Some Biochemical and Physical Changes during the Preparation of the Enzyme-Ripening Sufu, a Fermented Product of Soybean Curd

In this study, sufu, a fermented product of soybean curd, was prepared by ripening the salted tofu in the mash of Aspergillus oryzae-fermented rice−soybean koji possessing various hydrolytic enzymes for 16 days. It was found that protease, α-amylase, β-amylase, and lipase activities observed in the koji granules reduced most pronouncedly during the initial 4 or 8 days of ripening. Meanwhile, an increase in the activity of the various enzymes was noted in the ripening infusion and tofu cubes. During the ripening period, the content of total nitrogen, pH, and hardness of sufu decreased, while the titratable acidity, protein dissolution ratio, content of free fatty acid, and free amino acid increased with glutamic acid, showing the largest magnitude of increase at the end of the 16 day ripening period. Additionally, the color of tofu cubes changed from pale yellow to yellowish brown at the end of the ripening period.