Copyright 1999
Chapter 15
CLARIFICATION AND STABILIZATION
Our first impression of any wine is a visual one. Wine is seen before it is tasted, and wine is expected to be brilliantly clear and have an appropriate color. The consumer is always disappointed when a wine does not meet these visual expectations. Even zealous wine advocates shy away from turbid, dirty-looking wines. Judges at home wine competitions occasionally face this problem, and sometimes real courage is needed to taste a particularly ugly wine.
TYPES OF HAZES
Most wine haze problems are caused by grape particles and fragments, microbes, protein, tartrate, phenolic polymers, polysaccharide and metals. Wine clarity problems are not mysterious, and unless a wine has been grossly contaminated by the addition of some foreign material, wine haze is normally the result of one or more of these factors. Sometimes a winemaker will encounter a haze that is particularly difficult to remove, but these cases are rare. Most haze problems are solved by identifying the offending material and then acting accordingly.
Particles and Fragments
Grape particles seldom cause long-term haze problems. Even the smallest bits and pieces of grape pulp and skins are large enough to settle out of wine in a few weeks. However, ML fermentation can produce enough carbon dioxide gas to cause a significant turbulence in a small tank, and the turbulence prevents the smaller particles from settling out. ML fermentation can continue long after the sugar is gone, so winemakers check for the presence of carbon dioxide gas to make sure all fermentations have been completed. When the gas is gone, the particles will settle out, and after the wine has been racked a time or two, it will be clear and bright.
Microbial Hazes
Yeast cells are several microns in diameter, and if the wine is not disturbed, the yeast cells readily settle to the bottom of the container in a few weeks. Usually a little patience will take care of yeast haze problems, but the situation is not so simple with bacterial hazes. Bacteria are 10 to 100 times smaller than yeast cells, and bacteria are so small they never completely settle out of the wine.
Large wineries have sophisticated analytical equipment in their testing labs, but even then, some types of bacteria are difficult to identify and treat. Fortunately, many wine bacteria are sensitive to sulfur dioxide, and most bacterial problems can be prevented simply by maintaining the free sulfur dioxide content of stored wine at 30 milligrams per liter.
Once established, a bad bacterial haze can be difficult to overcome. The infected wine can be pasteurized, or the wine can be passed through a "sterile" membrane filter. Both techniques are effective, and both are common wine industry procedures. Unfortunately, few home winemakers have the sophisticated and expensive equipment needed to apply either of these treatments. Since gross bacterial infections are difficult to handle, home winemakers should maintain strict hygienic winemaking conditions, and they should always maintain reasonable amounts (20 to 30 mg/l) of free sulfur dioxide in their wines. Taking these two simple steps will reduce the occurrence of bacterial infections to a very low level. Sometimes home winemakers can rent sterile filtration equipment, and sterile filtration might be a feasible way of saving a particularly pleasing wine. Nevertheless, the equipment and sterilization procedures are complicated. For the average home winemaker, prevention is the most effective way of dealing with bacterial problems.
Protein
Grapes contain small quantities of protein, and some varieties (Sauvignon Blanc) sometimes contain large amounts. Protein is carried over from the grapes into the wine during fermentation. Originally, the protein molecules are much too small to be visible in the wine. However, under certain conditions protein molecules link together (polymerize) and grow larger. After many protein molecules have linked together, the protein particles are large enough to be visible, and the particles are too large to remain suspended in the wine. This growth process is very slow at normal cellar temperatures, but when wine becomes warm, the protein molecules grow more rapidly. At temperatures of about 120 degrees, protein molecules can link together and form large particles in a short time.
A bad protein haze is very unsightly in a bottle of white or blush wine. The protein particles are light and fluffy, and they produce a swirling cloud when the bottle is disturbed. When white or blush wines are subjected to warm storage conditions, protein hazes can form quickly, and all commercial white and blush wines are specifically treated to remove the excess protein before bottling.
