Saccharomyces cerevisiae is the most economically important yeast species, used for millennia in baking, brewing, winemaking, and distillation. It is a budding yeast (Ascomycete) that can grow both aerobically and anaerobically. Under aerobic conditions, S. cerevisiae oxidizes glucose to carbon dioxide and water via respiration. Under anaerobic conditions, it ferments glucose to ethanol and carbon dioxide through the Embden-Meyerhof-Parnas pathway followed by decarboxylation of pyruvate to acetaldehyde and reduction to ethanol. The Crabtree effect describes the repression of respiratory metabolism by high glucose concentrations, favoring fermentation even in the presence of oxygen.
In brewing and winemaking, yeast strain selection is critical for product quality. Ale yeasts (S. cerevisiae) are top-fermenting and operate at 15-25°C, producing fruity esters and phenolics. Lager yeasts (S. pastorianus, a hybrid of S. cerevisiae and S. eubayanus) are bottom-fermenting and work at 8-15°C, yielding a cleaner profile. In winemaking, S. cerevisiae is often inoculated after a spontaneous fermentation phase by indigenous yeasts (Hanseniaspora, Candida) that contribute complexity. Fermentation monitoring includes tracking specific gravity, ethanol concentration, pH, and volatile acidity.
Bread leavening relies on CO₂ production by S. cerevisiae, with commercial baker’s yeast available as compressed cream yeast, active dry yeast, and instant dry yeast. Dough fermentation also generates flavor precursors including organic acids, alcohols, and esters. Sourdough fermentation involves a symbiotic culture of LAB and yeasts (primarily Candida milleri or Kazachstania exigua), producing lactic and acetic acids that give sourdough its characteristic tang and extended shelf life.
Non-Saccharomyces yeasts are increasingly recognized for their contributions to fermented foods and beverages. Brettanomyces bruxellensis, while considered a spoilage organism in many wineries due to its production of volatile phenols (4-ethylphenol, 4-ethylguaiacol), is deliberately used in certain beer styles (Lambic, sour ales). Pichia, Kluyveromyces, and Torulaspora species are used in bioflavouring and biopreservation. These non-conventional yeasts exhibit diverse metabolic capabilities, including the production and degradation of organic acids, polyols, and aromatic compounds. Yeasts interact with lactic acid bacteria in many traditional fermentations, such as sourdough and kefir. Mold fermentations are used in soy sauce and cheese production, while yeasts are essential for fermented dairy products like kefir.
