How Malolactic Fermentation Softens Wine

malolactic fermentationMacerating red grapes during fermentation to extract color

Malolactic fermentation is frequently referred to as "MLF", or "malo" in winery terms. What is a malolactic fermentation and how does it affect a wine?

MLF normally occurs after the primary fermentation has completed, when the grape sugars have been converted by yeast, into alcohol and carbon dioxide (CO2). Lactic acid bacteria, Oenococcus oeni, and some other species of Lactobacillus and Pediococcus are responsible for this conversion.

The major function of these bacteria is to convert the main wine acid, (there are two main acids and many minor ones found in grapes) called L-malic acid, to another type, L-lactic acid. During this conversion CO2 is produced hence the term, fermentation.

Many wines that undergo a malolactic are improved by it, however those wines that rely on a higher acid level to show their full potential, such as Rieslings and Sauvignon Blancs, or similar younger varietal styles would never benefit from it. They require the crispness in the acidity and youthful freshness to show through.

The wines to benefit from MLF tend to be those fuller bodied dry whites as well as medium to full bodied reds. It is these heavier wines that need to be somewhat softened to ensure an attractive lower acid to fruit balance. MLF will have a dramatic effect in softening the tannins, especially in red wines. Tannins in wine are harsh and bitter, and can be felt during tasting as they make your mouth pucker.

Lactic acid bacterial growth is inhibited by cooler temperatures, and also the addition of sulfur dioxide (SO2) - an anti bacterial agent. Winemakers are able to stop the onset of MLF, and can therefore control the styles of wine being made by maintaining SO2 levels and lowering the wine temperature throughout the entire winemaking process.

Putting a wine through a malo, when in tank or barrel, is encouraged during winemaking as it will be less likely to try and repeat the process when it is in the bottle. If a wine unintentionally undergoes MLF when in bottle, it can be disastrous to the consumer. Apart from the wine losing its fruit integrity, it will appear to still be fermenting as CO2 will be being produced. An unpleasant lactic aroma would be detected.

How Malolactic fermentation effects taste

It is considered that MLF enhances the body and flavour of the wine, producing wines of greater palate softness and roundness. Most winemakers believe that there is much better integration of fruit and oak if MLF occurs while the wine is in barrel. This fuller mouthfeel is more pleasing to the palate.

It has been noted that malic acid resembles the taste of green apples, (malic comes from the Latin word for apple, malum). An appley taste is often to be found in wine descriptions. In contrast, lactic acid is prominent in milk and is much richer tasting, more like full-fat butter, (lactic is derived from the Latin word for milk, lac).

The mouthfeel of malic acid can be described as "hard and metallic" against the "softness" of the lactic acid. MLF is a natural de-acidification and softening of the wines palate.

MLF can happen naturally or be initiated artificially

In commercial winemaking, a malolactic conversion is often initiated by an inoculation of desirable strains of bacteria. This ensures that off-flavours will not be produced. Winemakers can also prevent malolactic conversion when it is not desired, so a higher acid profile can be maintained in the finished wine.

Malolactic fermentation is natures way of softening wine by making it much more acceptable to the palate and easier to drink.


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By Rob Hemphill