Winemakers call protein haze "hot instability" because warm storage conditions trigger the phenomena. A graphic demonstration of hot instability can be easily produced by leaving a bottle of Sauvignon Blanc in a car trunk on a hot summer day.
Excess protein seldom causes stability problems in red wine. Red wines contain phenolic compounds that react with the protein during primary fermentation, and the excess protein precipitates out of the wine. White and blush wines contain very little phenolic materials, so the winemaker must use a special treatment to remove the excess protein from these wines.
Potassium Bitartrate
Grapes contain several organic acids including tartaric acid, and they contain potassium. Vines manufacture tartaric acid through the photosynthesis process, and the vines obtain potassium from the soil. Potassium reacts with tartaric acid and forms a material called potassium bitartrate. Potassium bitartrate is a clear, crystalline material, and grapes always contain some of this material. Sometimes potassium bitartrate is called "cream of tarter," but winemakers often call this material "tartrate." Potassium bitartrate has several interesting physical properties. (1) Only small quantities of this material can be dissolved in grape juice. (2) After grape juice ferments, even less potassium bitartrate is soluble in the water-alcohol mixture. (3) The quantity of potassium bitartrate dissolved in wine is strongly dependent upon the temperature. Cold wine cannot hold as much potassium bitartrate as warm wine.
In combination, these three properties produce an interesting winemaking problem. Generally, grape juice contains all the potassium bitartrate it can hold when the grapes are picked. Alcohol begins to accumulate when the grapes are fermented. As the alcohol concentration increases, the new wine becomes saturated, and potassium bitartrate precipitates out of the wine. As fermentation continues, more alcohol is produced, and more tartrate is forced to precipitate out of wine. By the end of fermentation, the new wine is over saturated with potassium bitartrate. The tartrate continues to drop out of the solution, but at normal cellar temperatures, tartrate precipitation is very slow. Often the tartrate crystals continue to precipitate for a year or more, so potassium bitartrate causes serious long term stability problems for the wine industry.
The following example illustrates a common tartrate stability problem. A new white wine is clarified and aged for several months. The wine is then filtered with a 0.45 micron membrane and bottled. The newly bottled wine is clear and bright, but the wine is still nearly saturated with potassium bitartrate. Ultimately, a consumer puts a bottle of this wine into a refrigerator for a few hours before it is served. The wine cools rapidly in the refrigerator, and potassium bitartrate precipitates out of the cold wine. (See property number three above).
As tartrate drops out of solution, suspicious looking crystals are formed in the bottle, or dense hazes form. Tartrate hazes are very unsightly, and sometimes the consumer mistakes the tartrate crystals in the bottle for glass particles. In any case, the consumer is unhappy, and the winemaker is embarrassed. All commercial white and blush wines are cold stabilized sometime during the winemaking process to remove the excess tartrate material before the wine is bottled.
Phenolic Polymers
Phenolic compounds are present in wine in small amounts. The quantity may be small, but phenolic materials are very important wine ingredients. Phenolic compounds are responsible for color, bitterness, astringency and some odors and flavors. Many phenolic compounds polymerize just like protein molecules, and these phenolic molecules combine and slowly grow larger. Phenolic molecules carry an electric charge, and the molecules repel each other in the wine. Large phenolic molecules can remain suspended in the wine for a long time because of the electric charges.
Since phenolic molecules remain suspended for a long time, haze and bottle deposit problems often occur a few months after red wines are bottled unless the excess phenolic material is removed. To avoid phenolic problems, experienced winemakers remove excessive quantities of phenolic materials from red wines by fining or filtration.
Any protein fining material such as gelatin, casein, egg-white or isinglass can remove phenolic materials from wine. Many red wines are lightly fined with egg-whites or gelatin several weeks before bottling time. The quantity of fining material used is small, so the fining treatment does not alter other wine characteristics significantly. Practically all commercial red wines receive a light protein fining or a tight pad filtration to reduce bottle deposits.
Making red wines completely phenolic-stable is not practical, and most red wines will show some bottle deposit when several years old. Phenolic haze problems rarely occur in white or blush wines. These wines receive practically no skin contact, so they contain very little phenolic material.
