Acids Promoted by Fermentation and Roast
Acetic and lactic acid are two byproducts of fermentation. When vegetables are fermented in a sealed environment away from oxygen, it’s usually the lactic acid bacteria that predominate. This is very noticeable in sauerkraut and kimchi. Most vegetables are covered in what’s called lactic acid bacteria (LAB), which is also found on coffee cherry skins. The skins of coffee cherries are populated by yeasts, lactic acid bacteria, and acetic acid bacteria. In research from Weers et al., 1995, lactic acid was in such low levels in green samples as to be undetectable, but then by 6 and a half minutes of roasting, it had progressed to 1.4g/kg
When fermentation occurs with sweet fruits like grapes or coffee cherries, the process is usually a function of wild yeasts consuming sugars and converting them into alcohol. The alcohol is then converted into acetic acid (vinegar) by acetic acid-forming bacteria.
Over-fermentation can be a big problem resulting in sour bean defects which can result in an off tasting vinegary flavour taint and an overabundance of acetic acid in affected beans. However, acetic and lactic acids in roasted coffee can positively affect perceived fruitiness. Findings from Weer’s research saw acetic acid advanced from 0.29g/kg in green Colombian arabica, up to 4.98g/kg after roasting; an increase of 1700%!
How Roasting Increases the Abundance of Aliphatic Acids
It is a matter of conjecture whether the nuances in coffee processing affect acetic and lactic acid levels in the end beverage in any meaningful way. It is clear that roasting is the main player in increasing the levels of these two acids in the cup. The roaster affects the content of acetic and lactic acid by the breakdown of sucrose, glucose, and fructose which leads to the formation of aliphatic acids. Of these, the primary constituent in coffee is acetic acid.
Joseph Riviera states, “Depending on actual roasting conditions, acetic acid concentrations can increase up to 25 times its initial green bean concentration.