Winemaking: Malolactic Fermentation

So what makes Malolactic fermentation so important in the winemaking process? Most red wines and some whites go through a secondary fermentation, also known as Malolactic fermentation (MLF). Carried out by lactic-acid bacteria, it involves transforming malic acid into lactic acid. Malic is stronger than lactic, so this change softens the wine’s acidity. Normally start after the alcoholic fermentation is complete. Some winemakers prefer to add a ML culture, before the alcohol fermentation is complete when the alcohol concentration is still low. The alcohol and malolactic fermentations will proceed simultaneously. After the alcohol fermentation is complete, the wine needs to stay on lees, which will help to finish the MLF.



There are three general kinds of lactic-acid bacteria: Oenococcus (the most important), Leuconostoc, and Pediococcus. They are presented in all wines, but can be problematic. They can produce off-flavours and turbidity and have been reported to degrade components such as glycerol and tartaric acid into bitter substances and volatile acidity. In red wines, the lactic-acid bacteria remain high after completion of MLF, and can make large amounts of biogenic amines. Using selected starter cultures of bacteria that lack amino-acid decarboxylase activity and then adding SO2 at the end of MLF are popular control strategies. If the wines didn’t go thru malolactic fermentation, lactic-acid bacteria can cause secondary fermentation in the bottle, which will form small bubbles of CO2 and drop of some sediment. MLF also affects the color of red wines. The best strategy for color stabilization seems to be to inoculate with lactic-acid bacteria at the end of alcoholic fermentation, and then allow MLF to proceed slowly at low temperature (12-14 °C / 54-57 °F). MLF in barrel is now common for red white wines. The wines are pressed and transfer into barrel at the end of alcoholic fermentation; this is supposed to get a better integration of oak, which will improve the "texture" of the wine.



The lactic-acid bacteria didn’t just metabolize malic acid. They are also metabolizing some other organic acid, sugars, and aromatic compounds present in much smaller quantities. MLF also produces ethyl lactate, which enhances the sensation of "body", and diacetyl, which smell buttery at higher concentrations.



Control of lactic-acid bacteria is fairly straightforward. They are sensitive to SO2 and low pH in wine, and prefer high temperatures, so keeping temperatures down and maintaining decent free SO2 concentrations is quite effective. Also, frequent racking deprives these bacteria of nutrient resources from the decomposition of yeast lees.