There are many tools in a winemakers arsenal to make their wine more complex and nuanced. Some are well understood, others – such as malolactic fermentation (often referred to as ML, or malo) – are hugely important, but less well-understood by most wine drinkers.
Primary fermentation occurs in wine when yeast converts the sugars in the grape juice into alcohol. This is the fundamental process in winemaking – it is what makes wine wine. After that primary fermentation, however, a secondary fermentation can also take place. In this fermentation a type of lactic acid bacteria (Oenococcus oeni, for example) converts the L-malic acid in wine into L-lactic acid (along with a bit of carbon dioxide).
Malolactic fermentation has a number of possible consequences – some good, some bad, and some a matter of taste. First and foremost, malic acid is perceived as a much rougher acid – sharp, metallic, and hard are all words used to describe it. Lactic acid, by contrast, is often described as soft and round. So the process of malolactic fermentation often results in a ‘softer’ mouthfeel, and a more ‘drinkable’ wine.
At its best, malolactic fermentation produces a characteristic generally described as buttery. This is particular desirable in certain styles of chardonnay. At its worst, malolactic fermentation can produce the taste of curdled milk, pickled foods, or rancid butter.
Many winemakers feel that malolactic fermentation in barrel can help better marry the fruit and oak characteristics of an oak-aged wine. This is most apparent in the buttery-oaky style of chardonnay, where winemakers use malolactic fermentation to create a more integrated wine.
Running a wine through malolactic fermentation in the barrel has another advantage – the wine then will not go through ML while in bottle, which can produce undesirable carbon dioxide and destroy the fruit notes of the wine. For wines that cannot go through ML – or wines in which the winemaker wants to retain the sharp acid notes – the winemaker may use sulfur dioxide to stop ML from ever occurring. Generally this is used in lighter-bodied white wines which rely on their fruit notes – riesling is an excellent example of this. Malolactic simply dulls the acid needed to balance the wine, so virtually all rieslings have their malolactic fermentation process stopped before it begins.