Zygosaccharomyces rouxii
Japanese: 醤油酵母（Zygosaccharomyces rouxii）
A halotolerant yeast crucial for miso and soy sauce brewing. Adapted to high-salt and high-sugar environments, it produces aromatic compounds such as HEMF (4-hydroxy-2(5)-ethyl-5(2)-methyl-3(2H)-furanone).
Zygosaccharomyces rouxii is the leading yeast responsible for creating aroma in miso and soy sauce mash. It possesses remarkable salt tolerance, thriving even at 18% salinity.
HEMF (4-hydroxy-2(5)-ethyl-5(2)-methyl-3(2H)-furanone), a compound this yeast produces, imparts a sweet, rich fragrance to miso and soy sauce. It increases in the later stages of fermentation, signaling the progress of aging through scent.
To endure high osmotic pressure, it employs an elegant adaptation strategy: accumulating glycerol within its cells. It turns a harsh environment into its own home. This resilience is what leads to the aromatic richness of fermented foods.