Molecular Microbiology Research, 2025, Vol.15, No.2, 45-58 http://microbescipublisher.com/index.php/mmr 49 nucleotides (such as inosine and guanylic acid) also come from microbial cell lysis and show "umami-taste synergy" with amino acids (Huang et al., 2023). The salty taste is mainly provided by table salt, and high salinity not only gives salty taste but also a necessary condition for antibacterial. Sweetness and bitterness are weaker, and are contributed by some amino acids (sweet glycine, bitter leucine), sugars, bitter peptides, etc., but they are not prominent in the complex flavor of soy sauce. The aroma components of soy sauce are very complex, with hundreds of volatile organic compounds, but the overall aroma can be described as a coordinated combination of soy sauce (fermented black beans), burnt aroma and fruit vinegar aroma. The main aroma substances include: alcohols, organic acids, esters, phenols, furones, pyrazines and sulfides. The sources of these volatiles vary: alcohols, acids, esters, etc. are mainly produced by fermentation and metabolism of yeast and bacteria; most of them are the products of Maillard reaction and caramelization reaction, and are produced by reaction of amino acids and sugars during the long-term fermentation and later heating and concentration of the sauce mash. Especially the "frying soy sauce" step in the fish jumping process, gentle heating causes the residual sugar to react with the amino acids to Maillard, which significantly enhances the rich soy sauce aroma and attractive brown-red luster of the soy sauce. 4.2 The role of microorganisms in flavor formation The metabolic activities of microorganisms determine the types and quantities of flavor compounds produced during soy sauce fermentation. Molds (Aspergillus spp.) secrete hydrolytic enzymes that break down non-flavor macromolecular substrates (e.g., starch and proteins) into flavor precursors. These precursors not only contribute directly to the basic sweet and umami taste of soy sauce but also serve as substrates for subsequent microbial fermentation. In the early fermentation stage, lactic acid bacteria ferment part of the sugars into lactic acid and trace amounts of acetic acid. The mild acidity of lactic acid enhances flavor complexity and lowers the pH, thereby inhibiting undesirable microorganisms. Additionally, lactic acid bacteria produce certain volatile short-chain fatty acids, contributing to the fatty and mellow aroma of soy sauce. Furthermore, yeasts—particularly salt-tolerant species such as Zygosaccharomyces rouxii—are considered major contributors to soy sauce aroma. Yeasts ferment sugars into ethanol, which can react with organic acids to form various esters (e.g., ethyl acetate and ethyl lactate), imparting fruity and alcoholic aromas (Wu et al., 2022). Yeast also produces ester and phenolic aromatic substances through higher alcohol metabolism, such as 4-ethylguaia lignol, and so on in soy sauce smoked incense. However, different microorganisms have a synergistic effect on flavor: the ethanol produced by yeast can further react with the organic acid produced by lactic acid bacteria to form esters and enrich aroma; the large amount of amino acids provided by Aspergillus is more likely to undergo Maillard browning reaction to form calcified aroma substances under the conditions of lowering the pH of lactic acid bacteria (Wu and Yan, 2024). Therefore, a variety of fermented microorganisms have created a complete flavor spectrum of soy sauce. Studies have also found that certain minor microbial contaminants in traditional processes, such as species of Rhizopus and Penicillium, may positively influence flavor when present in small amounts. These microbes can produce unique enzyme systems and metabolites that enhance the aromatic complexity of soy sauce (Zhang et al., 2023). This may help explain why traditionally open-air fermented soy sauce often exhibits richer flavor profiles than those fermented in fully enclosed pure-culture systems. 4.3 Flavor control and process parameter optimization In modern soy sauce brewing, in order to obtain stable and excellent flavor, the key influencing factors in the fermentation process need to be regulated. The first is the fermentation temperature. Temperature not only affects the reaction rate, but also directly changes the composition of metabolites. A study of Bacillus subtilis related to soy sauce was cultivated at different temperatures. The results showed that at 37 ℃, the bacteria metabolize more ammonia odor substances, while above 45 ℃, more soy sauce aroma substances were generated. In actual production, appropriate high temperatures of soybean paste (such as 35 °C~40 °C) are beneficial to inhibit odor production and promote yeast fragrance production, while ripening at low temperatures in the later stage can reduce the accumulation of pungent odor (Gao et al., 2011).
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