Polysaccharides
Polysaccharides are very large molecules consisting of many simple sugar molecules (monosaccharides) linked together. Pectin and gums are common examples of polysaccharides. Pectin is the material that makes jam and jelly solidify, and pectin often produces hazes in fruit wines. However, pectin hazes are seldom a serious problem in wines made from grapes because grapes contain a naturally occurring enzyme that breaks down the large pectin molecules into smaller molecules that cause little trouble.
When they do occur, pectin or gum hazes can be difficult to remove from wine. These large polysaccharide molecules often carry electric charges, and the charges help hold the molecules in suspension. Most fining materials used for wine clarification are not very effective in removing this type of haze because of the chemical nature of these materials. Filtration is not always an effective way of removing pectin hazes because filter pads are plugged quickly by pectin and gums, and trying to filter wine with a bad pectin haze often becomes a frustrating and costly undertaking.
Winemakers often use pectic enzymes instead of trying to remove pectin hazes by fining or filtration. Pectic enzymes are available commercially, and they are added to the juice or the wine to break down the large troublesome pectin molecules. Alcohol interferes with enzyme action to some extent, so sometimes pectic enzymes are more efficient when added to juice before fermentation is started. Occasionally, just chilling the wine to a low temperature (28 degrees) will cause a bad pectin haze to speed out as if by magic.
Metals
Fifty years ago most winemaking equipment was made of iron or brass. Wine acids are strong enough to dissolve tiny amounts of these metals, and in the past, iron and copper hazes were common problems throughout the wine industry. Several proprietary fining materials were developed specifically to remove these excess metals from wine. Unfortunately, the effective products were based on poisonous, cyanide compounds, and great care and much testing was required when these products were used. In recent years, the prevalent use of stainless steel and plastic materials has virtually eliminated metal haze problems.
THE CLARIFICATION PROCESS
Large quantities of carbon dioxide gas are produced during primary fermentation, and considerable turbulence in the wine is produced as the bubbles rise to the surface. When fermentation is finished, bubbles are no longer produced, and the wine becomes still. When the wine is still, gravity slowly pulls the suspended material to the bottom of the container. Settling time depends on the size of the suspended material, and smaller particles require more time to settle than larger particles. Pulp and skin fragments settle out of small containers in just a few days. Yeast cells are much smaller, and a week or more is usually required for spent yeast cells to fall 24 inches. Bacteria are 10 to 100 times smaller than yeast cells, and bacteria are so small they never completely settle out of the wine.
New wines contain many different types of suspended particles, and these particles often have an electrical charge. The charges act just like magnets, and the "like" electrical charges repel each other. Then, even larger particles may never settle out of the wine until the electrical charges are neutralized.
Racking
The muck that slowly accumulates on the bottom of wine containers is called "lees." Clean wine is separated from the lees by a decanting process called "racking." After wine is racked two or three times, it becomes clean, clear and "bottle bright." Besides clarifying wine, racking helps remove other unwanted materials, so racking also contributes to long-term wine stability. Small containers like 5-gallon carboys are usually racked by syphoning the wine off the lees with a piece of clear plastic tubing. Barrels and drums are often racked with a small pump and plastic tubing. Wineries use powerful electric transfer pumps and large diameter hoses made from food grade materials to rack their large stainless steel wine tanks.
White wines are normally racked off the gross yeast lees shortly after the finish of alcoholic fermentation. These wines are racked a second time after they have been hot and cold stabilized. Red wines are often left on their gross lees until ML fermentation is finished. They are then racked for the first time. Red wines are usually racked two more times the first year and then at six month intervals. Most winemakers rack wines promptly (a week or so) after a fining treatment.
