Editing Malolactic fermentation (section)

==Wine faults==
[[File:Bottles being filled in a bottling truck.png|right|thumb|To prevent malolactic fermentation from happening in the bottle, wineries must have high levels of sanitation throughout the winemaking process.]]
The most common fault associated with malolactic fermentation is its occurrence when it is not desired. This could be for a wine that is meant to be acidic and fruity (such as Riesling) or it could be a wine that was previously thought to have gone through MLF and bottled only to have malolactic fermentation commence in the bottle. The outcome of this "in-bottle" fermentation is often gassy, hazy wine that can be unpalatable to consumers. Improvement in sanitation and control of lactic acid bacteria in the winery can limit the occurrence of these faults.<ref name="Wine Faults"/>

For early Vinho Verde producers, the slight effervesce that came from in-bottle malolactic fermentation was considered a distinguishing trait that consumers enjoyed in the wine. However, wineries had to market the wine in opaque bottles to mask the turbidity and sediment that the "in-bottle MLF" produced. Today, most Vinho Verde producers no longer follow this practice and instead complete malolactic fermentation prior to bottle with the slight sparkle being added by artificial carbonation.<ref name="Boulton"/>

While not necessarily a fault, malolactic fermentation does have the potential of making a wine "protein unstable" due to the resulting change in pH which affects the solubility of [[proteins in wine]]. For this reason, protein fining and heat stability tests on wine usually take place after malolactic fermentation has run to completion.<ref name="Zoecklein"/>

===Volatile acidity===
While [[volatile acidity]] (VA) is usually measured in terms of acetic acid content, its sensory perception is a combination of acetic ([[vinegar]]y aromas) and [[ethyl acetate]] ([[nail polish remover]] and [[model airplane]] [[glue]] aromas). High levels of VA can inhibit wine yeast and may lead to a sluggish or stuck fermentation. Several microbes can be a source for VA, including ''Acetobacter'', ''[[Brettanomyces]]'', and [[Biofilm|film yeast]] such as ''[[Candida (fungus)|Candida]]'', as well as LAB. However, while LAB usually only produce acetic acid, these other microbes often produce ethyl acetate, as well as acetic acid.<ref name="Wine Faults"/>

Most wine-producing countries [[wine laws|have laws]] regulating the amount volatile acidity permitted for wine available for sale and consumption. In the [[United States (wine)|United States]], the legal limit is 0.9 g/L for foreign wine exported to the United States, 1.2 g/L for white table wine, 1.4 g/L for red wine, 1.5 g/L for white [[dessert wine]], and 1.7 g/L for red dessert wine. [[European Union wine regulations]] limit VA to 1.08 g/L for white table wines and 1.20 g/L for red table wines.<ref name="Wine Micro"/>

Heterofermenting species of ''Oenococcus'' and ''Lactobacillus'' have the potential to produce high levels of acetic acid through the metabolism of glucose, though with most strains of ''O. oeni'', the amount is usually only 0.1 to 0.2 g/L.<ref name="Zoecklein"/><ref>Krieger, S., Triolo, G., and Dulau, L. ''"[http://www.lallemandwine.us/pdf/article_state_of_art_ml.pdf Bacteria and Wine Quality]"'' Lallemand. (2000) Accessed: 14 May 2013</ref> Several species of ''Pediococcus'' can also produce acetic acid through other pathways. Wines starting out with a high pH levels (above 3.5) stand the greatest risk of excessive acetic acid production due to the more favorable conditions for ''Lactobacillus'' and ''Pediococcus'' species.<ref name="Wine Faults"/><ref>Wibowo, D., Eschenbruch, R., Davis, C.R., Fleet, G.H., and Lee, T.H. ''"[http://www.ajevonline.org/content/36/4/302.short Occurrence and Growth of Lactic Acid Bacteria in Wine]"'' American Journal for Enology and Viticulture. Vol. 36 No. 4 302-313 (1985)</ref> ''L. Kunkeei'', one of the so-called "ferocious ''Lactobacillus''" species, has been known to produce 3 to 5 g/L of acetic acid in wines—levels which can easily lead to stuck fermentations.<ref name="Wine Micro"/>

