Monday, March 31, 2008

Malolactic fermentation

Every Spring, as the weather starts to warm up, the young wines which have been ageing peacefully in barrel since October suddenly start to change; their acidity softens and they become temporarily cloudy and a little fizzy. Incredibly the reason for this was totally unknown until explained by professors at UC-Davis in the late 1960’s…

The alcoholic fermentation had been well understood since the 19th century; yeast convert sugar to ethanol and CO2, but the professors at UC-Davis discovered that the mysterious Springtime changes were also due to a fermentation, this time conducted by bacteria, converting malic acid to lactic acid and CO2. Initially this ‘malolactic’ fermentation was considered a spoilage, and indeed if it takes place in bottle the wine is certainly spoiled (fizzy, with unpleasant sulphurous aromas). Conversely, if the fermentation takes place in barrel the CO2 and sulphurous aromas can escape and the wine is positively improved by the softer lactic acid. Now almost all red wines complete malolactic before bottling, and wineries everywhere follow the progress of their malolactic fermentations by paper chromatography...

A drop of each wine is placed at the bottom of a sheet of chromatography paper before the paper is stood up with its bottom edge in a thin pool of butanol, acetic acid and pH indicator solution. As the butanol wicks up the paper it carries with it the organic acids contained in the wine; the lighter acids being carried faster (and hence further up the paper) than the heavier acids. Once the butanol has reached the top the chromatogram is ‘developed’ by drying the paper. The drying process evaporates the acetic acid leaving the background blue and the acid spots yellow.
The developed chromatogram above shows the results for wines from 14 different barrels of my 2007 harvest. Reading from the left, first 5 barrels have finished malolactic, the 6th has just started, the next 3 are approaching completion, and the last 5 have also just started.


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