MALOLACTIC FERMENTATION

AND OTHER

FERMENTATIONS

Beginning winemakers often think the transformation of sugar into ethyl alcohol is the only fermentation process occurring in wine, but many other types of fermentation are possible in wine. Although the alcohol content and the high acidity makes wine a hostile environment for many microorganisms, several yeast and bacteria can exist and reproduce in wine. Besides the sugar transforming yeast, other microorganisms can convert one or more wine components into new materials. The microorganism might use the new material as a growth building block, or the transformation might be a source of energy for the microorganism. The organic acids, alcohols and glycerol are the wine ingredients most often metabolized by these other microorganisms. Winemakers call the transformation of grape sugars into ethanol by yeast primary fermentation, and they usually call the other transformations secondary fermentations.

MALOLACTIC FERMENTATION

Most high quality red wines are produced by two distinctly different fermentations. First, yeast converts the grape sugars into ethanol, and then bacteria in the wine convert malic acid into lactic acid. The conversion of malic acid into lactic acid by bacteria is called Malolactic (ML) fermentation, and ML fermentation produces significant changes in wine. Lactic acid is weaker than malic acid, so ML fermentation reduces wine acidity. This reduction in acidity is often used to improve the balance of wines with excessively high acid content. ML fermentation removes unstable malic acid from the wine, and when all of the malic acid is gone, the wine is more biologically stable. Small quantities of different byproducts are produced during malolactic fermentation, and some of these byproducts make positive contributions to the quality and complexity of the wine.

Several different types of wine bacteria can convert malic acid into lactic acid. These lactic bacteria consist of both cocci (round) and bacilli (rod shaped) microorganisms. The principal bacteria responsible for ML fermentation in wine belong to the Leuconostoc, Pediococcus and Lactobacillus

genera. Each genus contains several different species, so the term “malolactic bacteria” refers to a group of microorganisms. When wine undergoes spontaneous ML fermentation, several different kinds of bacteria may be involved, and these different microbes react in the wine in different ways. Depending upon conditions, the microbes produce a variety of byproducts.

Diacetyl

Diacetyl is one of several materials produced by ML bacteria. Diacetyl has an odor much like butter, and diacetyl in wine can modify aroma characteristics significantly. Generally, changes in wine aromas are subtle, but some wines like Chardonnay are often enhanced by very small quantities of diacetyl. Experienced winemakers produce specific wine styles by skillfully managing ML

fermentation.

Diacetyl is produced and metabolized by both ML bacteria and yeast. When ML fermentation occurs during primary fermentation or while wine is aging on active yeast lees, the yeast metabolizes much of the diacetyl and little of the diacetyl produced remains in the wine. A similar situation occurs when ML bacteria are allowed to remain in the wine after malolactic fermentation has been completed. Here, the bacteria consume the diacetyl, and after a few weeks little diacetyl remains in the wine.

Sometimes winemakers leave larger amounts of diacetyl in the wine to produce the buttery characteristics so typical of a full-bodied Chardonnay. When a winemaker wishes to leave larger quantities of diacetyl in the wine, he uses the following strategy. (1) When primary fermentation is complete and much of the yeast has settled, the wine is racked off the gross lees. (2) The wine is inoculated with ML bacteria. (3) The ML fermentation is carefully monitored at least once each week. (4) When ML fermentation is complete, the winemaker adds about 50 milligrams per liter of sulfur dioxide to the wine, and the sulfur dioxide kills the ML bacteria before they can metabolize the diacetyl. Considerable diacetyl can be left a wine using this technique.

At other times, the winemaker may choose to leave little diacetyl in his wine. Here, he uses a different strategy. (1) The wine is inoculated with malolactic bacteria either during or just after the alcohol fermentation when many viable yeast cells are present in the wine. (2) When ML fermentation is complete, both the yeast lees and the bacteria are allowed to remain in the wine for several weeks before any sulfur dioxide is added. During this time, the yeast and the bacteria consume much of the diacetyl. (3) When most of the diacetyl is gone, the winemaker adds about 50 milligrams per liter of sulfur dioxide to kill the bacteria. The winemaker uses standard winemaking procedures to clean up the wine.

Encouraging ML Fermentation

Often, winemakers wish to encourage ML fermentation even when diacetyl is not wanted. Red wines high in acid benefit from ML fermentation because the total acidity is reduced, and the wine has a better balance after ML fermentation. Red wines are more stable when the malic acid is gone, and many winemakers do not want to risk ML fermentation after the wine is bottled.

Spontaneous malolactic fermentation can be encouraged in several ways. (1) Only small amounts (20 to 30 milligrams per liter) of sulfur dioxide are added to the grapes when they are crushed. (2) Keeping wine pH values greater than 3.2 encourage the bacteria. (3) Keeping the wine temperature above 60 degrees encourages ML fermentation. (4) Keeping wine on yeast lees for several weeks can encourage ML fermentation.

Winemakers often inoculate their wines with malolactic bacteria to promote ML fermentation, and pure strains of bacteria are commercially available in both liquid and dry forms. Leuconostoc oenos are the bacteria most often used. Bacteria and yeast compete for nutrients in the juice, so ML fermentation is more likely to occur when the bacteria are added early in the sugar fermentation before the yeast has consumed all the nutrients.

Discouraging ML Fermentation

Wines produced from grapes grown in warm areas are often excessively low in acid, and these low acid wines may or may not benefit from ML fermentation. Many winemakers feel malolactic fermentation is not suitable for light, fruity wines because the bacterial fermentation decreases fruitiness.

The following steps are often taken to discourage ML fermentation. (1) Nominally, 50 mg/l of sulfur dioxide are added to the grapes when they are crushed. (2) When primary fermentation is complete, the free SO2 level is raised to about 30 milligrams per liter. (3) The wine is racked off the

yeast lees promptly, and the wine is clarified quickly. (4) Keeping wine cold will also discourage ML bacteria.

Wine Stability

The presence of malic acid in any wine represents a potential stability problem. When wine contains malic acid, ML fermentation can occur anytime, and when ML fermentation occurs after wine has been bottled, the results are often disastrous. ML fermentation in the bottle results in bottle deposits, off-odors, bad tastes and effervescent wine. Any red wine containing malic acid cannot be considered biologically stable, so before these wines are bottled, commercial winemakers take specific steps to improve long term, wine stability.

Commercial red wines containing malic acid are passed through a membrane filter at bottling time. These wines are often perfectly clear, and the filtration is not done to improve their appearance. The filtration is done to remove the ML bacteria from the wine, and sterile filtration is an effective means of preventing ML fermentation in the bottle. Fumaric acid can also be used to prevent ML fermentation in bottled wine. Before effective sterile filters were available, some winemakers added about 500 milligrams of fumaric acid per liter to their red wines just before bottling. Most home winemakers do not have sterile filtration equipment, and many home winemakers continue to use fumaric acid to inhibit ML fermentation in bottled red wine.