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Malolactic fermentation overview
Malolactic fermentation (MLF) is a process in which bacteria convert malic acid to lactic acid, with the release of CO2. It is sometimes called secondary fermentation because it typically starts after the (alcoholic) fermentation that is catalyzed by yeast. Malic acid has two acid groups, but lactic acid has only one acid group (and so has half the acidity). The result of MLF is to reduce the acidity of the wine, and to eliminate the sharp taste of green apples associated with malic acid.
Causes of malolactic fermentation
Several types of bacteria convert malic acid to lactic acid. MLF can be induced by adding bacteria to wine, and the principal bacteria that are used in this way are Oenococcus oeni (previously known as Leuconostoc oenos). Some bacteria that infect wine have harmful effects, including many Lactobacilli, which perform MLF but also generate excessive acidity. They are inhibited by low temperature (they function best at 18-25°C) or by SO2, and function above a pH of ~3.0 (wine has pH 2.9-4.2). In a very acid wine (where malolactic fermentation is most needed) it can be necessary to perform deacidification to reduce acidity to the point at which MLF can start. Malolactic fermentation must be completed before bottling, both to avoid trapping CO2 and other compounds that usually blow off, and to ensure stability (any unconverted malic acid could restart a fermentation with any remaining bacteria).
Controlling malolactic fermentation
MLF can be left to occur (or not) naturally, but more often is induced by inoculating wine with the bacteria at some time after fermentation. (This has the advantage of pre-empting the lactobacilli.) It can be blocked by low temperature or by treatment with sulfur dioxide. It is best performed at relatively low temperature to reduce risk of contamination with spoilage bacteria. When MLF is to be performed immediately, wine is not sulfited at running-off.
Timing of malolactic fermentation
MLF can be performed at any time from the end of alcoholic fermentation to later during maturation. It is considered inadvisable for MLF to start before alcoholic fermentation has finished, because there can be interference, and because the bacteria can convert some of the remaining sugar to acetic acid. Before the causes were understood, it used to occur in the Spring following the vintage, as the temperatures warmed up in the cellar and activated the bacteria. In Bordeaux, the wine was transferred to barriques after the malolactic fermentation was complete. A more recent tendency is to perform it in barrique soon after the alcoholic fermentation is finished, typically by running off the wine directly from the fermentation vats into the barriques. Proponents argue that the wine absorbs wood tannins more harmoniously and becomes better rounded. The wine certainly takes on a more oaky character and shows better in the Spring. This may be due to the two months' longer barrel aging (compared with wine where MLF is performed in vat), or to interactions of the bacteria with the wood, or possibly even to the fact that the wine is warm from fermentation when it enters the barrel. The question is what change this makes in the ultimate properties of the wine: proponents argue that it has no effect, while critics argue that it may make the wine less long-lived.
Effects of malolactic fermentation
Reduction of acidity is seen in several ways:
Malolactic fermentation of red and white wines
MLF is almost always performed with red wine. It is necessary to avoid too-sharp flavors in the wine and flavor clashes between malic acid and tannins. The reduction in acidity means that in a red wine, the color becomes less bright. (An exception is found with some very light red wines, e.g. simple Beaujolais produced by carbonic maceration.) MLF was originally used for white wines in order to reduce acidity, but for certain varieties, such as Chardonnay, is now used also for its contribution to the aroma profile. MLF is typically blocked with white wines that are products of hot climates (where total acidity is low, and it would therefore lead to a flabby wine). It may also be blocked in cases where "fresh" flavors are required, with a wine intended for immediate consumption (such as Muscadet). MLF can also be used for fine tuning, because commercial strains of bacteria are available with different characteristics (e.g. high or low diacetyl-producing). In exceptional circumstances, MLF may be blocked in (normally) cool climates: for example, Jadot blocked MLF in all their wines in the unusually hot vintage of 2003, because they felt this gave more natural results than acidification.
Malolactic fermentation of different grape varieties
The effects of malolactic fermentation depend on the grape. Some varietals lose their typical aromas with a malolactic fermentation; others, which have richer intrinsic aromas, are enhanced by it, such as Chardonnay. MLF is considered to give full-bodied white wines a significant part of their mouthfeel. It is more likely to be encouraged where white wines are intended for aging (such as crus in Burgundy). With other varieties, such as Sauvignon Blanc, Sémillon, Chenin Blanc, Riesling and the other aromatic varieties, MLF lowers the fruity character and is avoided, so other methods are used to reduce acidity if necessary. With Alsace Riesling, for example, MLF is encouraged only to reduce excess acidity in poor vintages. MLF often is used for base wine in production of Champagne, to give softness and more complex aromas. Rosés are like white wines; MLF may, or may not, be used depending on the circumstances.
