Describe the chemical mechanisms by which malolactic fermentation alters the mouthfeel and aroma of wine.

Malolactic fermentation (MLF) is a process where lactic acid bacteria (LAB), primarily Oenococcus oeni, convert malic acid to lactic acid. This conversion alters the mouthfeel and aroma of wine through several chemical mechanisms. The primary mechanism affecting mouthfeel is the deacidification effect. Malic acid is a dicarboxylic acid, meaning it has two carboxyl groups (-COOH), contributing to a sharp, tart taste. Lactic acid, on the other hand, is a monocarboxylic acid, possessing only one carboxyl group. The conversion of malic acid to lactic acid reduces the overall acidity of the wine, making it feel softer and rounder on the palate. This deacidification also raises the wine's pH, further contributing to a less acidic perception. MLF also influences mouthfeel through the production of glycerol and other polysaccharides. Some LAB strains produce glycerol as a byproduct of their metabolism. Glycerol contributes to the wine's viscosity, enhancing its perceived body and smoothness. Furthermore, LAB can release polysaccharides from yeast cell walls during MLF, which also contributes to a smoother mouthfeel. In terms of aroma, MLF generates several volatile compounds that contribute to the wine's complexity. Diacetyl is a key aroma compound produced during MLF, imparting a buttery or creamy aroma. While desirable at low concentrations, excessive diacetyl can be considered a fault. Esters are also produced during MLF, contributing to fruity and floral aromas. The specific esters produced depend on the LAB strain and the wine's composition. Finally, MLF can influence the perception of fruit aromas by altering the wine's acidity and pH. Lower acidity and higher pH can enhance the perception of ripe fruit aromas. However, it's crucial to manage MLF carefully to avoid the production of undesirable aroma compounds, such as those associated with spoilage bacteria like Brettanomyces.