By Harold McGee
Winemakers sometimes allow or even induce a second bacterial fermentation in the new wine after the main yeast fermentation. The bacterium Leuconostoc oenos consumes the wine’s malic acid and converts it into lactic acid, which is less strong and sour. This “malolactic” fermentation thus reduces the apparent tartness of the wine. It also produces a number of distinctive aroma compounds, among them buttery diacetyl. (A relative of L. oenos, L. mesenteroides, contributes the same compound to cultured butter itself!) Some winemakers work to prevent a spontaneous malolactic fermentation from developing, so that they can retain the sharpness and flavor of the original wine.