Optimizing the initial moromi fermentation conditions to improve the quality of soy sauce

•Optimum fermentation for soy sauce was at 10% w/w brine content, 40.7 °C for 4.6 days.•These conditions obtained first grade soy sauce based on the requirement of Taiwan.•Second-order polynomial models fitted the chemical properties of soy sauce very well.

Soy sauce is a traditional condiment consumed by the people in China, Korea, Japan and Taiwan. The annual production is about 5 million metric tons in China, accounting for over 50% of the world production. The objective was to optimize the process parameters for preparation of initial moromi fermentation of soy sauce, which was considered as an enzymatic hydrolysis for the further fermentation stage. Temperature (35, 40 and 45 °C), fermentation time (1, 3 and 5 days) and brine content (5 and 10% w/w) were studied as independent variables. The responses evaluated for deciding the optimum conditions were biochemical properties of total nitrogen, amino nitrogen, reducing sugar and total acid contents, as well as microbial properties of yeast and total bacterial counts. Experimental data were fitted well into second-order polynomial models. The simultaneously optimal conditions were 40.7 °C temperature, 4.6 days fermentation time and 10% w/w brine content. The optimized response values for total nitrogen, amino nitrogen, reducing sugar and total acid contents and pH were 1.43, 0.65, 2.79, and 1.28% and 5.48, respectively. These conditions could obtain total nitrogen (>1.4%) and amino nitrogen (>0.56%) contents satisfying the requirement of first-grade soy sauce within 5 days fermentation.