Process Optimization In Soy Sauce Fermentation And Its Effect On Soy Sauce Quality

Soy sauce is a kind of Chinese traditional condiment having a long history. Presently, China has the largest soy sauce production, distribution and consumption in the world. High-salt semi-liquid-state fermentation procedure is popular and is adopted by most of the leading enterprises. But the relationship between the process parameters and the quality of mature soy sauce is still not fully understood.The present works investigated the changes of colony morphology, acrocyst, spore, conidiophore and vegetative hyphen morphology of the Aspergillus oryzae responsing to various NaCl concentrations in the solid medium. Assay methods characterizing physicochemical properties of soy sauce were established. Herein, eight organic acids were chosen as the key factors representing the soy sauce quality and their quantification method was constructed by HPLC. The best moisture content of preculture medium for growing Aspergillus oryzae and the optimum inoculation time were 45%(w/w) and 72 h, respectively. Response surface methodology was used to optimize the medium formula of koji. It was shown that the optimum weight proportion of soy bean, fried wheat and water in koji medium is 11.3 : 10.5 : 19.6. At the optimal conditions, a high level of 2402 U/g koji protease activity was attained and increased by 20% compared with a control.During the fermentation process of sauce mash, the increase of fermentation temperature could result in drop in sauce mash pH. Activity and stability of protease produced by Aspergillus oryzae would be negatively influenced when pH decreased to 5.5. The amino nitrogen content in soy sauce product declined as protease activity and stability decreased. In addition, it was found that the high-salt enviroment could slow down the growth of microbes and retard the decline trend of pH in sauce mash. Such a high-salt environment evidently favors the high quality of soy sauce product. It was found that the low-temperature in fermentation could increase the content ofglutamic acid due to the slow growth of microbes and reduced transformation of glutamic acid to proline or arginine. At the end of the fermentation, the glutamic acid content reached as high as 23.07% of total amino acids, which is much higher than 20.15% and 15.49% when the temperatures were set at 30°C and 40°C, respectively. Moreover, the addition of salt-tolerant yeast could enhance the consumption of reduced sugars and bitter amino acids in sauce mash. But at the end of fermentation process, some umami amino acids would be improved due to autolysation of the yeast, which in turn favors the flavor of the soy sauce.