THE HOME WINEMAKERS MANUAL

by Lum Eisenman

Copyright 1999

Chapter 12

PRIMARY FERMENTATION

Practically all red grapes have clear, colorless juice. The red pigment is in the grape skins. Red wine is made by fermenting the juice, pulp, and skins together, and during fermentation, the red color is extracted from the skins. After several days of fermentation, the new red wine is pressed, and the liquid is separated from the solids. Besides color, many other materials are extracted from the skins during fermentation, and these materials produce the slight bitterness and astringency typical of red wine.

White and blush wines are produced differently. These wines are made by crushing and then pressing the crushed grapes. The liquid is separated from the solids before fermentation is started. White and blush wines are made by fermenting clear juice. Almost no skin contact occurs, and only small amounts of color, bitterness or astringency are extracted from the skins.

White wine can be made from red grapes if the contact between the juice and the skins is limited. French Champagne is made from Pinot Noir grapes, and Pinot Noir is a red grape. White Zinfandel and all blush wines are considered white wines because the juice is separated from the solids before fermentation. Rose wines, on the other hand, are considered red wines because they are fermented with the juice and the skins in contact for a short time. Winemakers use the terms "white" and "red" in two different ways. Besides describing wine color, these terms are also used to indicate the way wine is fermented.

YEAST GROWTH

Wine yeasts are microscopically small, single-cell organisms. Like every living organism, yeasts need energy to survive, and the necessary energy is obtained by metabolizing grape sugars. Ethyl alcohol is produced as an end product, but the yeasts do not use the alcohol. They are only concerned with the energy produced when the sugars are converted into alcohol. Besides sugar, yeasts must have access to many other materials to reproduce new cells, and yeasts are sensitive to their environments.

The conversion of glucose into alcohol is a complicated, multi step, biochemical process. Several different enzymes are needed to convert the sugars, and yeast must have access to vitamins, minerals, oxygen, nitrogen, etc. to produce the required enzymes. Grape juice normally contains all the necessary materials, but sometimes fermentations lack one or more of these critical growth factors. Then the yeast cannot convert all of the grape sugars into alcohol, and sugar remains in the wine. Most stuck fermentations are caused by deficiencies in the starting juice or by excessively high or low temperatures.

Oxygen

Yeast reproduces rapidly when sufficient oxygen is available. When the environment is optimum, yeast populations can double in less than an hour. This rapid period of yeast growth is called the "exponential" growth phase, and an enormous yeast population (10 million cells per milliliter of juice) can develop in less than 24 hours. Rapid cell growth occurs during the exponential growth phase, but little alcohol is produced. The situation is different when oxygen is restricted. With little oxygen, yeast cell reproduction is much slower, but the yeast produces larger amounts of ethyl alcohol.

From the winemakers point of view, having oxygen available early in the fermentation process is always desirable. Yeast cells then multiply rapidly, and a large yeast population is produced quickly. Later in the fermentation, oxygen is deliberately restricted to promote alcohol production. This simple technique allows winemakers to encourage early yeast growth, and the large yeast population will convert the grape sugars in a dependable way.

A lack of oxygen is seldom a problem when wine is fermented under typical, home winemaking conditions. The quantity of oxygen needed by the yeast is small, and more than enough oxygen is introduced when grapes are subjected to the normal winemaking processes of crushing and pressing. Nitrogen

Yeast must have protein to make new cells, and yeast must have nitrogen to produce the protein. Normally, grapes contain enough nitrogen to meet the yeast's needs. However, vineyards needing fertilization often produce fruit excessively low in nitrogen content, and then the yeast has problems producing the large numbers of cells needed to complete fermentation. Winemakers often add small quantities of diammonium phosphate (or other sources of nitrogen) to juices low in nitrogen. The diammonium phosphate gives yeast the nitrogen needed to produce new cells and complete the task of fermentation. The yeast and the winemaker are then happy.

Sometimes, a problem develops when nitrogen is added near the end of fermentation. A significant amount of alcohol has accumulated by this time. The alcohol seems to prevent the intake of nitrogen by the yeast cells, and sometimes the fermentation sticks. To avoid this problem, winemakers monitor their fermentations carefully and correct any nitrogen deficiencies early in the fermentation cycle before large amounts of alcohol accumulate.

