During soy sauce brewing, various organic substances in the raw materials undergo complex biochemical reactions under the action of different enzyme systems of various microorganisms, forming a variety of components in soy sauce. Among these, proteins in the raw materials undergo a series of hydrolysis processes under the successive action of proteases and peptidases, generating peptides of different molecular weights. The optimal temperature for the action of proteases and peptidases is 40–45℃.
Plant proteins contain 18 amino acids; glutamic acid and aspartic acid contribute umami flavor, glycine, alanine, and tryptophan contribute sweetness, tyrosine contributes bitterness, and peptides also contribute to umami and flavor. Amino acid nitrogen accounts for approximately 50% of the total nitrogen in finished soy sauce. If insufficient salt is added or the brine is not mixed evenly, putrefactive bacteria can easily develop in the fermented mash, decomposing amino acids to produce ammonia and amines, resulting in a loss of umami flavor and the production of foul and unpleasant odors. Therefore, prevention is crucial. The optimal temperature for the growth of putrefactive bacteria is around 30℃, and the suitable environment is neutral or slightly alkaline.
Amylase hydrolyzes starch to produce sugars, providing nutrients for fermentative bacteria (such as lactic acid bacteria). These bacteria gradually multiply and undergo lactic acid fermentation, creating a slightly acidic to acidic environment that effectively inhibits putrefactive bacteria and creates favorable conditions for yeast growth and fermentation. Proteins continue to hydrolyze under the action of acidic proteases. Simultaneously, other types of fermentation also occur, producing corresponding products such as organic acids and alcohols.
During fermentation, cellulose, hemicellulose, pectin, and fats in the raw materials also undergo enzymatic changes, forming their respective decomposition products.
Among these products, organic acids and alcohols undergo esterification reactions to form various esters.
Besides amino acid products, soy sauce contains sugars such as dextrin, maltose, glucose, and pentose; organic acids such as lactic acid, acetic acid, citric acid, succinic acid, propionic acid, and malic acid; alcohols such as ethanol, methanol, propanol, butanol, pentanol, and hexanol; and esters such as ethyl acetate, ethyl lactate, butyl acetate, and ethyl propionate. Each of these contributes to the unique flavor of soy sauce. Browning, or color development, also occurs during soy sauce brewing. There are two types of browning: enzymatic browning and non-enzymatic browning. The former involves the oxidation of tyrosine into brown pigments (melanin) under the catalysis of microbial phenol hydroxylase and polyphenol oxidase, primarily occurring in the later stages of fermentation. The latter involves no direct enzyme involvement; it occurs when fermentation products such as glucose and amino acids undergo a Maillard reaction to produce melanoidins. Extending the fermentation period and increasing the temperature can enhance this reaction but affect enzymatic fermentation. Therefore, this browning reaction can only be used in the later stages of fermentation. The above browning only produces light-colored soy sauce. If a dark brown soy sauce is desired, caramel coloring (made by caramelizing sugar at 150-200℃, a process not involved in soy sauce fermentation) must be added. Therefore, mixing the prepared koji (fermentation starter) with brine during the incubation fermentation process accelerates the chemical changes of various enzymes at suitable temperatures, producing a mixture of umami, sweetness, alcoholic flavor, sourness, and the saltiness of the brine, resulting in the unique color, aroma, flavor, and body of soy sauce. The formation of soy sauce aroma components is even more complex, with more than 80 trace aroma components discovered, mainly including esters, alcohols, hydroxy compounds, acetals and phenols. Their sources include those generated from raw materials, those composed of Aspergillus metabolites, those generated by salt-tolerant lactic acid bacteria metabolites, and those generated by chemical reactions.
