If Saccharomyces cerevisiae is the king of wine and beer, then Zygosaccharomyces rouxii is the king of soy sauce. The production of soy sauce is a multi-stage fermentation with many different microbes involved. But the production of the essence of soy sauce, that caramel-like odor that is the heart of any high-quality soy sauce, has been mastered by the yeast Zygosaccharomyces rouxii.
Macro- and Microscopic Appearance
When grown on plates in the lab, colonies of this yeast can have a very wrinkly appearance, as is seen below. The colonies are white or light cream in color. Compared to Saccharomyces, these colonies tend to grow more slowly on standard yeast medium, such as YPD.
Under the microscope, the cells of this yeast are circular to elliptical. In the photo below, you can see pegs or nipples coming off from some of the cells. This is where one cell is budding to produce another cell.
Soy sauce starts with moldy soybeans. The mold Aspergillus oryzae (known as koji) grows on the beans and provides enzymatic power to break down the components of soybeans. A brine solution is added to the moldy beans to create a moromi mash and fermentation by lactic acid bacteria proceeds. Once the pH is lowered by the lactic acid bacteria, Zygosaccharomyces rouxii flourishes. As it undergoes alcoholic fermentation, this yeast also produces chemicals that we perceive as the essential flavors of soy sauce. After several months of aging, the moromi is pressed to obtain the liquid soy sauce.
While Zygosaccharomyces rouxii can contribute beautiful flavors to soy bean fermentations, it does have a dark side. Foods with high amounts of sugars, such as jams and honey, appear moist to us, but don’t actually have a lot of free water because it is bound to sugars. We say that these foods have a low water activity. Most organisms will not survive in these environments because their cells cannot stay intact with so little water in the environment. As one of the most xerophilic (dry-loving) organisms known on the planet and a lover of sugars, Zygosaccharomyces rouxii can grow in these foods with incredibly low water availability leading to the production of alcohol and off-flavors. Numerous studies have tried to increase the sugar content of preserved fruits to prevent the growth of this yeast, but nothing can stop this sugar-loving beast. Who doesn’t like a little booze with their toast and jam?
The most important role of this yeast is producing the flavor molecules of soy sauce. The main aroma compounds produced by Zygosaccharomyces rouxii are 4-hydroxy-3[2H]-furanones. Furanones are widespread flavor compounds that can result from non-biological processes such as Maillard reactions and non-enzymatic browning. They are also produced by microbes and plants (4-hydroxy-3[2H]-furanones are the major flavor components of strawberries). Furanones have very low aroma thresholds meaning that even small quantities of the compounds can be easily detected by humans. In the 19070’s, scientists in Japan discovered that these were the most important flavor elements of soy sauce and that they originated from the biological activities of Zygosaccharomyces rouxii. Despite the importance of this flavor, the details on how the yeast creates these flavors still allude food scientists.
It’s difficult to say where Zygosaccharomyces rouxii spends it’s time when it’s not in a moromi mash pumping out flavor. The natural habitat of the yeast isn’t well understood, but any habitat rich in sugars, such as rotting fruits and plant nectars, are possible hangouts for this yeast. It’s also difficult to understand how it made it into the soy sauce production process in the first place given that fruits aren’t added in the process. Most large-scale soy sauce production factories inoculate the yeast into their mash to ensure consistent flavor development. Zygosaccharomyces rouxii also makes an appearance in other fermentation processes such as balsamic vinegar production.
Post written by Benjamin Wolfe. This is an excerpt from a longer salute to food yeasts that originally appeared in Edition 41 of the World of Fine Wines. It’s a lovely magazine about wine. Be sure to check it out.
Want to learn more about this delicious yeast? Check out these papers:
Tobias Hauck, Fredi Brühlmann, and Wilfried Schwab, “Formation of 4-Hydroxy-2,5-Dimethyl-3[2H]-Furanone by Zygosaccharomyces rouxii: Identification of an Intermediate,” Applied and Environmental Microbiology 69:7 (2003), pp. 3911-18. http://www.ncbi.nlm.nih.gov/pubmed/12839760
Koji E. Aidoo, M.J. Rob Nout, and Prabir K. Sarkar, “Occurrence and Function of Yeasts in Asian Indigenous Fermented Foods,” FEMS Yeast Research 6:1 (2006), pp. 30-9. http://www.ncbi.nlm.nih.gov/pubmed/16423068