The main yeast species is Saccharomyces cerevisiae. Other microbes important in wine making include lactic acid bacteria Oenococcus oeni (Leuconostoc oenos) that perform malolactic fermentation, the “conversion” of malic acid into lactic acid. responsible for the roundness of Cabernet and some of the buttery qualities of Chardonnays. Lactobacillus can be also used in malolactic fermentations (e.g., Lactobacillus plantarum for Lallemand) but some species are contributing to spoilage. Lactobacillus kunkeei, for example, can increase the volatile acidity of wine in 48 hours. Lactobacillus fructivorans, aka “Fresno mold,” can fill a bottle of wine with cotton candy-like filaments. And Lactobacillus hilgardii may be responsible for the aroma of dead mouse that plagues the rare bottle of red.
Schizosaccharomyces pombe is another yeast that could contribute to winemaking or produce off-flavors. And so is Brettanomyces (Bret) that can perfume a wine with rank notes of sewage and animal sweat or infuse it with aromas of leather and spice as it produces 4-ethylphenol, 4-vinylphenol, and isovaleric acid from grape-derived compounds. Pediococcus (Pedio) adds excessive diacetyl (a buttery or butterscotch aroma), biogenic amines, dirty sock aromas and, worst of all, “ropiness:” when microbes create long polymers of sugar molecules, dextrans. Pediococcus is very important in fermenting veggies like sauerkraut and may add desirable “complexity” to wines in these cases.
Acetobacter produces large quantities of acetic acid (vinegary flavor), acetaldehydes (“bruised apple” aroma), and ethyl acetate, which contributes the notorious “nail polish remover” scent. Yeast Kloeckera/Hanseniaspora covers the surface of good, unspoiled grapes at harvest time and could contribute substantially to the early stages of fermentation, adding to the rich fragrance of spontaneous wines that start off low, slow, and cool.
Botrytis cinerea (Botrytis) is a fungus that, for all its grey, fuzzy unpleasantness, often goes by the rarefied name “noble rot” as it could add special flavors to wines under the right conditions. Botrytis infects grapes during cool, wet weather and countributes to some of the world’s most prized wines: Sauternes, Tokaji, and trockenbeerenauslese. It helps to concentrate the sugars, discharging metabolites such as glycerol, and producing an enzyme (laccase) that contributes to the deep golden color of these wines. Specific film-dorming strains of Saccharomyces cerivisiae: beticus, montuliensis, cheresiensis, and rouxii known as Flor act like living plastic wrap, protecting the wine from direct exposure to air. These yeasts metabolize ethanol producing the huge amounts of acetaldehyde that bestow that unique nutty, bruised-apple quality to Sherry-style wines.