Lactic acid bacteria shape wine character through malolactic fermentation

In the world of winemaking, yeast often takes center stage for its role in transforming grape sugars into alcohol. However, another group of microorganisms, lactic acid bacteria, play a crucial part in shaping the final character of many wines. These bacteria, similar to those found in cheese and yogurt production, are responsible for a process known as malolactic fermentation. This process is essential for reducing the natural acidity of wine and enhancing its smoothness and complexity.

Malolactic fermentation involves more than a dozen types of bacteria, including Lactobacillus casei and Pediococcus damnosus. Their main function is to convert malic acid, which is the dominant acid in wine and has a sharp, tart taste, into lactic acid, which is softer and less acidic. Chemically, malic acid is a diacid while lactic acid is a monoacid, making the latter much gentler on the palate.

This transformation typically begins after the primary alcoholic fermentation has finished. The primary fermentation is driven by yeast converting sugar into alcohol and carbon dioxide. Once this stage ends—either after racking or when temperatures rise in spring—malolactic fermentation can start if conditions are right. The optimal temperature for this bacterial activity is around 68°F (20°C). During this process, some carbon dioxide is also produced, which may cause bubbling or slight cloudiness in the wine.

Malolactic fermentation is more common in red wines than whites and tends to occur more frequently in wines from cooler climates. In these regions, grapes often have higher levels of malic acid, resulting in wines with pronounced acidity that can be harsh or difficult to drink. By converting malic acid to lactic acid, winemakers can soften these wines, making them rounder and more approachable. The process also affects color and aroma: as acidity drops, red wines may lose some vibrancy in hue but gain body and smoothness. Aromatically, primary fruit notes—especially those reminiscent of green apple—are subdued, while secondary aromas like butter or vanilla become more noticeable.

For white wines, the decision to allow malolactic fermentation depends on style and climate. In cool regions where acidity can be excessive even in whites, this process can help balance the wine. However, for many white wines where freshness and crispness are desired, winemakers may avoid malolactic fermentation to preserve acidity.

Careful management of malolactic fermentation is essential for ensuring wine stability and quality. If not properly controlled by the winemaker or enologist, unwanted side effects such as sediment formation or spoilage can occur. Monitoring the process helps prevent surprises for consumers and ensures that the wine maintains its intended character.

In some styles of wine aging under flor yeast—such as certain sherries—there is also a notable reduction in malic acid and an increase in lactic acid during maturation. This contributes to the unique texture and flavor profile of these wines.

The most common lactic acid bacteria involved in winemaking include both homolactic species like Pediococcus damnosus and heterolactic species such as Lactobacillus plantarum, Lactobacillus casei, Leuconostoc mesenteroides, and Oenococcus oeni. Each plays a specific role depending on grape variety, region, and desired wine style.

The presence and activity of lactic acid bacteria are fundamental to modern winemaking practices across many regions worldwide. Their ability to transform acidity not only makes certain wines more enjoyable but also adds layers of complexity that define some of the world’s most celebrated bottles.