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starting levels as high as 108 cells per gram. In contrast, lactic acid bacteria and various yeasts are present at much lower initial cell concentrations (101 to 104 per gram).

Although the initial pH of the mash (6.5 to 7.0) is perfectly fine for all of these organisms, the high salt concentration poses a substantial barrier.A mash containing 18% salt will have an osmolality of more than 6 Osm and a water activity (aw) of about 0.88, which is lower than most of the moromi organisms can tolerate. Thus, the salt-sensitive organisms, including the Micrococcus, wild yeasts, and most of the Bacillus, will not survive for long and will be undetectable in the mash within a month or two. In contrast, the restrictive conditions will select for those organisms capable of tolerating a high salt, high osmotic pressure, low aw environment. Among the lactic acid bacteria that can tolerate these conditions and predom inate the early stages of the mash fermentation are strains of Lactobacillus delbrueckii and Tetragenococcus halophilus (formerly Pedio-coccus halophilus).Although they may be present initially at low levels (<103/ml) in the mash, they can reach cell densities of 107 cells/ml or higher after six to eight weeks. It is now common to add selected strains of these organisms to the moromi, rather than rely on their natural occurrence.

Salt-tolerant yeasts, Zygosaccharomyces rouxii and Candida versitalis, are also typically added to the mash.These yeasts are even more salt- and acid-tolerant than the lactic acid bacteria. When the mash pH falls below 5.0, growth of L. delbrueckii and T. halophilus (in the presence of more than 18% salt) will be inhibited, and yeasts will dominate the rest of the fermentation. Eventually, when the pH reaches 4.0, even Z. rouxii will stop growing.

The manner in which the lactic acid bacteria and yeasts grow during the soy sauce fermentation, therefore, resembles that of a succession-type fermentation.

A wide variety of fermentation end products accumulates in the mash and is responsible for the complex flavor of the finished soy sauce (Table 12-4). Growth of the homofer-mentative T.halophilus on glucose results primarily in lactic acid formation. Fermentation of other sugars, particularly pentoses, however, may yield heterofermentative products including ethanol, acetate, and CO2, as well as other organic acids and alcohols. Acids, amines, ammonia, and other products may be produced from amino acid metabolism. The subsequent sugar fermentation by the yeasts also generates ethanol and CO2 via the ethanolic pathway, as well as many other higher alcohols, esters, furanones, and other flavor volatiles.

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