User:TriNotch/Zygosaccharomyces rouxii

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Zygosaccharomyces rouxii

Formally known as Saccharomyces rouxii, Zygosaccharomyces rouxii is a homothallic diploid yeast and fructophilic spoilage and fermenting agent. This organism is known to cause spoilage of foods as it is salt-tolerant and osmotolerant. The Zygosaccharomyces genus consists of six species, where the Z. bailii, Z. bisporus, and Z. rouxii are the most relevant regarding the spoilage of foods and beverages. Colonies of this yeast have a wrinkly, white, or light cream appearance and grow more slowly on standard yeast medium. Although there continues to be research on investigating the natural habitat of the yeast, it is well accepted that any habitat rich in sugars, such as rotting fruits and plant nectars, are optimal locations for this yeast. Additionally, Zygosaccharomyces rouxii is used in the cultural staple of soy sauce production and other fermentation processes such as balsamic vinegar production. Advantages of the development of the halo- and osmo-tolerant adaptations are what detects where the microbe can survive, including many allegedly non-perishable food items.

Taxonomic Position:
Zygosaccharomyces rouxii is a genus of yeasts in the family Saccharomycetaceae. It was first described under the genus Saccharomyces, but in 1983, was reclassified to its current name.

Chemistry:
Zygosaccharomyces rouxii’s incredible resistance to high osmotic pressure is due to the intracellular accumulation of the polyols glycerol and d-arabitol, which maintain osmotic equilibrium. This equilibrium stabilizes cellular structure and metabolism. High NaCl concentrations increase osmotic pressure and create a toxic effect on cell metabolism and membrane function. Therefore, to compensate for the harmful effects and ensure intracellular cation homeostasis, the extrusion of sodium cations occurs. Z. rouxii resistance to the stress imposed by high salt concentrations involves three plasma-membrane transporters (ZrSod2-22p, ZrSod2p, and ZrSod22p). The transporters rely on the H+-gradient, which explains why plasma membrane H+-ATPase is activated under salt stress. There was a different response of Z. rouxii to osmotic stress at the level of plasma membrane structure which decreased membrane fluidity, caused by increased fatty acid saturation degree.

Distribution and Habitat:
The habitat-metabolism relationship in Z. rouxii is extraordinarily unique. The ideal conditions for the microbes are extreme growth conditions such as extreme salinity or sugar. Ironically, Z. rouxii can also be found in food products generally considered “non-perishables.” Z. rouxii is the most prominently found microbe in spoiled syrups, honey, fruit juice concentrates, jams, jellies, sauces, sodas, and various shelf-stable condiments like ketchup. In addition, Z. rouxii can grow in foods with low water availability leading to the production of alcohol and off-flavors. There have been numerous scientific attempts to increase the sugar content of preserved fruits to prevent yeast growth, but many have been unsuccessful.

Uses:
The yeast Zygosaccharomyces rouxii is responsible for the production of soy sauce. The moldy soybeans that are critical for soy sauce contain the mold Aspergillus oryzae (known as koji), which provides enzymatic power to break down the components of soybeans. A brine solution is then added to the moldy beans to create a moromi mash where fermentation by lactic acid bacteria proceeds. Zygosaccharomyces rouxii requires a low pH. When the pH is in ideal conditions caused by the lactic acid bacteria, the yeast produces chemicals that are essential flavors of soy sauce. Like other fermentation tactics, several months of aging are required, where then, eventually, the moromi mash is pressed to obtain what consumers view as soy sauce. Surprisingly, foods with high amounts of sugars, such as jams and honey, although they may appear moist to the average consumer, are not as the free water is bound to sugars. Therefore, foods with high amounts of sugars have low water activity and are an extreme and unlivable condition to most organisms. Most organisms cannot survive as their cells cannot stay intact with the bit of water present in the environment. Alas, the persistent xerophilic, meaning dry-loving and sugar-tolerant Zygosaccharomyces rouxii, can thrive and reproduce. ￼