What is acetic acid in coffee?

Acetic acid is the organic acid responsible for the sharp, vinegary taste and smell of vinegar. In coffee processing, it is produced by acetic acid bacteria (AAB) - obligate aerobic organisms that convert ethanol (produced by yeast fermentation) into acetic acid in the presence of oxygen.

In most processing contexts, the presence of acetic acid at significant levels is considered a defect - a symptom of fermentation that has progressed too long, at too warm a temperature, or in conditions where AAB have proliferated uncontrollably. Coffees with ferment defect taste sharply sour, vinegary, or like rotting fruit - what cuppers sometimes describe as compost bin or salad dressing notes.

However, in low concentrations, acetic acid contributes to the perception of fruity and floral brightness in the cup - research has shown it can actually enhance the sensory character of coffee at controlled levels. This is the basis of intentional acetic processing, where producers deliberately encourage AAB growth under controlled aerobic conditions to produce complex fruitiness rather than defect. The line between controlled acetic complexity and unpleasant ferment defect is narrow and determined primarily by concentration. Understanding acetic acid helps explain both why fermentation faults taste the way they do and why some experimental processes deliberately court the same bacteria that cause them.