Difference between revisions of "Acetic acid"
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| − | 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 | + | 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 6 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 g/l [1], at which point the lambic is referred to as "hard", though it is still useful for blending [2]. |
==As a precursor to other molecules in Lambic== | ==As a precursor to other molecules in Lambic== | ||
Revision as of 18:39, 9 January 2015
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.
Chemical and physical properties
Acetic acid is a low molecular-weight organic acid, with a lower acidity than lactic acid. It is the acid conjugate to ethanol and is often formed by the oxidation therof. Along with water, it is the primary component in vinegar, and thus its flavor and odor are characteristic of vinegar. As a pure liquid, it has a boiling point of ~120 C and a vapor pressure at room temperature around 1000 Pa, indicating moderate volatility.
In Lambic
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 6 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 g/l [1], at which point the lambic is referred to as "hard", though it is still useful for blending [2].
As a precursor to other molecules in Lambic
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"