Micro Nutrients

Yeasts also need an assortment of vitamins, minerals and other growth factors. Yeasts require very small quantities of these substances, so winemakers often call these materials "micro nutrients." Normally, grapes contain adequate quantities of these micro nutrients, but some vineyards consistently produce grapes deficient in some particular growth factor. In these cases, winemakers try to avoid fermentation problems by adding a complete "yeast food" to the juice. Several commercial products such as Superfood are produced specifically to supply needed micro nutrients.

Yeasts often produce excessive amounts of hydrogen sulfide when they lack pantothenic acid. Hydrogen sulfide produces the familiar "rotten egg" smell, and even small quantities of hydrogen sulfide can damage wine quality, and commercial and home winemakers routinely add minute quantities of pantothenic acid to fermentations. Pantothenic acid is a common vitamin, and it can be purchased in any drug store.

Handling Dry Yeast

Dry yeast looks almost indestructible. However, dry yeast consists of live cells, and it must be handled with care. Yeast weakened by mishandling often requires an unusually long time to start fermenting, and sometimes damaged yeast has trouble fermenting the juice to dryness. Several ways of damaging dry yeast are listed below.

(1) By prolonged storage at temperatures above 95 degrees.
(2) By storage in a freezer.
(3) By prolonged exposure to air after the package is opened.
(4) By rehydrating yeast in water that is too hot or too cold.
(5) By excessive amounts of sulfur dioxide in the juice.
(6) By juice temperatures either below 60 degrees or above 90 degrees.

Dry yeast will remain viable for at least two years when unopened packages are stored in a cool, dry place. However, once a package has been opened, the yeast should be used within a few months. Using open packages of dry yeast the next crush season is risky even when the open packages are stored under optimum conditions. Old, dry yeast should be saved. It is useful for fining wines containing ethyl acetate.

Rehydrating Dry Yeast

Nine times out of ten satisfactory fermentations can be started just by sprinkling dry yeast on the must. To avoid problems the tenth time, all yeast manufacturers recommend rehydrating dry yeast before it is added to the must. Yeast rehydration is a simple procedure. Just add the dry yeast to a small amount of warm water. However, the temperature of the water is important, and a thermometer should be used to adjust the water temperature to 100 degrees. About one cup of water is needed for a tablespoon of dry yeast. Stir the yeast mixture until it is smooth and then let it rehydrate. After 20 to 30 minutes, pour the yeast mixture into the must or juice. Most winemakers use about one gram of dry yeast for each gallon of must.

RED FERMENTATIONS

Wine contains phenolic compounds in quantities ranging from 0.03 percent for white wines to about 0.5 percent for red wines. Although these quantities are small, phenolic compounds are among the most important wine ingredients because phenolic materials are responsible for wine color, some bouquet and flavor components, bitterness, astringency, browning characteristics, etc.

Extraction

Tannins (large condensed polymers) and anthocyanins (color pigments) are the phenolic compounds of greatest interest to winemakers. Most of the phenolic materials in wine come from the grape skins, seeds and stems. Some phenolic compounds are more soluble than others. The more soluble compounds are extracted from the grape solids quickly, but longer soak times are necessary to extract the less soluble phenolic materials.

Other phenolic materials (tannins) are more soluble in alcohol than in water, and these phenolic materials are extracted late in the fermentation cycle when the alcohol level is high. Pigment compounds and some flavor compounds are quite soluble in water, and these materials are extracted earlier in the fermentation. The Table shows how pigment and tannin compounds accumulate as a function of skin contact time. These data were obtained from a typical Cabernet Sauvignon fermentation, and the data show four important features. (1) More than 90 percent of the total available color was extracted in the first four days of the fermentation. (2) The color intensity of the liquid started to decrease after about eight days of skin contact time. (3) After 18 days of skin contact, the color dropped to about three fourths of the maximum value. (4) Tannins continued to accumulate over the entire 28-day interval.

Tannins and pigments are extracted from grape solids differently. The data in Table below show that practically all of the color was extracted from the skins by the fourth day, but the tannin content increased throughout the 28-day period. The color pigments (and some flavor materials) are more soluble in water, so these materials are extracted early. Tannins are more soluble in alcohol, and the harsh, biter materials are extracted later in the fermentation when more alcohol has accumulated.

