The Magic of Malolactic Fermentation

Simple grape juice is transformed into the enjoyable elixir we call wine by the work of invisible microscopic organisms. Primary alcoholic fermentation depends on yeast to convert the sweet sugar in grape juice to heat, bubbly carbon dioxide and alcohol. Yet grapes contain a lot more chemical variety than just sugars. The predominant acids tartaric and malic still present in the fermenting juice can also be acted upon to add to the complexity and balance in wine.

Now here’s where another alchemical process is decided upon by the winemaker – to do malolactic transformation or not.

What is malolactic conversion?

Often referred to as ‘secondary fermentation’ which follows or occurs simultaneously with primary fermentation, MLF changes the acid chemistry and the mouth-feel of a wine.

With the help of benign bacteria (smaller than yeasts), malic acid, the aggressive sharp acid typical of unripe fruit is converted into the softer less sharp lactic acid. When I first started exploring wine through the microscope I was surprised to see how the shapes of these acids matched our biochemical understanding – malic was larger and more hard-edged while lactic revealed much smaller forms.

Malic acid on top, lactic acid on the bottom:

I understood this process to be part of the transformation of a simple chardonnay to a more complex buttery expression. What I didn’t know until decades later was that malolactic conversion is almost universally done with red wines. Not just chardonnay? Why?

Malolactic Changes in Red Wines

In this case, helpful lactic acid bacteria are usually inoculated into the barrel of red wine at the end of primary fermentation. If used barrels are the ‘holding tank’ for the wine, bacteria are still present. Now the sour tart malic acid is converted into the less sour lactic acid. This process lowers the acid content of the wine (natural deacidification) and softens its mouth feel. This results in a smoother, more balanced wine.

In addition to a change in the mouth feel is a change in the wine’s aroma and flavor. It may smell more vinous with a buttery finish.

Looking again into the chemical changes during MLF, in addition to lactic acid is the formation of diacetyl, which gives a buttery feel to the wine.

In essence, for both red and white wines MLF adds to a greater softness and roundness to the wine. Some winemakers say that when MLF is done during oak aging following fermentation it contributes to better integration of oak and fruit.

A further advantage for controlled MLF in red wine, it won’t occur accidentally and spontaneously in the bottled wine. Think explosion of the built-up carbon dioxide.

Which Wines Benefit?

In general, medium to full-bodied reds destined for aging are improved by MLF. In addition to the stylistic qualities, MLF confers to a chardonnay, it is sometimes used to soften pinot blanc and pinot gris. On the other hand, lighter fruit-driven wines like Beaujolais or sauvignon blanc require crisp acidity to express their character, MLF is not for them.

In addition, wines that have undergone MLF are more stable and require less sulphur dioxide (SO ₂) to ensure stability.

The next time you’re exploring your palate try comparing the same varietal with and without MLF to add to your tasting knowledge and experience.

Still curious? Here are some additional resources to explore for more:

An Introduction to Malolactic Fermentation in Wine

Malolactic Fermentation

Wine’s Hidden Beauty

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