Malolactic fermentation and Diacetyl
Diacetyl is produced by many lactic acid bacteria including Oenococcus oeni. It smells like warm butter and is responsible for the buttery aroma that develops in barrel-fermented Chardonnays. (It is commonly used to flavor "movie" popcorn.) It is a metabolic by-product of citric acid. Citric acid metabolism begins at the same time that malic acid degradation occurs, but degradation of the citric acid is slower; maximum diacetyl concentration tends to coincide with the exhaustion of malic acid during MLF. The amount produced depends on the bacterial strain, the bacterial concentration, the supply of citric acid, the SO2 level (SO2 binds to remove diacetyl), the pH (higher pH reduces production), and the redox potential. At low levels, diacetyl may be considered desirable; at higher levels the buttery aromas become overwhelming and undesirable. Estimates of threshold levels for these effects vary widely, but very roughly up to 1 mg/l (1 ppm) may be a typical acceptable level. Yeast, and continued action of the lactic bacteria, reduce diacetyl, so levels can be controlled by adjusting exposure to yeast and bacteria. To maximize diacetyl levels, the wine is racked before MLF and is stabilized as soon as MLF is completed. To minimize diacetyl levels, MLF is performed at the end of alcoholic fermentation, and the wine is kept on the lees. As diacetyl is usually considered (relatively) undesirable in red wines, MLF is usually performed on the lees. The process can also be controlled by stopping the lactic bacteria with SO2 at the appropriate time.
Malolactic fermentation and sulfur dioxide
Because sulfur dioxide inhibits the bacteria that perform MLF, it can be used to control the process. In order to perform MLF, SO2 levels are minimized following alcoholic fermentation. Bringing SO2 levels to >30 ppm after alcoholic fermentation will prevent MLF from occurring. If high levels of diacetyl are desired, increasing SO2 levels immediately after MLF will stop the process while the diacetyl is still high. If low diacetyl is desired, SO2 is not added until much later, when the bacteria have had time to continue to the stage of metabolizing the diacetyl. (Before the causes of MLF were understood, use of too much sulfur dioxide kept acidity high by preventing MLF, although this was attributed to a direct acidifying effect.)
Oak and malolactic fermentation
Performing the malolactic fermentation in barrel gives wines that are richer, more colored, less oaky, and more aromatic. This has a particularly strong effect on the wines at the start of their life, especially after a long élevage, and makes red wines appear especially attractive at the point at which they are sold en primeur (beginning of the summer the year after the vintage). With this, the wheel has come full circle. Originally, malolactic fermentation occurred (or failed to occur) in the oak barrels in which fermentation had occurred, then there was a transition to controlling the process by using stainless steel tanks, now it is preferred to encourage it in the original barrel.
Overview of fermentation
Fermentation involves the breaking down of complex organic substances into simpler ones. Wine is made by an alcoholic fermentation that is catalyzed by yeasts that convert sugar to alcohol. The process converts grape juice into wine. Red wines and some white wines may also pass through a second fermentation, called the malolactic fermentation, in which bacteria convert malic acid to lactic acid in the young wine.
Yeast genetic modification and fermentation
One of the most dramatic developments in genetic engineering is to produce strains of yeast that can perform both the alcoholic fermentation and the malolactic fermentation. This has been accomplished in two separate ways. The pathway for converting malic acid to lactic acid has two essential components: one is responsible for importing the malic acid into the cell, and the other then converts it into lactic acid. Yeast actually does have the pathway, but it is not very efficient. One approach has been to increase its efficiency by changing its location and the way it is controlled. Another approach has been to transfer the two genes from a bacterium into yeast. In both cases, the malolactic fermentation occurred simultaneously with alcoholic fermentation instead of separately. This has major implications for big commercial producers, who can cut time required for production. But it is almost certainly going to result in changes in the aroma and flavor spectrum of the wine in ways that are hard to predict. The ML01 strain is already in commercial use: it is claimed to reduce the content of histamine in wine (reducing risk of migraines), because biogenic amines are usually produced by bacteria during MLF.