==="Ferocious" ''Lactobacillus''===
In the late 20th century, among American winemakers, seemingly healthy fermentation were reported becoming rapidly inundated with high levels of acetic acid that overcame wine yeasts and led to stuck fermentations. While a novel species of ''Acetobacter'' or wine spoilage yeast was initially thought to be the culprit, it was eventually discovered to be several species of ''Lactobacillus'', ''L. kunkeei'', ''L. nagelii'', and ''L. hilgardii'', collectively nicknamed "ferocious" ''Lactobacillus'' for their aggressive acetic acid production, how quickly [[Bacterial growth|they multiply]], and their high tolerance to sulfur dioxides and other microbiological controls.<ref name="Boulton"/>

Ferments of high-pH wines (greater than 3.5) that spent time [[cold soak]]ing prior to yeast inoculations and received little to no sulfur dioxide during [[crushing (wine)|crushing]] seem to be at the most risk for "ferocious" ''Lactobacillus''. While infection seems to be vineyard-specific, currently, none of any of the implicated lactobacilli has been reported as being found on the surface of freshly [[harvested (wine)|harvested]] wine grapes.<ref name="Boulton"/>

===Acrolein and mannitol taint===
[[File:Raisin botrytisé.JPG|left|thumb|Wine grapes infected with ''Botrytis'' bunch rot tend to have higher levels of glycerol which can be metabolized by LAB into acrolein. Especially in red wine grapes with their high phenolic content, this can lead to the development of bitter-tasting wines as the acrolein interacts with these phenolics.]]
The degradation of glycerol by some strains of LAB can yield the compound [[acrolein]]. Glycerol is a sweet-tasting [[polyol]] present in all wines, but at higher levels in wines that have been infected with ''[[Botrytis cinerea]]''. An "active-[[aldehyde]]", acrolein can interact with some [[phenolic compounds in wine]] to create highly bitter-tasting wines, described as ''amertume'' by Pasteur. While at least one strain of ''O. oeni'' has been shown to produce acrolein, it is more commonly found in wines that have been infected by strains of ''Lactobacillus'' and ''Pediococcus'' species such as ''L. brevis, L. buchneri'', and ''P. parvulus''. Acrolein taint has also shown to be more common in wines that have been fermented at high temperatures and/or made from grapes that have been harvested at high [[Brix]] levels.<ref name="Wine Micro"/>

Heterofermenting species from the genus ''Lactobacillus'', as well as some wild strains of ''O. oeni'', have the potential to metabolize fructose (one of the main sugars in wine) into the sugar alcohols [[mannitol]] and (less commonly) [[erythritol]]. These are sweet-tasting compounds can add sweetness to a wine where it is not desired (such as [[Cabernet Sauvignon]]). Mannitol taint, described as ''mannite'' by Pasteur, in wines is often accompanied by other wine faults, including the presence of excessive levels of acetic acid, diacetyl, lactic acid, and [[2-butanol]], which can contribute to a "vinegary-[[ester]]y" aroma. The wine may also have a slimy sheen on the surface.<ref name="Zoecklein"/>

===Fresno mold and ropiness===
In the mid-20th century, a cottony [[mycelium]]-like growth began appearing in the bottles of some sweet [[fortified wines]] produced in [[California (wine)|California]]'s [[Central Valley (California)|Central Valley]]. Being fortified, these wines often had alcohol levels in excess of 20% which is usually a level that discourages growth of most spoilage organisms associated with winemaking. Nicknamed "Fresno mold" due to where it was first discovered, the culprit of this growth was determined to be ''L. fructivorans'', a species which can be controlled by sanitation and maintaining adequate sulfur dioxide levels.<ref name="Wine Micro"/>

Some ''Lactobacillus'' and ''Pediococcus'' species (particularly ''P. damnosus'' and ''P. pentosaceus'') have the potential to synthesize [[polysaccharides]] that add an oily [[viscosity]] to the wine. In the case of ''Lactobacillus'', some of these saccharides may be [[glucan]]s that can be synthesized from glucose present in the wine as low as 50–100&nbsp;mg/L (0.005 to 0.01% residual sugar) and afflict seemingly "dry" wines. While "ropiness" can occur in the barrel or tank, it is often observed in the wines several months after they are bottled. Wines with pH levels above 3.5 and low sulfur dioxide levels are at most risk for developing this fault.<ref name="Boulton"/>