Fining
Most white and blush wines will be almost clear after being hot stabilized with bentonite, but additional clarification steps are usually necessary to produce a bright, clear wine. These additional clarification steps might consist of fining with Sparkolloid or gelatin/Kieselsol or filtering the wine through a course pad or cartridge. Commercial wineries use large filters to clarify their wines because filtration is safer and quicker when the proper equipment is available. Small wineries often clean up their wines by fining before any filtration is done to reduce production cost. Most home winemakers do not have filtration capability, so they rely solely on fining to clarify their wines.
Something is amiss when a white or blush wine does not come clear after being hot stabilized with bentonite and then fined with Sparkolloid. Excessive fining or filtering can devastate wine quality, so care must be taken when attempting to clarify a stubborn wine. The first step is to identify the specific problem. Second, appropriate steps should be taken to eliminate the problem, and then the wine clarification and stabilization process can be continued.
Sound red wines come clear without any fining or filtration treatments because the tannin in red wine acts as a fining agent. Red wines are usually clear and bright after being racked a couple of times and aged a few months. Although clarity is seldom a problem in red wine, long term bottle stability is always an issue. Red wines generally produce significant amounts of bottle deposit unless they are filtered or lightly fined with a protein material. Light fining or filtration with a medium pad will not significantly alter flavors or bouquets, but either procedure can effectively reduce bottle deposits. The bottle deposits are unsightly, so most commercial red wines are stabilized by either fining and/or filtration before they are bottled.
Filtration
Today, consumers demand brilliantly clear white and blush wines. Unfortunately, maximum wine clarity is difficult to produce by fining alone. In addition, fining wine is time consuming, produces lees and always involves some risk. Consequently, filtration plays an important role in the clarification of commercially produced wines.
Pad and frame type filters and cartridge type filters are extensively used throughout the wine industry. Both styles of filters have advantages and disadvantages. Pad and frame type filters are best suited for filtering large quantities of wine. Filter pads are inexpensive, but the frame assembly is very expensive. Pad and frame filters are best suited for commercial applications because of the large initial capital investment. Home winemakers often use cartridge type filters because inexpensive, plastic filter housings are available in most hardware stores. Filter cartridges are more expensive than filter pads per gallon of wine filtered. However, since home winemakers produce smaller quantities of wine, the cost of filter cartridges is not prohibitive. Both types of filters are discussed in greater detail in the following chapter.
WINE STABILIZATION
Wine is often exposed to considerable heat when shipped long distances in the summer time. Most white and blush wines are chilled to about 50 degrees for several hours before they are served. Practically all commercial wines are specifically treated to make them stable, and after stabilization, wine appearance or quality will not be altered by reasonable temperature extremes. Most commercial winemakers consider wine stable if the wine does not show significant changes when exposed to storage temperatures ranging from 40 to 100 degrees.
Stabilizing a light, fruity white table wine is not trivial. Light, fruity wines can be damaged easily by over processing, excessive handling or oxidation, and producing good long term bottle stability without reducing the quality of a delicate wine requires considerable winemaking skill.
Occasionally, a winemaker bottles a wine without doing stability tests. The wine has been brilliantly clear for several months, so the winemaker assumes the wine is stable. A few weeks after bottling, the wine develops a bad haze or drops an ugly sediment in the bottles. Now the winemaker has little recourse because un-bottling, treating and re-bottling would destroy wine quality. Bottling unstable wine can be a discouraging event for any winemaker, and it can be an economic disaster for a commercial producer.
Cold Stabilization
Practically all new wine contains excessive quantities of potassium bitartrate, and the tartrate precipitates out of cold wine as crystals or hazes. All white and blush wines require cold stabilization before bottling, and most commercial producers cold stabilize their red wines as well.
Wine can be effectively cold stabilized in several ways. A few large wineries use ion exchange columns to remove potassium from the wine. Ion exchange columns are filled with resin and work on the same principle as domestic water softeners. This type of wine cold stabilization requires large, expensive equipment, and a trained chemist is needed to establish the proper operation of the exchange column. However, once the equipment is operating, the ion exchange method is a fast and economical cold stabilization process. Unfortunately, wine quality can be reduced when the ion exchange method is used inappropriately.
