Copyright 1999
Chapter 13
MALOLACTIC AND OTHER
FERMENTATIONS
Webster defines fermentation as "a chemical change accompanied by effervescence." Beginning winemakers often think the transformation of sugar into ethyl alcohol by yeast is the only wine fermentation process. However, many other fermentations are possible in wine. Although the alcohol content and the high acidity makes wine a hostile environment for microorganisms, several different kinds of yeast and bacteria can exist and reproduce in wine.
Besides the sugar transforming yeasts, 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 excessively high in acid. 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 large 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 ages on active yeast lees, most of the diacetyl produced is metabolized by the yeast, and little diacetyl 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 - 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 encourages 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 micro 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 micro 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) Nominal amounts (30 - 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 discourages 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 commercial winemakers take specific steps to improve long term, wine stability before these wines are bottled.
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, and before effective sterile filters were available, winemakers often 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.
OTHER FERMENTATIONS
Most other secondary fermentations have a detrimental influence on wine quality. Some secondary fermentations produce subtle changes in wine characteristics, but other fermentations often produce gross wine spoilage. The changes depend upon when the fermentations occur in the winemaking cycle, on conditions and on the types of byproducts produced. Several common wine microorganisms and some problems produced by these microbes are discussed below.
Lactic Souring
Spontaneous malolactic fermentation is a common red winemaking phenomena. However, several different kinds of ML bacteria exist, and different types of bacteria produce different byproducts in wine. Unfortunately, many byproducts produced by malolactic bacteria (other than the ML fermentation) are detrimental to wine quality.
The French enologist, Emile Peynaud, gives several rules for making red wine in his excellent book "Knowing and Making Wine." His cardinal rule is ". . . make sure the sugars are fermented by yeast, and the malic acid is fermented by bacteria." This is sage advice because most types of lactic bacteria can and will ferment sugar. When lactic bacteria attack grape sugars, the glucose is converted into lactic acid and acetic acid, and the fructose can be converted into a nasty material called mannitol. When lactic bacteria ferment sugar, the volatile acidity of the infected wine can increase rapidly, and the wine often takes on a characteristic sweet-sour taste. This type of bacterial spoilage occurs most often when wines have an excessively high pH.
Lactic souring and vinegar formation are quite different. Lactic bacteria produce acetic acid by fermenting the sugar. Unlike acetobacter, lactic bacteria produce little ethyl acetate and large amounts of air are not required. Sometimes this type of spoilage is difficult to diagnose because the wine often has a good bouquet and the flavor may be good. However, a hot, burning characteristic is always present in the aftertaste. A lingering hotness in a wine always suggests excessive acetic acid, and sweet-sour tastes sometimes occur when considerable sugar remains in the wine. Lactic bacteria are probably the culprits rather than acetobacter when wine contains excessive acetic acid and little ethyl acetate.
Controlling bacteria early in the fermentation is important because some types of lactic bacteria prefer sugar to malic acid. Most experts recommend treating grapes with 30 milligrams per liter of SO2 at the crusher, even if ML fermentation will be encouraged later in the winemaking process. Even at this low level, the sulfur dioxide is effective in limiting early bacterial growth. After a few days of fermentation, the sulfur dioxide level in the wine will be very low, and it will no longer inhibit the desired ML fermentation.
Residual Sugar & Lactic Bacteria
Stuck fermentations often occur in the following way. First, a neglected, red fermentation overheats, and the high temperature kills the yeast. Fermentation abruptly stops, and considerable sugar remains in the wine. The wine is warm, and the wine contains sugar. In this condition, wine is extremely vulnerable. Some lactic bacteria are always in wine, and when the wine is in this condition, these bacteria often attack the remaining sugar. The wine is warm, and large amounts of acetic acid can be produced quickly. A wine can spoil in a short time under these condition.
Lactic souring is the great danger always associated with stuck fermentations, and any significant amount of residual sugar places wine in jeopardy. Consequently, stuck fermentations should be restarted promptly before lactic bacteria can multiply to excessive levels. This is why some winemakers prefer to inoculate their wines with ML bacteria late in the alcohol fermentation when very little sugar remains in the wine.
Restarting a stuck fermentation of this kind can be very difficult. Large amounts of acetic acid are in the wine, and acetic acid is toxic to wine yeast. Large wineries remove the acetic acid by using a special reverse osmosis process. When the excess acetic acid is gone, new yeast is added to restart fermentation, but reverse osmosis is a complicated process, and applying it to small quantities of wine is seldom feasible.
Small wineries and home winemakers deal with lactic souring problems primarily by prevention. This is one reason 30 to 50 milligrams per liter of sulfur dioxide should be added when the grapes are crushed. Then tartaric acid is added to the crushed grapes, and the pH is lower to less than 3.5 before fermentation is started.
