How does the malolactic fermentation (MLF) process affect the acidity and flavor profile of wine or cider?

Malolactic fermentation (MLF) is a secondary fermentation process in winemaking and cidermaking where malic acid is converted to lactic acid by lactic acid bacteria (LAB), primarily *Oenococcus oeni*. This conversion significantly affects both the acidity and the flavor profile of the finished product. The primary impact of MLF on acidity is a reduction in the perceived tartness and overall acidity of the wine or cider. Malic acid is a dicarboxylic acid, meaning it has two carboxyl groups (-COOH), which contributes to a sharp, green apple-like tartness. Lactic acid, on the other hand, is a monocarboxylic acid with only one carboxyl group, resulting in a softer, smoother acidity. The conversion of malic acid to lactic acid therefore decreases the total acidity and increases the pH of the wine or cider, making it taste less tart and more mellow. In addition to reducing acidity, MLF also alters the flavor profile through the production of various secondary metabolites. Diacetyl is one of the most well-known of these metabolites, imparting a buttery or butterscotch aroma and flavor. The amount of diacetyl produced depends on the LAB strain and fermentation conditions. Other flavor compounds produced during MLF include esters, which can contribute fruity and floral notes, and acetaldehyde, which can add a nutty or sherry-like character. MLF can also affect the mouthfeel of the wine or cider, making it feel smoother and rounder due to the reduction in acidity and the production of polysaccharides by the LAB. Whether MLF is desired depends on the style of wine or cider being produced. In some red wines, such as Cabernet Sauvignon and Merlot, MLF is almost always performed to soften the acidity and enhance complexity. In some white wines, such as Chardonnay, MLF can add desirable buttery notes and a richer mouthfeel. However, in other white wines and ciders, such as Riesling and crisp dry ciders, MLF is often avoided to preserve the fresh, crisp acidity. The control of MLF is therefore an important aspect of winemaking and cidermaking, allowing producers to tailor the acidity and flavor profile of their products.