Cold Soaking

Winemakers have developed several techniques to help control the phenolic content of wine. Sometimes a method called "cold soaking" is used to produce red wines with a softer, more fruity character. Wines produced this way require less aging, and these wines can be consumed a few months after bottling.

Small producers often use the following procedure to produce light, fruity red wines. (1) Sound grapes are crushed, and a small amount (30 mg/l) of sulfur dioxide is added. (2) The crushed grapes are placed in a closed container, and the head space is blanketed with carbon dioxide gas. (3) The must is cooled to 50 degrees or lower. (4) The crushed grapes are held at the cold temperature for a time ranging from one to several days. (5) The refrigeration is then removed, and the crushed grapes are allowed to warm. (6) When the crushed grapes reach room temperature, they are inoculated with yeast and fermented in the usual way.

Significant quantities of color and flavors are extracted from the grape solids during the prolonged, cold skin contact time. However, little tannin is extracted during the cold soak because the juice contains no alcohol. This technique can be used with any variety of grapes, but it is particularly effective when used with Pinot Noir.

Fermentation Temperature

Red wines are normally fermented at temperatures ranging from 70 to 90 degrees. Within this temperature range, fermentations lasting from four to ten days are typical. Small wineries and home winemakers often use open fermenters, and most small red fermentation tanks are not fitted with elaborate temperature control equipment. Red fermentations can become fast and generate large quantities of heat in warm weather, and sometimes winemakers get into trouble. Yeast is sensitive and cannot survive for long when fermentation temperatures exceed 90 degrees. Consequently, small producers watch their fermentations carefully, and when necessary, they use cold water, ice or any means available to cool their fermentation tanks. Red fermenters in large wineries are temperature controlled, and fermentation temperatures are established simply by adjusting a thermostat.

Cap Management

Many small bubbles of carbon dioxide gas are formed during active fermentation. In red fermentations, the carbon dioxide bubbles stick to the grape skins, and the bubbles make the skins more buoyant. Soon, some skins float to the surface of the fermenting liquid, and a thick layer of skins, pulp and seeds accumulates after a few hours. As the layer of skins rises, liquid slowly drains away, and this "cap" of skins becomes dry. A dry cap in an open fermenter creates problems because vinegar bacteria can grow in the cap. The vinegar bacteria convert alcohol into acetic acid, and the acetic acid spoils the wine. Winemakers prevent this very undesirable condition by breaking up and submerging the cap periodically to keep the skins wet.

Caps on small fermentations are not very thick, and small caps can be managed easily by stirring the must with a large, wood spoon two or three times each day. In larger fermentations, the cap becomes several inches thick, and a special tool is needed to push the cap back down into the juice. Most winemakers "punch down" the cap at least twice a day. Several studies show more color and flavors are extracted when the cap is gently punched down several times a day during the first few days of fermentation.

Large, commercial wineries ferment red wine in closed stainless steel tanks, and they use a technique called "pumping over" to keep fermentation caps wet. A powerful pump and a large hose are used. Liquid is pumped from the bottom of the tank and sprayed over the cap with the hose.

Early Pressing

The phenolic content of red wine depends on the length of time the liquid is in contact with the grape solids, and skin contact time is a common method used to control the astringency of red wines. The term "early pressing" is used when red fermentations are pressed before they reach dryness, and early pressing is a common and effective technique for producing soft, fruity red wines. Skin contact times are short, so the tannin content is low. Usually, four to seven days of skin contact will produce ample extraction when red wines are fermented at normal temperatures.

Winemakers often produce light, fruity red wines by pressing the fermentation when the sugar content drops to eight or ten degrees Brix. However, wines pressed too early are often light in structure, and these wines can lack complexity. Full-bodied red wines are normally pressed when the hydrometer reads zero, and some "big" red wines are kept on the skins for three or four weeks.

Sometimes novice winemakers attempt to make a "big" red wine by using an extra long skin contact time. Unfortunately, such attempts usually fail. High quality wines can only be made from very high quality grapes, and home winemakers seldom have access to outstanding fruit. More than long skin contact times are required to make exceptional wines.

Skin contact time is always something of a compromise. There are no fixed rules, and knowing just when to press a red fermentation is part of the winemaking art.