Acetaldehyde Production
Candida mycoderma can oxidize ethyl alcohol in the wine into a very volatile liquid called acetaldehyde. In small quantities, acetaldehyde gives wine a distinctive "nut like" aroma. Acetaldehyde is the material that gives sherry its distinctive characteristics. Although the nutty quality is highly desirable in sherry, excessive quantities of acetaldehyde give table wine a tired, oxidized quality that most people do not appreciate. Like vinegar bacteria, Candida mycoderma needs large amounts of air. This yeast is often seen as a thin, patchy film floating on the surface of wine, so winemakers call Candida mycoderma "film yeast." Fortunately, film yeast is sensitive to sulfur dioxide.
The following is an effective treatment for wine infected with film yeast. Mix an appropriate quantity of sulfite crystals in a small amount of water then pour the sulfite solution onto the top surface of the wine. Pour carefully so mixing of the sulfur dioxide solution and the wine is reduced, and a large amount of sulfur dioxide is concentrated at the surface. Since the bacteria are growing on or near the surface, this is a potent treatment. Such dramatic treatments are seldom needed if the sulfur dioxide is maintained at reasonable levels and the wine containers are kept completely full and tightly sealed.
Glycerol Fermentation
Under certain conditions, some types of lactic bacteria ferment glycerol in the wine into lactic and acetic acids. Small amounts of acrolein are also produced during glycerol fermentation. The infected wine increases in volatile acidity, and the wine takes on an unpleasant, bitter taste. This type of glycerol fermentation occurs most often in wines low in acid and with high values of pH. Glycerol fermentation is also more common in wine made from heavy press fractions or wines made from moldy grapes.
Glycerol fermentation is not very common because adjusting wine pH to 3.5 or less and maintaining 30 milligrams of sulfur dioxide per liter of wine usually provides adequate protection against this type of wine infection.
Tourne
Tartaric is one of the more stable organic acids. However, a few species of lactic bacteria can ferment tartaric acid into lactic acid, acetic acid and carbon dioxide gas. When tartaric acid is fermented, the fixed acidity of wine decreases and the volatile acidity of wine increases. The wine takes on a strange, dull appearance, and the color turns brown. A strong and disagreeable "mousy" odor often develops.
The French name for this unfortunate wine condition is "Tourne," and when the disease is advanced, the wine becomes undrinkable. Tourne is a gross transformation, and this sickness is more prevalent in wines having high pH values. Fortunately, bacteria that attack tartaric acid are very sensitive to sulfur dioxide, and maintaining reasonable levels of molecular sulfur dioxide in wine will prevent the development of Tourne.
Vinegar Formation
Vinegar bacteria (Acetobacter) are found in the vineyard, on the grapes, in the cellar, on the equipment, in the wood of used wine barrels and in the wine. Acetobacter is the bug that converts ethyl alcohol into acetic acid (vinegar). Sometimes, very small amounts of acetic acid in a red wine make a positive contribution to the aroma, but when acetic acid exceeds about 0.04 percent, it produces a burning aftertaste that quickly decreases wine quality. In a warm cellar and with sufficient air present, acetification can progress rapidly, and fine wine can be spoiled in a short time. Along with the acetic acid, vinegar bacteria also produce ethyl acetate, and ethyl acetate has a strong, aromatic smell like finger nail polish remover. This odor is easy to identify, and the smallest hint of ethyl acetate suggests Acetobacter activity in a wine.
Vinegar bacteria must have access to large quantities of air to oxidize the alcohol into acetic acid. Colonies of bacteria often develop as thick, wrinkled mats on the surface of the wine where they have direct access to the air. However, vinegar bacteria can and do exist in the bulk of the wine. Barrels infected with vinegar bacteria should be taken out of service because disinfecting materials like wood is nearly impossible. Acetobacter activity is controlled by keeping the sulfur dioxide content of the wine at a reasonable level and by keeping wine containers completely full and tightly sealed.
SUMMARY
Besides the primary alcoholic fermentation, several other microbial transformations often occur in wine. Malolactic fermentation reduces wine acidity and improves the long term stability of red wine. ML fermentation produces the butter-like quality prevalent in heavier style Chardonnay wines, and it adds pleasing complexity to red wines. ML fermentation is generally encouraged in red wines because red wine containing malic acid is biologically unstable.
Besides yeast, many other microorganisms exist in wine. These organisms can ferment normal wine components into significantly different materials, and the products of some of these other fermentations can reduce wine quality.
When winemakers are aware of the microorganisms present, they can take effective measures to prevent spoilage. Most types of bacteria are sensitive to small quantities of sulfur dioxide, and many bacterial problems can be avoided by maintaining 20 to 30 milligrams per liter of free SO2 in the wine. Wine bacteria can also be controlled by other simple techniques. Adding tartaric acid to decrease wine pH and maintaining the wine at low temperatures are both effective control techniques. Keeping wine containers full and excluding air is usually all that is necessary to control vinegar bacteria. Winemakers often use sterile filtration to remove bugs from wine mechanically, and under adverse condition, pasteurization is sometimes necessary.
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