What is lactic acid in coffee?

Lactic acid is an organic acid produced by lactic acid bacteria (LAB) during the fermentation of coffee. In controlled fermentation, LAB convert sugars into lactic acid - the same process that produces yogurt, cheese, and sourdough bread. In coffee processing, lactic acid contributes a soft, creamy, mild acidity that is distinct from the brighter, sharper character of citric acid or the fruity sharpness of malic acid.

Lactic acid is the target compound in lactic processing - the method popularised by La Palma y El Tucán where fermentation conditions are deliberately managed to favour LAB over other organisms. The resulting coffees are often described as having a clean, dairy-like sweetness, smooth acidity, and creamy mouthfeel rather than sharp fruit character.

In standard washed and natural processing, lactic acid is produced as a natural by-product of fermentation alongside many other compounds. Its concentration in the final cup depends on fermentation duration, temperature, microbial population, and how much lactic acid survives into the dried bean and through roasting. Some lactic acid is produced during roasting itself. At moderate concentrations it adds smoothness and roundness to a coffee's acid profile; at high concentrations it can produce a yogurt-like or dairy ferment note that's distinctive but can be polarising.