Press Pressure

More tannin is extracted from the solids when press pressures are high. Because of this pressure effect, "press wine" (the wine obtained late in the pressing operation) contains more phenolic material than "free run" wine (the wine obtained before much pressure is applied). Consequently, winemakers also use pressing technique to control red wine astringency. Most home winemakers mix the free run wine and the press wine together, but most professional winemakers ferment and age the more astringent press wine separately. If the winemaker feels more body and astringency are needed, the press wine is blended into the main batch later. Holding press wine separate from the main batch gives these winemakers a simple, convenient way to adjust red wine astringency.

WHITE FERMENTATIONS

White wines are different from red wines. White wines contain less phenolic material than red wines, and consumers can tell the difference even when the wines are tasted in complete darkness. White wines lack the slight bitterness and the astringency of red wines because the phenolic content is lower.

Quality red wine can be made with rudimentary equipment, but high quality white table wine is much more difficult to make with simple equipment. White wine oxidizes easily, and the effects of oxidation are more apparent. Any off-odors or off-tastes are very apparent in white or blush wines. More and better winemaking equipment is needed to make high quality white table wine, and home winemakers must be prepared to expend more time and effort.

Pressing White Grapes

White grapes are more difficult to press than red grapes. The pulp of some white varieties is very slippery and slimy, and extracting juice from slimy pulp is difficult. Modern, wine presses can be programed to apply low pressures and execute many press cycles automatically. The low pressures and many cycles remove juice efficiently from white pomace without extracting large amounts of astringent materials. However, most home winemakers use small, vertical basket presses for white wines, and separating juice from slippery pulp with a small hand press is a difficult and time-consuming job.

Cold Settling White Juice

Cleaner tasting, fruitier wines are produced when juice contains less than 2 percent solids, so removing solids before fermentation is an important step in producing high quality white and blush wines. Large wineries remove juice solids with a large filter system or with a centrifuge. Either method requires expensive equipment.

Smaller wineries use a simple, cold settling procedure to remove the solid materials from their white juices. First, the juice is cooled to less than 50 degrees, and the solids are allowed to settle. After 12 to 48 hours, the clear juice is racked off the residue. The juice is then allowed to come to room-temperature, and fermentation is started.

This is a simple, effective procedure, but unless the juice is well chilled, it will start to ferment. When spontaneous fermentation starts, turbulence is created by the carbon dioxide gas, and the turbulence stirs up the juice and prevents the solid material from settling.

Fermentation Temperature

Fermentation temperature is an important quality factor in white wine production. Producing high quality, white table wine is very difficult unless fermentation temperatures can be kept below 60 degrees. Often, novice winemakers do not appreciate the need for cold fermentation temperatures, and large amounts of poor quality white table wine are made each season because of high fermentation temperatures. Low fermentation temperatures are necessary to retain the fruity characteristics of the grapes.

Light, fruity, white or blush wines like Riesling, Chenin Blanc or white Zinfandel are produced by fermenting well-clarified juice at temperatures ranging from 40 to 55 degrees. Fermentation is very slow at these low temperatures. The carbon dioxide gas is produced slowly, and the bubbles are small. Little turbulence is produced, and the volatile materials are retained in the juice instead of being blown away by violent bubbling.

Steinberg yeast ferments well at low temperatures, and it is often used to cold ferment Riesling wines. Prise de Mousse yeast also ferments well at low temperatures, but some types of yeast do not. For example, Epernay yeast seldom ferments to dryness at temperatures much lower than 50 degrees.

Sometimes full-bodied white table wines like Chardonnay or Sauvignon Blanc are fermented in barrels, and malolactic fermentation is often encouraged. These wines are often aged both in bulk and in the bottle to produce depth and complexity. Full-bodied white wines are usually fermented at temperatures ranging from 55 to 65 degrees.

HYDROGEN SULFIDE

Hydrogen sulfide gas (H2S) produces the familiar "rotten egg" smell. This noxious gas can be produced by yeast during fermentation. H2S can also be formed from decaying yeast cells when wine is left on gross lees for a long time. Most people can detect one part per million of this gas, so very small quantities of hydrogen sulfide can completely spoil a fine wine. Most often, hydrogen sulfide is produced from elemental sulfur during fermentation. The sulfur enters the juice as a residue on grapes treated with a sulfur spray (to control powdery mildew). The sulfur is converted into hydrogen sulfide by the reducing atmosphere of the fermentation.

