How does fermentation preserve food and change its flavor?

Fermentation preserves food primarily through the production of acids and alcohol by microorganisms like bacteria, yeasts, and molds. Lactic acid bacteria, for example, convert sugars into lactic acid, dropping the pH and creating an environment too acidic for harmful pathogens to survive. Alcohol produced by yeasts similarly inhibits spoilage organisms. The flavor changes come from the same metabolic byproducts. Acids add sourness, as in yogurt or sauerkraut. Yeasts produce esters and other compounds that create fruity or complex aromas in beer and wine. Proteolysis and lipolysis, where proteins and fats are broken down by microbial enzymes, generate savory, pungent, or funky flavors found in aged cheeses and cured meats. The specific microbes and conditions determine the final flavor profile.