Malolactic conversion

Lactic acid bacteria convert malic acid (the acid in green apples) into lactic acid (the acid in milk). The wine becomes softer and rounder, and its total acidity falls.

The process can produce diacetyl, which smells of butter — the same compound used to flavour popcorn. This is where "buttery Chardonnay" comes from. It is not the grape and not the oak.

Nearly all red wine goes through it. For white wine it is a choice, and blocking it is how crisp, citrusy styles are preserved.

It is commonly called malolactic fermentation, though it is a bacterial conversion rather than a fermentation in the yeast sense.

Malolactic conversion is not a fermentation by yeast, and calling it one causes persistent confusion. Lactic acid bacteria — principally Oenococcus oeni — convert malic acid into lactic acid and carbon dioxide.

The effect is a fall in total acidity and a change in its character. Malic acid is the sharp acid of green apples; lactic acid is the softer one of milk. A wine that has been through it tastes rounder and less angular.

It also produces diacetyl, the compound responsible for the buttery note associated with oaked Chardonnay. That note is a product of this process, not of the oak, which is one of the more useful things to know about white winemaking.

Almost all red wine goes through it, both for the softening and because it stabilises the wine — leaving malic acid in a bottled red invites the conversion to happen later, in the bottle, producing haze and fizz.

For white wine it is a choice. Blocking it, by chilling and filtering or by adding sulphur dioxide, preserves acidity and keeps the wine crisper — which is why almost no Sauvignon Blanc or Riesling goes through it and most Burgundian Chardonnay does.

Partial malolactic conversion has become the common middle path: enough for texture, not so much that the freshness goes.