Sometimes hydrogen sulfide is produced by yeast when the grapes contain no residual sulfur. Here, the smell is usually detected near the end of fermentation. During the later part of fermentation, yeast often runs short of some needed material, and hydrogen sulfide can be produced when yeast does not have enough nitrogen, micro nutrients or vitamins. Hydrogen sulfide is produced when fermentations do not contain enough pantothenic acid, but hydrogen sulfide can be produced any time yeast is subjected to stressful conditions.

Winemakers often add extra nitrogen and micro nutrients to their fermentations specifically to avoid the production of hydrogen sulfide gas. Diammonium phosphate is a commonly used source of nitrogen, and proprietary yeast foods are added to provide the yeast a variety of micro nutrients. Adding extra yeast nutrients is a simple and inexpensive way of avoiding problems with stinking fermentations. Unfortunately, hydrogen sulfide problems are occasionally encountered even when the best winemaking techniques are used.

Removing Hydrogen Sulfide

Some home winemakers use the following procedure to remove hydrogen sulfide from wine. (1) About 50 milligrams per liter of sulfur dioxide is added to the wine when fermentation is complete. (2) The wine is then aerated by racking with a great deal of splashing and bubbling. This treatment converts the hydrogen sulfide back into elemental sulfur, and the sulfur settles to the bottom of the container. Sometimes stinky wine needs to be racked two or three times to remove the stench completely. (3) After a week or two, the wine should be racked or filtered to remove the elemental sulfur. The smell may reappear unless the sulfur is carefully removed. However, aeration can convert hydrogen sulfide into disulfides, so this procedure must be used carefully.

Commercial wineries and some advanced home winemakers use copper to remove hydrogen sulfide from their wines. A 1 percent solution of copper sulfate is commonly used. When 150 milliliters of 1 percent copper sulphate pentahydrate solution is added to 1000 gallons of wine, 0.1 milligrams per liter of copper is produced in the wine. The copper converts the hydrogen sulfide into copper sulfide. Copper sulfide is not soluble in wine, so it settles to the bottom of the tank. A few days later, the winemaker racks or filters the wine off the copper sulfide residue.

Copper is a heavy metal, and only very small quantities can be added to wine safely. Additions often range from 0.05 to 0.2 milligrams of copper per liter of wine (mg/l), and home winemakers should never add more than 0.5 mg/l. Bench testing and careful measurements are required when copper is used. Little hydrogen sulfide and very little copper will remain in the wine when just the right amount of copper sulfate solution is used.

COMPLETING FERMENTATION

Winemakers monitor fermentations carefully to tell if the sugar is being converted at a reasonable rate and to detect any problems early. Small producers measure the temperature and the Brix of ongoing fermentations once a day. In larger wineries, fermentations are usually tested twice a day. Each time the sugar is tested, winemakers also check for potential problems by carefully smelling and tasting the sample. This close attention allows any fermentation problems to be detected early. The winemaker can then take prompt corrective action and avoid catastrophic wine failures.

Testing for Residual Sugar

Sometimes novice winemakers have trouble deciding when fermentation is complete. Fermentation may be complete when the following three conditions are met: (1) all bubbling has stopped, (2) the Brix has dropped to less than minus one and (3) the hydrometer readings have remained constant for several days.

Even when all three conditions have been met, some sugar can remain in the wine. Consequently, most winemakers measure the residual sugar content in all wines shortly after the end of primary fermentation. Low levels of residual sugar can be quickly and easily measured using a Clinitest kit. These inexpensive kits can be purchased at most large drugstores for just a few dollars. Use the "five drop" method and follow the directions supplied carefully.

STUCK FERMENTATIONS

Winemakers use the term "stuck" when active fermentation stops before all the sugar is gone. Generally, residual sugar in wine is a dangerous and undesirable condition. Residual sugar in wine represents major biological instability because fermentation can restart anytime. When fermentation restarts late in the winemaking cycle, much of the work done to clarify and stabilize the wine must be repeated. Then more processing is required, and the additional handling will not help wine quality. Sometimes, fermentation resumes after a wine is bottled, and the yeast produces an unsightly sediment in the bottle. The wine becomes effervescent, and sometimes the bottles explode. When the Clinitest measurement shows significant sugar remains in wine, appropriate steps must be taken to insure future wine stability.

