Malolactic (Secondary) Fermentation
Malolactic fermentation is the process by which the sharper malic acid is converted into the softer lactic acid through the influence of non-spoilage bacteria. Putting your red or white wine through this process might be desirable if the total acidity of your new wine is so high that wine is almost too tart to drink. In addition, malic acid is a food source for a spoilage bacteria, so getting the malic acid out of the system might be a good idea even if your wine does taste balanced.
Malolactic fermentation can take place on its own, but adding a prepared culture will likely yield better results. Usually, malolactic starter cultures are packaged in such a way that one package is good for between five and ten gallons of wine. Once the amount of malic acid drops below .03 grams per liter, malolactic is finished and the wine can be aged further.
It is perfectly fine to rack your wine before malolactic fermentation has ended. The key is to keep your sulfur dioxide addition low enough such that malolactic fermentation can finish.
A by-product of malolactic fermentation is diacetyl, a compound that creates a butterscotch or butter aroma in white wines. While diacetyl is present in red wines, its concentration is typically not high enough to be detected. The levels of diacetyl produced depend on factors ranging from the bacteria strain used to whether citric acid was used to acidify.
