malolactic fermentation

Malolactic fermentation (or sometimes malolactic conversion) is a process of a change used in winemaking where tart-tasting malic acid, naturally present in grape must, is converted to softer-tasting lactic acid.

Use in winemaking

Malolactic fermentation is commonly referred to as "MLF", or (in winemaker's speak as) "malo" (pronounced may-low). MLF usually occurs shortly after the end of the primary fermentation (when the grape sugar is converted to alcohol by yeast). It is undertaken by the family of lactic acid bacteria (LAB); Oenococcus oeni, and various species of Lactobacillus and Pediococcus. The primary function of all these bacteria is to convert one of the two major grape acids found in wine called L- malic acid, to another type of acid, L+lactic acid. This conversion is accompanied by the production of carbon dioxide (so hence the term, fermentation).

Wines that typically undergo, and are improved by MLF, are the full-bodied dry whites and medium to full bodied dry reds. A prominent example of this is the prevalence of malolactic fermentation in California chardonnays.^{[citation needed]} But it must be stressed that not all wines benefit from MLF. Rieslings are a classic case in point. As a general rule, the quality of lighter-bodied fruit-driven wines that require crisp acidity are reduced by the action of MLF. The growth of all LAB are inhibited by cool temperatures and the anti-microbial agent, sulfur dioxide (SO2). Winemakers are therefore able to arrest the onset of MLF when making these styles by maintaining both low temperatures and reasonable SO2 levels during winemaking and subsequent bottling.

There is also a major practical reason why MLF is encouraged during the making of many wines, and in particular reds wines that have previously undergone malo in tank or barrel are far less likely to go through malo when in bottle. The onset of MLF in the bottle is disastrous as the wine will appear to the consumer to still be fermenting (as a result of CO2 being produced). The wine may also lose its fruit integrity and take on the unpleasant lactic aroma of cured meats.

Effect of taste

Malolactic fermentation tends to create a rounder, fuller mouthfeel. It has been said that malic acid tastes of green apples – indeed, malic comes from the Latin word for apple, mālum, and is present in apple juice – and this can be tasted in the wine. By contrast, lactic acid is richer, even unctuous, and more buttery tasting – corresponding to its presence in milk, as reflected in the word lactic being derived from the Latin word for milk, lac, and it is present in sour milk.

In addition lactic acid has a mouthfeel "softness" about it in comparison to the oft described "hard" and "metallic edged" malic acid. In short, MLF results in a natural de- acidification and softening of the wine's palate. Grapes produced in cool regions tend to be high in acidity much of which comes from the contribution of malic acid. For wines produced from such grapes, de-acidification via MLF is particularly useful as it results in a more balanced and palatable wine.

Although acid reduction is the most obvious result of the growth of lactic acid bacteria in wine, their action can also significantly modify the wine's aroma, flavour and mouthfeel. These changes may be either good, bad or positively ugly depending to a large extent on which of the lactic acid bacteria dominates the MLF. Some of the Lactobacillus species have been implicated in the production of fetid milk, sauerkraut and sweaty characters. Whilst many high quality Old World wines are characterized and complexed by lactic nuances such as these, when dominant they are rather unpleasant. Some forms of Lactobacillus are also responsible for the production of "mousy taint" which is arguably the most unpleasant of all wine faults. Oenococcus oeni on the other hand is a far more desirable LAB as it typically produces substances that have pleasant and wine sympathetic aromas and flavours. Diacetyl is the most important of these substances, as it provides the most recognizable and characteristic of all MLF characters; butteryness. However, when in excess, diacetyl imparts strong caramel and rancid butter like characters, which can easily dominate the wine. Luckily, the more oenologically desirable Oenococcus oeni generally dominates the MLF as it has a greater tolerance to the high acid and high alcohol environ- ment of wine than the other lactic acid bacteria.

MLF is also thought to generally enhance the body and flavour persistence of wine, producing wines of greater palate softness and roundness. Many winemakers also feel that better integration of fruit and oak character can be achieved if MLF occurs during the time the wine is in barrel.

Process

Malolactic conversion is accomplished by lactic acid bacteria (such as Oenococcus oeni), which consume malic acid to liberate energy. This can occur naturally. However, in commercial wine making, malolactic conversion typically is initiated by an inoculation of desirable bacteria. This prevents undesirable bacterial strains from producing off-flavors. Conversely, commercial winemakers actively prevent malolactic conversion when it is not desired, to prevent accidental initiation and maintain a tarter, more acidic profile in the finished wine.^[1]

Chemically, malolactic fermentation is a decarboxylation, which means that carbon dioxide is liberated in the process.

Sometimes malolactic conversion can occur unintentionally after the wine is bottled. This is almost always a fault, and the result is a slightly carbonated wine that typically tastes bad. The carbonation from this type of change should not be confused with benign carbonation, known as spritz.

Because it consumes malic acid, which is present at the time the grapes are crushed, malolactic conversion can take place at any time during or after alcoholic fermentation. A wine undergoing malolactic conversion will be cloudy due to the presence of bacteria, and may have the curious smell of buttered popcorn, due to the production of diacetyl.

See also

• Acids in wine

References

External links

• Purdue University - "The Joy of Malolactic Fermentation" Accessed Dec. 27th, 2007