Called ''graisse'' (or "grease") by the French<ref name="Wine Faults"/> and ''les vins filant'' by Pasteur, this fault has been observed in [[Cider|apple wines]] and [[cider]]. It can also be potentially be caused by other spoilage microbes such as ''[[Streptococcus mucilaginous]]'', ''[[Candida krusei]]'', and ''[[Acetobacter rancens]]''.<ref name="Boulton"/>

===Mousiness and geranium taint===
Wines infected with ''L. brevis, L. hilgardii'', and ''L. fermentum'' have been known to occasionally develop an aroma reminiscent of rodent [[droppings]]. The aroma becomes more pronounced when the wine is rubbed between the fingers and, if consumed, can leave a long, unpleasant [[finish (wine)|finish]]. The aroma can be very potent, detectable at a sensory threshold as low as 1.6 parts per billion ([[μg]]/l). The exact compound behind this is [[Derivative (chemistry)|derivatives]] of the amino acid [[lysine]] created through an oxidation reaction with ethanol.<ref name="Wine Faults"/> While undesirable LAB species have been most commonly associated with this fault, wine infected by ''Brettanomyces'' yeast in the presence of [[ammonium phosphate]] and lysine have also been known to exhibit this fault.<ref name="Wine Micro"/>

[[Sorbate]] is often used as a yeast-inhibitor by home winemakers to stop alcoholic fermentation in the production of sweet wines. Most species of lactic acid bacteria can synthesize sorbate to produce [[2-ethoxyhexa-3,5-diene]] which has the aroma of crushed [[geranium]] leaves.<ref name="Wine Faults"/>

===Tourne===
Compared to malic and citric acids, tartaric acid is usually considered microbiologically stable. However, some species of ''Lactobacillus'' (particularly ''L. brevis'' and ''L. plantarum'') have the potential to degrade tartaric acid in wine, reducing a wine's total acidity by 3-50%. French winemakers had long observed this phenomenon and called it ''tourne'' (meaning "turn to brown")<ref name="Wine Faults"/> in reference to the color change that can occur in the wine at the same time likely due to other processes at work in addition to the tartaric loss. While ''Lactobacillus'' is the most common culprit of ''tourne'', some species of the spoilage film yeast ''[[Candida (fungus)|Candida]]'' can also metabolize tartaric acid.<ref name="Wine Micro"/>

===Health-related faults===
{{see also|Health effects of wine}}
[[File:Cadaverine synthesis.svg|right|thumb|Cadaverine is one of the biogenic amines that some LAB species, particularly from the ''Lactobacillus'' and ''Pediococcus'' genera, have the potential to produce.]]
While the presence of [[ethyl carbamate]] is not a sensory wine fault, the compound is a suspected [[carcinogen]] which is subjected to regulation in many countries. The compound is produced from the degradation of the amino acid [[arginine]] which is present in both grape must and released in the wine through the autolysis of dead yeast cells. While the use of [[urea]] as a source of [[yeast assimilable nitrogen]] (no longer legal in most countries) was the most common cause of ethyl carbamate in wine, both ''O. oeni'' and ''L. buchneri'' have been known to produce [[carbamyl phosphate]] and [[citrulline]] which can be [[precursor (chemistry)|precursor]]s to ethyl carbamate formation. ''L. hilgardii'', one of the "ferocious ''Lactobacillus''" species, has also been suspected of contributing to ethyl carbamate production. In the United States, the [[Alcohol and Tobacco Tax and Trade Bureau]] has established a voluntary target limit of ethyl carbamate in wine to less than 15&nbsp;μg/L for table wines and less than 60&nbsp;μg/L for dessert wines.<ref name="Wine Micro"/>

Biogenic amines have been implicated as a potential cause of [[red wine headache]]s. In wine, [[histamine]], [[cadaverine]], [[phenylethylamine]], [[putrescine]], and [[tyramine]] have all been detected. These amines are created by the degradation of amino acids found in grape must and left over from the breakdown of dead yeast cells after fermentation. Most LAB have the potential to create biogenic amines, even some strains of ''O. oeni'', but high levels of biogenic amines are most often associated with species from the ''Lactobacillus'' and ''Pediococcus'' genera. In the [[European Union]], the concentration of biogenic amines in wine is beginning to be monitored, while the United States currently does not have any regulations.<ref name="Wine Faults"/>