Lactic acid
Lactic acid is an organic compound formed by lactic acid bacteria, such as Lactobacillus, Pediococcus, and Enterococcus, and to a lesser degree by Brettanomyces in lambic. It is chemically similar to acetic acid, differing in that lactic acid has an alcohol group attached to the second carbon. Lactic acid is chiral and posesses two optical isomers, (S)-lactic acid and (R)-Lactic Acid. In nature, the (S)- isomer is by far the most common.
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Chemical and physical properties
Lactic acid is a low molecular-weight acid, with a boiling point of 122 C under standard conditions and a vapor pressure around 10 Pa at room temperature. With a lower vapor pressure than acetic acid, lactic acid is less volatile and only contributes slightly to the odor of Lambic. Lactic acid is ten times more acidic than acetic acid, as measured by the propensity to lose a proton. This gives lactic acid a soft, acidic taste even in low concentrations. Lactic acid is a common part of many food products and is primarily responsible for the tart, fresh taste of yogurt, keffir, cottage cheese, and other sour milk products as well as many non-dairy products such as kombucha, sourdough bread, and Lambic.
In Lambic
Lactic acid is an important component of lambic and is normally found in concentrations of 4-8 g/l in the final product. Much of the "sourness" of lambic can be directly attributed to the presence of lactic acid. Inital quantities of lactic acid are produced by the enteric bacteria in the first weeks of fermentation, usually increasing to around 1 g/l, which remains roughly constant until 4-8 months, at which point lactic acid bacteria begin to dominate the bacteria flora in the lambic [1]. Excessive activity at this point can lead to concentrations of lactic acid of up to 13 g/l[ref] as well as sugar/protein chains that cause the Lambic to take on a "ropy" texture. Normally, these ropes are hydrolized by Brettanomyces, but in excessively ropy lambic, the condition may persist.
As a precursor to other molecules in Lambic
Lactic acid can combine with ethanol to form the ester ethyl lactate, via the reaction:
CHOHCOOH + CH3CH2OH -> CHOHCOOCH2CH3 + H2O
Ethyl lactate has a buttery, coconut like scent in the concentrations that are likely to be found in lambic [ref].
References
- ↑ D. Van Oevelen, M. Spaepen, P. Timmermans and H. Verachtert, ASPECTS OF SPONTANEOUS WORT FERMENTATION IN THE PRODUCTION OF LAMBIC AND GUEUZE, 1977