Malolactic Fermentation Explained

Spring has well and truly arrived here. When we wander down to the winery and see bouncing lambs, daffodils, and fruit tree blossoms, I know it’s also the season when a few barrel bungs might start popping. Why? Because some of the beneficial, naturally occurring bacteria in the maturing wines also spring to life—devouring malic acid (naturally present in grapes) and converting it into lactic acid. This transformation releases carbon dioxide, which bubbles to the surface and can even lift the bungs right out of the barrels. It becomes a daily ritual to wander through the cellar, lightly replacing them.

Malic acid is formed in grapevine leaves during the growing season and transported into the berries, where it fuels respiration—a process that helps create those beautiful ripe flavours we love in mature fruit. Respiration is driven by temperature: in warmer climates, malic acid is consumed faster; in cooler regions, more remains by harvest.

To me, the winemaking year that follows harvest is like an extension of ripening, a second act where everything continues to evolve. Grape sugars “ripen” into alcohol, and in certain wine styles, malic acid “ripens” further into lactic acid. Think of malic acid as the sharp tang of green apples (it’s the principal acid in apples) and lactic acid as the softness of milk. Lactic acid is gentler—both chemically and on the palate—bringing roundness and calm where malic once brought edge.

The bacteria responsible for this transformation are also temperature sensitive. Some winemakers warm their wines in autumn and add commercial cultures to induce MLF. We have always preferred to let nature take its course, so it’s not until spring, when the barrels naturally warm, that the wild bacteria complete malolactic fermentation on their own.

For red wines, this process is almost universal. Because reds already have tannin and structure, any leftover malic acid would exaggerate bitterness and make the wine seem green.  However, for white wines it’s a stylistic choice—guided by the season, fruit, and desired balance. Most full-bodied, cool-climate Chardonnays undergo some degree of MLF for creaminess and depth, while un-oaked whites like Riesling, Pinot Gris, and Sauvignon Blanc usually skip it, keeping their bright, fruit-driven freshness intact.

There’s a fringe benefit too: once MLF is complete, one more fermentable element is gone. For dry wines—those without sugar and with no malic acid remaining—there’s no risk of re-fermentation in bottle. This allows lower preservative levels and gentler filtration, which is partly why dry red wines tend to need less sulphur than whites.

So that’s malolactic fermentation in a nutshell.  For us, it’s time to dodge the lambs, step carefully around the daffodils, and gently replace those bungs.

Cheers,
Rod