Why do winemakers want a second fermentation after the first?

Winemakers want a second fermentation because it swaps a harsh acid in the wine for a softer one. Yeast turn grape sugar into alcohol, and bacteria are then given a turn (when both are started together, the yeast usually wins the race for nutrients and the bacteria start late). They convert tart-tasting malic acid, naturally present in the grape must, into softer-tasting lactic acid. This is the malolactic conversion, standard for most red wines and common for some whites such as Chardonnay.

The two acids taste different. Malic acid is typically associated with green apples, and lactic acid is richer and more buttery. After conversion the wine's titratable acidity drops by 1 to 3 g/L and its pH rises by 0.3 units, and the wine tends to feel rounder and fuller in the mouth. The bacteria, chiefly Oenococcus oeni, also make a by-product called diacetyl, which in Chardonnay can add a buttery flavour. In low amounts diacetyl is nutty or caramel-like, but above 5 mg/L it becomes an intense butterscotch that is seen as a flaw.

Not every wine wants this. Winemakers actively block malolactic conversion in fruity, floral whites such as Riesling and Gewürztraminer, to keep a more tart profile. And if it begins in the bottle, it is usually a fault, because the wine looks as if it is still fermenting.

For centuries winemakers saw it without understanding it. In spring, wine in barrels seemed to come back to life, releasing carbon dioxide and turning cloudy. Pasteur, who isolated the first bacteria from wine in 1866, concluded that all bacteria in wine cause spoilage. It took the 20th century to show that this one was a friend.