Molecular Microbiology Research, 2025, Vol.15, No.2, 45-58 http://microbescipublisher.com/index.php/mmr 48 koji. Modern large soy sauce factories generally use selected Aspergillus oryzae strains to make koji to ensure consistent enzyme activity and fermentation performance of the koji. At the same time, during the high-salt fermentation stage, some companies also add purely cultured functional bacteria, such as inoculating mixed bacterial agents with salt-resistant lactic acid bacteria and yeast in the sauce mash to speed up fermentation and inhibit miscellaneous bacteria. Compared with natural inoculation, artificially domesticated bacteria have the advantages of strong fermentation, high enzyme production and safe and controllable. In recent years, scientists have further carried out bacterial breeding work and cultivated soy sauce fermented strains with optimized performance. For example, studies have successfully screened out salt-resistant lactic acid strains with low-yield acetic acid and used for soy sauce fermentation to reduce the inhibition of acetic acid on yeast and improve the fermentation alcohol production and flavor quality. Qi et al. (2021) reported a T. halophilus mutant strain that lacks the key enzyme of the citric acid metabolism pathway, which does not produce excessive acetic acid. The use of this bacterial fermentation makes the alcohol fermentation of Z. rouxii yeast more vigorously, and the soy sauce produced has a more fragrant and lower sour taste (Qi et al., 2021). This shows that through strain breeding and improvement, certain fermentation characteristics can be improved in a directional manner, thereby optimizing the flavor and quality of soy sauce. 3.3 Microbial diversity and synergistic interactions Whether traditional natural fermentation or modern inoculation fermentation, soy sauce brewing is the result of the joint action of multiple microorganisms. In the early stage of fermentation, aerobic fungi such as Aspergillus multiply in large quantities, providing saccharification and protein decomposition products; then, under high-salt anaerobic conditions, halophilic lactic acid bacteria reproduce in large quantities, fermenting some sugars to produce acids (such as lactic acid), reducing the pH of the mash, creating conditions for subsequent yeast activities.Studies have shown that in naturally fermented moromi at room temperature, lactic acid bacteria and Bacillus species are actively involved in acid production during the initial phase. In contrast, temperature-controlled fermentation tends to enrich more lactic acid bacteria, leading to a more stable acidification process. In the mid to late stages, salt-tolerant yeasts such as Zygosaccharomyces rouxii utilize the sugars and amino acids generated to carry out alcoholic and ester fermentations, producing alcohols and esters that contribute to the aromatic profile of the soy sauce (Devanthi et al., 2018). At the same time, facultative anaerobic bacteria (e.g., Staphylococcus spp.) continue to degrade proteins, releasing amino acids and small peptides that enhance the umami taste. During open-air fermentation processes, diurnal temperature variations and seasonal changes also influence the succession of dominant microbial communities. High summer temperatures favor aroma-producing yeasts, while colder winter conditions help preserve moromi by supporting psychrotolerant bacteria. Through genomic and transcriptomic analyses, Chun et al. (2021) further revealed that in the natural fermentation of traditional Korean soy sauce, anaerobic glycolysis by lactic acid bacteria is the dominant metabolic pathway. Meanwhile, some aerobic halophilic bacteria (such as Chromohalobacter and Halomonas) participate in the oxidative metabolism of amino acids and other compounds. Yeasts (e.g., Debaryomyces, Wickerhamomyces) are closely associated with the production of biogenic amines and specific flavor compounds. 4 Formation Mechanism and Regulation Strategy of Flavor Substances 4.1 Analysis of main flavor ingredients The reason why soy sauce has a unique taste is due to the combined action of a large number of flavored substances and aromatic compounds produced during the fermentation process. Its basic salty and fresh flavor comes from the superposition of high-salt environments and amino acids. Among them, umami flavor (quality) is mainly contributed by amino acids and small peptides, especially the high content of free amino acids such as glutamic acid and aspartic acid, which will bring umami flavor and thick flavor. When soy sauce fermentes, the protein deteriorates in the synergistic action of Aspergillus protease and bacterial enzymes, releasing more than 20 free amino acids and short peptides, forming a nitrogen source substance rich in soy sauce. In addition,
RkJQdWJsaXNoZXIy MjQ4ODYzNA==