How does fermentation preserve food and create flavor?

Fermentation preserves food primarily through the production of acids and alcohol by microorganisms like bacteria, yeast, and molds. Lactic acid bacteria, for example, convert sugars into lactic acid, dropping the pH of the food and creating an environment too acidic for harmful pathogens to survive. Alcohol produced by yeast similarly inhibits spoilage organisms. The flavor development comes from the same metabolic processes. Bacteria and yeast produce a complex mix of organic acids, esters, alcohols, and other compounds as byproducts of breaking down sugars, proteins, and fats. In cheese, proteolysis breaks proteins into savory amino acids. In sourdough, acetic and lactic acids create tang. Each fermentation environment produces a distinct chemical signature, which is why kimchi, wine, and yogurt all taste so different despite sharing the same basic biological process.