Editing Malolactic fermentation (section)

==Role in winemaking==
The primary role of malolactic fermentation is to deacidify wine.<ref name="Oxford"/> It can also affect the sensory aspects of a wine, making the [[mouthfeel (wine)|mouthfeel]] seem smoother and adding potential complexity in the flavor and [[aroma of the wine]]. For these other reasons, most red wines throughout the world (as well as many [[sparkling wine]]s and nearly 20% of the world's white wines) today go through malolactic fermentation.<ref name="Jacobson"/>

Malolactic fermentation deacidifies the wine by converting the "harsher" [[diprotic]] malic acid to the softer [[monoprotic]] lactic acid. The different structures of malic and lactic acids leads to a reduction of [[titratable acid]]ity (TA) in the wine by 1 to 3 g/L and an increase in [[pH]] by 0.3 units.<ref name="Zoecklein"/> Malic acid is present in the grape throughout the [[growing season (vine)|growing season]], reaching its peak at ''[[veraison]]'' and gradually decreasing throughout the [[ripe (wine)|ripening process]]. Grapes harvested from cooler climates usually have the highest malic content and have the most dramatic changes in TA and pH levels after malolactic fermentation.<ref name="Oxford"/>

[[File:Malolactic fermentation.svg|right|thumb|The chemical process of malolactic fermentation is actually a decarboxylation instead of a fermentation. The bacterial cell takes in malate, converts it into lactate, and releases carbon dioxide in the process. The lactate is then expelled by the cell into the wine.]]
Malolactic fermentation can aid in making a wine "microbiologically stable" in that the lactic acid bacteria consume many of the leftover nutrients that other spoilage microbes could use to develop wine faults. However, it can also make the wine slightly "unstable" due to the rise in pH, especially if the wine already was at the high end of wine pH. It is not unusual for wines to be "deacidified" by malolactic fermentation only to have the winemaker later add acidity (usually in the form of [[tartaric acid]]) to lower the pH to more stable levels.<ref name="Boulton"/>

===Conversion of malic into lactic===
Lactic acid bacteria convert malic acid into lactic acid as an indirect means of creating energy for the bacteria by [[chemiosmosis]] which uses the difference in [[Electrochemical gradient|pH gradient]] between inside the cell and outside in the wine to produce [[Adenosine triphosphate|ATP]]. One model on how this is accomplished notes that the form of L-malate most present at the low pH of wine is its negatively charged [[Ion#Anions and cations|monoanionic]] form. When the bacteria move this anion from the wine into higher pH level of its cellular plasma membrane, it causes a net-negative charge that creates [[electrical potential]]. The decarboxylation of malate into L-lactic acid releases not only carbon dioxide but also consumes a proton, which generates the pH gradient which can produce ATP.<ref name="Wine Micro"/>

Lactic acid bacteria convert L-malic acid found naturally in wine grapes. Most commercial malic acid additives are a mixture of the [[enantiomer]]s D+ and L-malic acid.<ref name="Wine Faults"/>

===Sensory influences===
[[File:Chardonnay-UVa.jpg|left|thumb|The sensory characteristic of "buttery" Chardonnay comes through the process malolactic fermentation.]]
Many different studies have been conducted on the sensory changes that occur in wines that have gone through malolactic fermentation. The most common descriptor is that acidity in the wine feels "softer" due to the change of the "harsher" malic acid to the softer lactic acid. The perception of [[sour]]ness comes from the titratable acidity in the wine, so the reduction in TA that follows MLF leads to a reduction in perceived sour or "tartness" in the wine.<ref name="Boulton"/>

The change in mouthfeel is related to the increase in pH, but may also be due to the production of [[polyols]], particularly the [[sugar alcohol]]s [[erythritol]] and [[glycerol]].<ref name="Wine Micro"/> Another factor that may enhance the mouthfeel of wines that have gone through malolactic fermentation is the presence of [[ethyl lactate]] which can be as high as 110&nbsp;mg/L after MLF.<ref name="Zoecklein"/>

The potential influence on the [[aroma of the wine]] is more complex and difficult to predict with different [[strains (biology)|strains]] of ''Oenococcus oeni'' (the bacterium most commonly used in MLF) having the potential to create different [[aroma compounds]]. In Chardonnay, wines that have gone through MLF are often described as having "[[hazelnut]]" and "[[dried fruit]]" notes, as well as the aroma of freshly baked [[bread]]. In red wines, some strains metabolize the amino acid [[methionine]] into a derivative of [[propionic acid]] that tends to produce [[roasted]] aroma and [[chocolate]] notes.<ref name="Wine Micro"/> Red wines that go through malolactic fermentation in the barrel can have enhanced spice or smoke aromas.<ref name="Jacobson"/>

However, some studies have also shown that malolactic fermentation may diminish primary fruit aromas such as [[Pinot noir]], often losing [[raspberry]] and [[strawberry]] notes after MLF.<ref name="Wine Micro"/> Additionally, red wines may endure a loss of color after MLF due to pH changes that causes a shift in the equilibrium of the [[anthocyanins]] which contribute to the stability of [[color in wine]].<ref name="Boulton"/>