Beyond yeast: the essential role of lactic acid bacteria in wine production

The rich tradition of winemaking is not solely a product of the renowned yeast fermentation process. While yeast has always been acknowledged as the primary catalyst in converting grapes into wine, the entire process is significantly more complex and nuanced. Among the myriad of elements that contribute to the transformation of grapes into this celebrated beverage, a group of bacteria known as lactic acid bacteria (LAB) play an essential role. These bacteria, often associated with cheese and yogurt, have a critical function in reducing the natural acidity of wine. This process, typically referred to as malolactic fermentation (MLF), is paramount in the flavor development of various types of wines, particularly reds.

Malolactic fermentation: more than just fermentation

The term "malolactic fermentation" may seem slightly misleading, as many experts, especially chemists, argue that it is not a fermentation process but a metabolic pathway or a degradation. This process begins after the normal alcoholic fermentation (the transformation of sugar into alcohol and carbon dioxide by yeast) has concluded. It is either triggered post pressing or in spring, with a temperature increase in the wine. The optimal temperature for this degradation to occur is around 20ºC.

During malolactic fermentation, the bacteria transform malic acid, the predominant acid in wine, into lactic acid, a milder form. The malic acid is diacidic, while lactic acid is monoacidic, hence less acidic. This process often leads to the release of carbon dioxide, sometimes misleadingly suggesting that the yeast has reappeared to break down untransformed sugars.

The influence on taste and aroma

Malolactic fermentation is more common in red wines than in whites and in wines from cold climates than in those from warm regions. When properly conducted, this transformation rounds out the wine's quality by replacing the strong, acidic taste of malic acid with a smoother lactic flavor.

In northern climates, the importance of this phenomenon becomes evident for wines born with high fixed acidity, even in white wines, or those whose high acidity makes them difficult to consume. As the acidity decreases, the color is enhanced, and red wines acquire less vibrant shades. Even the aroma changes, attenuating primary fruity scents, particularly those reminiscent of apple, and emphasizing secondary aromas like butter or vanilla.

Taste-wise, wines gain body and creaminess after malolactic fermentation, becoming denser and softer. However, the benefits of MLF for red wines may not always be desirable for white wines, as the loss of acidity can diminish their freshness. Nevertheless, in very cold climates, malolactic fermentation may be advisable for certain white wines.

The importance of stability and monitoring

Proper supervision by the winemaker is vital to ensure the necessary biological stability of the wine and avoid unpleasant surprises for the consumer, such as sedimentation or wine death due to lack of acidity. Guiding the malolactic fermentation process enologically, once normal fermentation is complete, can mitigate these risks.

In aging processes with flor yeast, a notable decrease in malic acid and an increase in lactic acid are also observed, undoubtedly enhancing the quality of specific wines like finos and manzanillas.