What is acetic process coffee?

Acetic process is a named processing method that deliberately encourages the growth of acetic acid bacteria (AAB) during fermentation - organisms that produce acetic acid as their primary metabolic by-product. Unlike most fermentation methods that aim to minimise acetic acid (which in excess produces undesirable vinegary, sour defect notes), acetic processing uses it intentionally as a flavour variable.

AAB are obligate aerobes - they require oxygen for their metabolism. Acetic process therefore involves fermentation in an oxygen-rich environment, typically with the coffee being stirred or mixed regularly to maintain aerobic conditions. La Palma y El Tucán in Colombia popularised the naming convention, using a vessel where pulped coffee is continuously mixed during fermentation to encourage AAB alongside aerobic yeasts.

The goal is to harness acetic acid in controlled, low concentrations where it contributes fruity or floral character rather than the vinegary defect associated with out-of-control fermentation. The line between intentional acetic complexity and fermentation fault is genuinely narrow - temperature control, timing, and cherry quality all determine which side of that line a lot lands on. For buyers, acetic process coffees are typically intensely flavoured and fruit-forward, and knowing the process helps set expectations for what you're likely to find in the cup.