During the wet process microbes within mucilage consume sugars to produce acetic acids as well as a number of other compounds. Since the reaction is dependent on time and temperature, final concentrations of acetic acid in will vary depending on environmental factors as well as the nature of the bean. It's estimated that after fermentation only a small portion (0.01%db) of acetic acid produced during fermentation will remain within the bean.
Coffee Fermentation Tank - photo courtesy of Edwin Martinez
However, it is during the roasting process where the formation of acetic acid significantly increases. During this stage small to medium chained carbohydrates like sucrose begin to breakdown, resulting in the formation of aliphatic acids such as acetic, formic and others.
Depending on actual roasting conditions, acetic acid concentrations can increase up to 25 times its initial green bean concentration. Overall acetic acid concentrations reach a maximum at light to medium roasts, then quickly dissipate as roasting progresses due to its volatile nature.
The following table summarizes acetic acid concentrations in green and roasted coffee:
Chemically, acetic acid is a weak volatile organic acid, but plays a significant role in both perceived acidity and aroma formation. According to Sivetz, a noticeable increase in acidity can be gained by the use of a pressure-roaster, which is believed to retain volatile acids and improve quality.
Sensorially in low concentrations acetic acid imparts a pleasant clean, sweet-like characteristic, but can quickly become ferment-like at higher concentrations.