Qualitative analysis of soy sauces made from fresh okara using two fermentation methods

Meju, a brick made of fermented soybeans, is the main ingredient of soy sauce. It was prepared without (S) or with (OS) 40% okara fermented using Bacillus subtilis (traditional fermentation, TF) or a mixture of Aspergillus oryzae and Aspergillus niger (modified fermentation, MF). MF soy sauces (8.39% and 7.22%) showed a lower salinity than the TF soy sauce (15.41% and 16.19%), whereas the total nitrogen (TN) in MF soy sauce was twice higher than in the TF soy sauce. Amino-type nitrogen in TF-S (55.0%) was higher than in TF-OS (32.6%), but was lower in MF-S (97.9%) than in MF-OS (113%). The MF soy sauce carried a higher content of total free amino acids (82.7 vs. 85.4 mg/ml) than the TF (23.3 to 50.98 mg/ml). The level of glutamic acid was the highest in MF-OS (31.2 mg/ml), followed by the levels in MF-S (29.4 mg/ml), TF-S (17.1 mg/ml), and TF-OS (5.9 mg/ml). In sensory test, MF-S (4.26) showed the highest acceptability, followed by MF-OS (3.65), TF-S (3.22), and TF-OS (3.17). Our results revealed that the quality of okara soy sauce can be enhanced when okara meju was fermented by Aspergillus rather than Bacillus. The okara fermented by Aspergillus decreased the fermentation period from 90 to 30 days. Practical applications To increase the utilization of okara, which is a tofu byproduct, soy sauce was developed using okara meju as the main ingredient. To select effective microorganisms for okara meju fermentation, the okara meju was inoculated with a mixture of A. oryzae and A. niger to produce soy sauce. The quality of soy sauce produced by inoculating a mixture of A. oryzae and A. niger was then compared with that of soy sauce produced by inoculating B. subtilis, the traditional method used to ferment meju for soy sauce in Korea. Our results revealed that the quality of okara soy sauce can be enhanced when okara meju was fermented by A. oryzae and A. niger rather than B. subtilis. Further, the MF method using A. oryzae and A. niger reduced the salt content in soy sauce and the fermentation period.