Wine FAQ: What is malolactic transformation?

The Wine FAQ section answers your burning wine questions. Have a question? Let me know via email or Instagram. This time it's about malolactic transformation .

Conversion, yes. Even though it's often called malolactic fermentation in the wine world, that's incorrect, because technically, it's not fermentation. Fermentation involves converting sugar into alcohol, and that's not what happens during malolactic conversion. What does happen is the conversion of one acid into another.

Who would have ever thought I'd ever voluntarily delve into chemistry? I honestly never did a great job in high school. Yet here I am, with three books and five websites in front of me, trying to learn the ins and outs of malolactic chemistry.

What is malolactic metabolism and what does it do?

It's a lactic acid bacterium that converts the naturally occurring, sharp malic acid in wine into milder lactic acid (and a little carbon dioxide). Almost all red wines undergo malolactic fermentation. White wines are different. Some white wines, such as Riesling and Sauvignon Blanc, have a significant acidity—that's their signature—so they're not altered.

In his book "I Taste Red," James Goode says that—as far as he knows—the only red wine that doesn't undergo a MO is the Portuguese red Vinho Verde. That explains a lot. I have to tell you, it's the worst wine I've ever tasted.

Why is malolactic conversion used?

In The Oxford Companion – my rock – Jancis lists three reasons: reducing acidity, adding flavor and complexity, and stabilizing the wine.

Reducing acidity

Malolactic acid reduces acidity (and raises pH). Moreover, lactic acid tastes less sour than malic acid, as James Goode writes in his book "I Taste Red."

Adding flavor and complexity

Malolactic fermentation produces diacetyl, a substance with a distinct buttery aroma. It's also used to flavor popcorn, for example. It can be very pleasant in wine, provided the flavors are balanced. Chardonnay, in particular, can have a buttery aroma. In red wine, it's much harder to detect diacetyl.

Stabilizing the wine

In addition, malolactic conversion provides stabilization. Something must be done to the bacterial balance in the wine before it is bottled. This can be achieved by either performing malolactic conversion or by blocking it (see the following questions). Doing nothing is not an option, as there's a chance that malolactic conversion will still occur in the bottle.

When and how does malolactic transformation occur?

Lactic acid bacteria are present on the skins of the grapes, although their population diminishes somewhat after pressing. As a bacterium, it's no fun living in grape pomace; it's too sour and too sweet.

If the winemaker then also adds a spoonful of sulfites, it's game over. In the best-case scenario (for the bacteria), about ten percent of the lactic acid bacteria remain after alcoholic fermentation. That's far too little.

There are several factors that contribute to the growth of the bacterial population, including the temperature of the wine (around 20 degrees), the pH level (between 3.2 and 3.5 is ideal), the amount of nutrients and the right bacterial strains in the wine.

Harvest takes place in the fall and alcoholic fermentation in the winter. By the time it's ready, it's still far too cold in the cellar. It can then take until spring for MO to begin. This is risky, as spoilage lurks (including Brettanomyces and acetic acid bacteria). That's why some winemakers choose to add cultivated bacteria. These are readily available; ask for Oenococcus oeni .

Can MO be blocked?

Certainly, the winemaker has several options. A low temperature is one way to block MO. A low pH (below 3.1) also works well. After alcoholic fermentation, the winemaker can add sulfur dioxide again. This kills bacteria, including lactic acid bacteria.

Another option is to clarify the wine. If there are no nutrients, the lactic acid bacteria won't function.

Want to read more about malolactic transformation?

Want to learn more about this exciting topic? Check out these resources.

• GuildSomm – The history and science of malolactic conversion
• James Goode – I Taste Red
• Jancis Robinson – The Oxford Companion to Wine