Acetic Acid |
Acetic acid, or more commonly known as the active ingredient in vinegar, is produced during two stages of coffee processing: post-harvest fermentation and roasting. During wet processing microbes within the bean consume sugars via fermentation to produce acetic acid.The concentration of acetic acid during fermentation is highly dependent on ambient temperature and conditions of the batch. Once processing has been completed, only a small portion (0.01%db) of acetic acid remains within the bean after fermentation.
However, it is during the roasting process that the formation of acetic acid significantly increases. During roasting, small to medium chained carbohydrates - namely sucrose - breakdown resulting in the formation of aliphatic acids such as acetic, formic and over twenty organic acids1,2. Depending on actual roasting conditions, acetic acid concentrations can increase up to 25 times its initial green bean concentration. Acetic acid concentrations reach a maximum at light to medium roasts, or roughly 240oC, then quickly dissapates with further roasting due to its volatile nature2. Table 1 summarizes acetic acid concentrations in green and roasted coffee.
Acetic acid is a volatile, weak organic acid dissociating only 7% in solution but plays a signifiant role in both perceived acidity and aroma formation4. According to Sivetz, a noticable increase in acidity can be gained by the use of a pressure-roaster which is believd to retain volatile acids3. In low concentrations acetic acid imparts a pleasant clean, sweet-like charateristic to coffee infusions, but can quickly become ferment-like at higher concentrations.
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