Fermentation is the process by which microorganisms, usually bacteria or yeast, break down sugars and other compounds in food, producing new flavors, aromas, textures, and often alcohol or acid as a byproduct. It has been used for thousands of years to preserve and transform food; the earliest known evidence of a fermented beverage dates to around 7000 BC in China.
Different fermentations use different organisms for different results: lactic acid bacteria sour foods like yogurt, sauerkraut, kimchi, and sourdough; yeast drives alcoholic fermentation in wine, beer, and sake; and koji mold underlies soy sauce, miso, and sake production. Many fermented foods contain live probiotics and can make certain nutrients easier to absorb. Note that fermented does not mean the same thing as pickled or cured: vinegar pickling, for instance, preserves food with acid rather than live microbial activity.