OCCASIONALLY one comes across what is really a third fermentation, the malo-lactic fermentation. This occurs usually after the wine has been bottled, and often as much as a year or more after it was made. It is something which should be welcomed, when it does occur, for it imparts a very pleasant freshness to a white wine, and does reduce the acidity a little. For this last reason it is important to the winemakers of Austria, Germany and Switzerland, whose grapes tend to contain slightly more malic acid than those from sunnier regions, where the sun will have accounted for most of it before the wine is even made.
Malic acid is the acid to be found in apples, and what happens during the malo-lactic fermentation, as the name indicates, is that a bacterium to be found in all fresh wines (b. gracile) sets to work on the malic acid and converts it into lactic acid. This might not seem much of an improvement, but lactic acid is much less acid than malic, and the acidity of the wine is thus reduced, to say nothing of the very pleasant, clean, freshness with which this slight fermentation endows the wine.
Occasionally, one can bring about such a fermentation by agitating any yeast deposit and bringing the wine into the warm, but usually one can only be duly grateful if it occurs of its own accord
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