Acetic acid

Acetic acid is an organic compound formed in lambic by the oxidation of ethanol. An organic acid, it is a colorless liquid in its pure state with a strong acidic odor. In more dilute solution, the odor can take on a smell of overripe fruit. Acetic acid is best known as the primary flavor component of vinegar.

The synthesis of acetic acid can occur through direct oxidation of ethanol, though this process is very slow in anaerobic conditions [ref]. Some bacteria, noteably Acetomonas and Acetobacter can catalyze the conversion of ethanol to acetic acid, greatly speeding up the process. The former can continue the oxidation process to fully convert ethanol to carbon dioxide and water, but the latter species is incapable of progressing beyond acetic acid. Both of these bacteria require oxygen for the reaction to take place, so the production of acetic acid occurs early in the fermentation process, when the wort is still well-oxygenated. After about ___ weeks, the production of acetic acid trails off and the concentration will remain the same until the end of fermentation [1]. As acetic acid bacteria are ever-present in the wort, exposure to excessive oxygen at any time during fermentation will lead to the formation of large amounts of acetic acid, sometimes up to 4,000 mg/l [1], at which point the lambic is referred to as "hard", though it is still useful for blending [2].

Acetic acid can combine with ethanol to form the ester ethyl acetate, via the reaction:

CH3COOH + CH3CH2OH -> CH3COOCH2CH3 + H2O

Ethyl acetate has a green-apple like scent in the concentrations that are likely to be found in lambic [ref].

[1] MICROBIOLOGICAL ASPECTS OF SPONTANEOUS WORT FERMENTATION [2] Guinard, "Lambic"