Stuck fermentations can be due to a lack of nitrogen, the lack of an essential yeast nutrient, the use of damaged yeast, excessively low or high fermentation temperatures, etc. Whatever the causes, prompt action is needed, and the stuck fermentation should be restarted as quickly as possible.

First, a thermometer should be used to make sure the temperature of the stuck fermentation is neither too high nor too low, and the stuck fermentation should be racked. Sometimes racking with a little splashing and bubbling will be enough to rejuvenate the yeast and restart a stuck wine. If nitrogen deficiency is expected, diammonium phosphate should be added to the stuck wine. If the fermentation stopped early with lots of sugar remaining, the additional nitrogen may restart fermentation. On the other hand, when little sugar remains, the stuck fermentation should be re-inoculated with a fresh batch of alcohol tolerant yeast (Prise de Mousse or Pasteur Champagne) after the nitrogen addition.

More effort is often required to restart stuck fermentations. The following method is often successful if the original problem has been corrected. (1) Make a gallon of "starter" using either Pasteur Champagne or Prise de Mousse yeast. (2) When the starter is active, add a gallon of the stuck wine. (3) Wait until the starter becomes active again, and then add two gallons of stuck wine. (4) Wait until the starter is active again then add four gallons of stuck wine. (5) Continue this doubling process until all of the stuck wine has been added.

Once active fermentation is underway, the wine should be monitored carefully by measuring Brix twice a day. When the hydrometer is steady and reads less than zero, the wine should be tested with a Clinitest tablet to be sure all the sugar is gone.

Excessive Acetic Acid

Acetic acid is toxic to all strains of Saccharomyces (wine) yeast. Yeast activity is curtailed, and fermentation slows when the acetic acid content of fermenting juice exceeds about 0.1 percent. When the acetic acid exceeds 0.2 to 0.3 percent, few viable yeast cells can be found and fermentation stops.

A subtle fermentation problem can develop in the following way. Low acid, high pH grapes are common in warm growing regions. Controlling native bacteria with sulfur dioxide is difficult when the pH of the juice is high, and a large population of Lactobacillus bacteria sometimes develops during the primary sugar fermentation. The bacteria convert grape sugars directly into acetic acid, and the acetic acid content of the fermentation becomes excessive.

Under these conditions, little or no ethyl acetate is produced, and without ethyl acetate, the winemaker is often unaware of the problem. Undetected, the lactic bacteria can quickly raise the acetic acid level of the juice into the range of 0.1 to 0.4 percent. The wine yeasts are unable to tolerate such high concentrations of acetic acid, and the unhappy winemaker is left with a fermentation high in volatile acid and high in residual sugar. This kind of stuck fermentation is almost impossible to restart because of the excessive amounts of acetic acid, and the fermentation is often a total loss.

Winemakers accustomed to working with low acid grapes avoid this type of lactic bacterial problem by adding tartaric acid before starting fermentation. The tartaric acid lowers the pH of the juice, and sulfur dioxide becomes more effective in controlling the bacteria.

SUMMARY

Red wine is made by fermenting the juice, pulp and skins together, and during fermentation, the red color is extracted from the skins. Fruity red wines can be produced by using several well known winemaking techniques such as cold soaking, short skin contact times, careful cap management and low press pressures. White and blush wines are produced by crushing and then pressing the grapes before fermentation is started. Low fermentation temperatures are essential for producing light, fruity style white table wines.

A few simple precautions will avoid most fermentation problems. Active, dry yeast should be stored in a cool, dry place (not in a refrigerator), and yeast in packages from packages that have been open for more than a few months should be avoided. A yeast suitable for the fermentation conditions, temperatures and wine style should be chosen, and the directions supplied by the manufacturer should always be followed when storing and rehydrating dry yeast.

Fermentation progress should be monitored by measuring the Brix each day with a hydrometer and by smelling and tasting the juice. If fermentation appears sluggish, nitrogen should be added before much alcohol has accumulated.

Home winemakers remove hydrogen sulfide from wine when fermentation is complete by adding about 50 milligrams per liter of sulfur dioxide. The wine is then racked with a great deal of splashing and bubbling. Commercial wineries use copper sulfate to remove hydrogen sulfide from their wines.

Moderate quantities of sulfur dioxide may not control bacteria in high pH juices effectively. Large populations of Lactobacillus bacteria sometimes develop during the primary fermentation and produce excessive amounts of acetic acid and kill wine yeast.

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