malolactic fermentation

When people hear of malolactic fermentation, they first think of deacidification. That is true, but it only describes the surface. In fact this second fermentation step changes a wine's inner order: its texture, its stability and the way acidity, alcohol and fruit relate to one another.

What happens chemically

With the help of an enzyme, lactic acid bacteria split a molecule of carbon dioxide off malic acid. Malic acid, which has two acidic groups, becomes lactic acid, which has only one, and a little CO₂ is released. Because lactic acid tastes milder, both measured and perceived acidity fall, and the pH rises slightly. In practice this work is almost always done by Oenococcus oeni, because this bacterium tolerates low pH, alcohol and sulfur better than others. As with spontaneous fermentation the naturally present bacteria can carry the process here too. Diacetyl, which recalls butter, may form as a by-product – usually only as a fine undertone in red wine, more pronounced in some white wines.

Why it changes more than the acidity

Lactic acid feels broader and less pointed than malic acid. As a result the acidity stands out less in isolation, the alcohol seems better integrated, the finish calmer. The change therefore affects not a single element but the relationship of the components to one another – a wine's structure and its texture. There is also a practical benefit: if the malic acid is broken down in the cellar, bacteria can no longer convert it later in the bottle. The wine becomes microbiologically safer and its development more reliable.

You taste it above all as a feeling in the mouth. A wine that has undergone malolactic fermentation often seems creamier and less angular; in white wines such as some Chardonnays a buttery note can be added, which goes back to diacetyl. In red wine the effect usually remains subtler and shows itself more in a calmer mouthfeel than in a distinctly new aroma.

A decision about time

Whether malolactic fermentation happens early, late, partially or not at all is not an inevitable consequence of alcoholic fermentation but a decision. An early course promotes approachability and speeds up integration. A later or only partial course preserves more acid tension, but requires more careful protection so that the wine stays stable. In some white wines it is deliberately held back to preserve freshness and clear fruit. Which path is right depends on grape variety, vintage, desired texture and the planned maturation time.

How we understand it

That this step can be controlled was long unclear. From the 1930s Jean Ribéreau-Gayon showed in the Bordeaux region the oenological significance of the conversion of malic into lactic acid; in the 1950s Émile Peynaud, who worked under him, understood its bacterial cause and made it predictable. This knowledge is part of our estate's history: Aux Conseillans was for decades the wine estate of Jean Ribéreau-Gayon, one of the co-founders of modern oenology. We therefore do not treat malolactic fermentation as automatic, but decide anew for each parcel and cuvée whether and when it serves a wine. The real question is rarely whether it takes place, but why.