souring

Souring is the development of a sour taste or acidity in a substance, typically achieved through microbial metabolism or chemical reactions. In food and drink, souring can be deliberate, as in fermentation, or it can indicate spoilage when undesirable organisms dominate.

The main mechanisms involve acid production. Lactic acid bacteria convert sugars into lactic acid, contributing tanginess to dairy products like yogurt and sour cream, as well as to fermented vegetables such as sauerkraut and kimchi. Acetic acid bacteria oxidize ethanol to acetic acid, giving vinegar its characteristic sharpness. Other pathways include the production of propionic, butyric, or malic-derived acids during various fermentations, which shape the final sourness and flavor profile. Souring can also occur in beverages through spontaneous or controlled fermentation, as seen in sour beers and lambics, or during malolactic fermentation in wine, which softens acidity by converting malic acid to lactic acid.

Contexts of souring vary. In dairy, soured milk or cultured dairy products arise from deliberate fermentation or unintended spoilage. In vegetables and grains, fermentation builds complexity and preservation. In beverages, souring can be a defining feature or an unintended spoilage that alters flavor and aroma. Spoilage souring is typically associated with off-putting smells, curdling, or visible spoilage, and is usually considered unsafe for consumption.

Overall, souring describes a broad set of phenomena in which acidity and sour flavors emerge from biological or chemical processes, with outcomes ranging from culinary techniques to food safety concerns.