production of salted tofu and the preparation of rice-soybean koji mash with Aspergillus oryzae separately, and the two are combined during fermentation. This study was conducted to investigate some biochemical changes of Taiwanese sufu during fermentation at different fermentation temperatures (25, 37 and 45℃). The results revealed that the enzyme activity (protease, lipase, α-amylase and β-amylase) detected in rice-soybean koji significantly decreased (p < 0.05) with the extension of fermentation. The activity of protease and lipase in the infusion slightly increased during the early stage of fermentation (0-8 days), but decreased thereafter. Generally, the activity of α-amylase and β-amylase detected in the infusion was the highest after 4-day of fermentation, regardless of fermentation temperature. No significant change in their activity was detected as the fermentation was further proceeded. Activity of these hydrolyte enzymes was also the highest in the tofu after 4-day of fermentation. While protease and lipase activities reduced, and the activities of α-amylase and β-amylase remained constant as the fermentation was extended. At the end of fermentation, the activities of protease and lipase detected in both infusion and tofu were significantly lower (p < 0.05) when fermentation was performed at 45℃ than at 25 or 37℃. After a 16-day of fermentation, sufu fermented at 37℃ and 45℃ showed a protein hydrolysis ratio of 20%, while that of 25℃-fermented sufu was only 12%. The content of free amino acid of sufu after 16-day of fermentation was significantly higher (p < 0.05) than that without fermentation. The free amino acid content of the 37℃- and 45℃-fermented sufu showed no significant difference (p > 0.05) while they were significantly higher (p < 0.05) than that of the 25℃-fermented sufu. During fermentation, the color of tofu and infusion changed from ivory white to reddish brown. This phenomenon was most obvious with that fermented at 45℃. As the fermentation time extended, the content of amino nitrogen, titratable acid and free fatty acid of the tofu increased gradually, while the total nitrogen, pH and hardness decreased slightly. Sensory evaluation showed that the sufu fermented at 45℃ had the highest score in total evaluation, while the score of that fermented at 25℃ is the lowest.eywords: sufu, enzyme activity, free amino acid, Aspergillus oryzae, fermentation temperature.