In the second part of our article series on malolactic fermentation, Westgarth Wines wine specialist Maurizio Broggi looks at the reasons why winemakers would use malolactic fermentation and its effects on the wine.

In winemaking, there is a choice to undergo full or partial malolactic fermentation or avoid it completely. For red wines, malolactic fermentation is almost universally encouraged, while for many white wines and many sparkling base wines, it is a winemaking and stylistic choice. There are several reasons why the process can be used.

There are three main reasons for winemakers to promote malolactic fermentation in their wines. The most obvious effect is to lower the wine’s acidity. Malic and tartaric acids account for over 90% of all acids in grapes. Tartaric acid is by far the most dominant, while malic acid accounts for approximately only 10 to 25% of the acids (depending on climate and level of grape ripeness).

Malic acid has a sharp, strong, green-appley taste (the name malic comes from malum – Latin for apple) that significantly contributes to the acidic taste of wine. Lactic acid bacteria degrade malic acid into the softer and less acidic lactic acid, resulting in an overall reduction in wine acidity. Depending on the initial levels of malic acid, the drop in acidity following malolactic conversion can be significant, reducing titratable acidity by 1 to 4g/l and raising the pH by as much as 0.1 to 0.3 units (as acidity in wine is inversely related to pH, higher pH means lower acidity). The combination of lower acidity and an increase in milder lactic acid also results in a softer, creamier mouthfeel.

The second reason why malolactic fermentation may be encouraged is microbial stability, i.e. to prevent potential spoilage and off-flavors. Malic acid is an energy source for several microorganisms and, as such, its presence in wine that has already been bottled can result in malolactic fermentation happening in-bottle. This causes the wine to become cloudy and fizzy, gaining unpleasant lactic off-odors. Consequently, winemakers let their wines undergo malolactic fermentation to protect them from potential malolactic fermentation occurring in the bottle.

Last but not least, malolactic fermentation contributes to the flavors of wine. Among the several flavor compounds produced by lactic acid bacteria during malolactic fermentation, one of the most prominent and well-known is diacetyl. In small quantities, it adds complexity to wines giving a buttery, creamy, toasty note. Indeed, diacetyl is largely responsible for the distinctive buttery character typical of Chardonnay. However, when in excess, diacetyl may negatively affect the organoleptic profile of the wine, giving an overwhelmingly buttery character or even a strong butterscotch note.

The flavor contribution of diacetyl is much more noticeable in white wines due to the compound’s lower sensory threshold. Studies have shown that the sensory threshold for diacetyl is 15 times higher in Chardonnay compared to Cabernet Sauvignon. This explains why in red wines, which almost always go through malolactic conversion, the buttery-diacetyl note is usually not perceived.

In white winemaking, malolactic conversion may be encouraged for those wines with high acidity levels such as those from cool climate regions like Chablis and Champagne. However, the decision also very much depends on the specific grape variety and wine style. Some grape varieties like Chardonnay are naturally well suited to malolactic conversion whose buttery and creamy notes complement the soft, full-bodied character of Chardonnay well, often in combination with barrel maturation.

On the other hand, malolactic fermentation tends to be detrimental for aromatic cultivars such as Riesling and Sauvignon Blanc which are distinguished by purity of fruit, freshness, and crisp acidity. For white wines with insufficient levels of acidity, malolactic conversion is detrimental to the overall balance of the wine, making them flat and flabby. Interestingly, some winemakers take a hands-off approach to malolactic conversion, they don’t encourage but they also don’t try to prevent it, and whether it happens or not it is embraced as an expression of the vintage.

Malolactic conversion in red wines is almost always carried out because it provides microbial stability and makes wines smoother, more supple, and less angular. Particularly when done in oak barrels, malolactic fermentation in red wines is said to improve texture, decrease astringency, and enhance color intensity.

Want to read more? Take a look at some of our other blogs:

• • Malolactic fermentation: lactic acid bacteria
  • Alcoholic fermentation: yeasts
  • Grape processing: from picking to sorting