Malolactic Fermentation(book excerpts)
Malolactic fermentation, also known as secondary fermentation or malolactic deacidification, is the process in which the harsh malic acid (as found in apples) is converted into softer-tasting lactic acid (as found in milk). Note that malolactic fermentation is not really fermentation as we would name the multi-step conversion of sugar to alcohol by fermentative yeast, but is actually the decarboxylation of L-malic acid to yield L-lactic acid and carbon dioxide. The principal effects of malolactic fermentation are a rise in pH and a reduction in perceived acidity. Malolactic fermentation can occur naturally, or winemakers will initiate it intentionally by inoculating the bacteria into wine. The genera Lactobacillus and Pediococcus are able to conduct malolactic fermentation, but Oenococcus oeni is the preferred species for the malolactic fermentation of all wines (Lerm et al., 2010). It is the bacterium best adapted to the distinctive conditions of winemaking - pH, ethanol, phenolic compounds, sulfur dioxide, etc. - conditions that are too restrictive for other species. It is also the most desirable lactic acid bacteria as it typically produces pleasant wine sensory attributes, whereas other lactic acid bacteria tend to be associated with producing undesirable flavors or hazardous metabolites. For wines grown in cool climates that contain high levels of malic acid, this decrease in acidity is essential to the balance of the wine. Malolactic fermentation plays an integral role in the production of the majority of red wines, as well as some white wines including Chardonnay, and some champagnes/sparkling wines. Oenococcus oeni, a member of the lactic acid bacteria family, is the main bacterium responsible for conducting malolactic fermentation. Apart from its main sensory effect (depleting malic acid from wine) this secondary fermentation can modify the aromatic properties and flavor complexity of wines by releasing notable concentrations of diacetyl and other carbonyl compounds. Another less readily accepted consequence of malolactic fermentation is enhanced microbiological stability. Although malolactic fermentation is the most widely accepted deacidification method of wine, it is a difficult and often time-consuming process and is often unpredictable and it is difficult to control or manipulate. Winemakers can inhibit malolactic fermentation by adding sulfur dioxide to wines post-fermentation or through the use of enzymes, such as lysozyme.
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