Fermentation

Fermentation is the metabolic process by which microorganisms (bacteria, yeasts, molds) convert sugars into other compounds—primarily alcohol, acids, and gases—in the absence of oxygen. It is one of humanity's oldest food technologies, predating written history. Alcoholic fermentation (by yeasts like Saccharomyces cerevisiae) converts glucose into ethanol and CO2—this makes bread rise and beer ferment. Lactic acid fermentation (by Lactobacillus bacteria) converts sugars into lactic acid—this makes yogurt tangy, sauerkraut sour, and kimchi pungent. Both pathways preserve food by creating environments hostile to spoilage organisms.