Smaller wineries use a much simpler method to stabilize their wines. The wine is cooled to about 27 degrees and held at this low temperature for a week or two until the excess potassium bitartrate precipitates. This method of cold stabilizing wine also has advantages and disadvantages. Low temperatures are beneficial to new wine in several ways. Besides causing the potassium bitartrate to precipitate, the cold temperature helps other unwanted materials settle out of the wine. Sometimes suspended pectin and gums can be removed by chilling the wine. In addition, several days of low temperature storage can be helpful in developing long term wine stability.
Unless the wine is carefully handled, considerable oxygen can be absorbed while the wine is cold. The oxidation problem can be managed by purging wine containers with an inert gas, keeping the containers completely full and by maintaining adequate levels of sulfur dioxide in the wine. The high cost of energy needed to operate the large capacity refrigeration system raises production costs, and many wineries use specially insulated tanks to stabilize their wines.
Tartrate crystals also form in red wines, but the dark color obscures small deposits of tartrate crystals. Red wines are not chilled before serving, so a haze seldom forms. Often the tartrate crystals in red wines are found adhering to the cork, and the crystals are removed when the cork is pulled. Tartrate crystals are not so noticeable in red wine, so a few smaller wineries and many home winemakers do not bother to cold stabilize their red wines. However, most commercially produced red wines are cold stabilized before being bottled.
Hot Stabilization
Commercial wine is shipped long distances in warm weather, and under these conditions, protein instability causes hazes to form in white or blush wine. Protein hazes are very unsightly, and the wine industry considers excess protein removal an indispensable treatment for all white and blush wines.
Excess protein is not difficult to remove from most wines, but sometimes Sauvignon Blanc wines can be difficult to stabilize completely without damaging aromas and flavors. The standard treatment for all new white and blush wines is to fine with bentonite. The bentonite fining can be done anytime during the winemaking process, but the procedure is more efficient when the bentonite fining is done after the new wine has been rough filtered. Nevertheless, many winemakers find it more convenient to stabilize their wines by removing the protein earlier in the winemaking process (see below).
Bentonite additions rang from one to ten pounds per thousand gallons of wine. However, high dose rates can strip desirable flavors, so bench testing should always be done to measure the minimum quantity of bentonite needed. The treated wine is allowed to settle for a week or so before it is racked.
Tannin in red wine reacts with protein and causes the protein to precipitate out of the wine during fermentation. Little protein remains at bottling time, so protein hazes are seldom a problem in red wine.
Combined Hot & Cold Stabilization
All white and blush wines require both hot and cold stabilization treatments, and some winemakers combine both stabilization procedures into a single operation to reduce handling. First, the wine is fined with bentonite, and then the wine is immediately chilled to about 27 degrees. The wine is held at the cold temperature for a week or so while the tartrate precipitates. When the excess tartrate is gone, the cold wine is racked or filtered off the bentonite and tartrate lees. This combined procedure has some advantages. The tartrate crystals settle on top of the fluffy bentonite lees forming a crusty layer, and the wine is much easier to rack off the compacted lees. Both procedures are accomplished in a single winemaking operation, so labor is reduced. Wine manipulation is reduced, and the risk of wine oxidation is reduced.
Finishing White Wines
Many of the desirable qualities of light bodied, white table wines come from the characteristics of the grapes. These wines are only a few months old when bottled because lengthy aging destroys the desirable fruit qualities.
For example, light fruity Riesling wine is often finished in the following way. After fermentation, the free sulfur dioxide content is adjusted to about 30 milligrams per liter, and then the wine is hot and cold stabilized. After cold stabilization, the acid level is adjusted (if necessary), and the wine is allowed to rest for a few weeks. Then the wine is critically evaluated, and any other necessary adjustments are made. If large adjustments were made, the winemaker would test the wine again for both hot and cold stability.
At bottling time, the free sulfur dioxide content of the wine is raised to about 30 milligrams per liter. Riesling wine normally contains residual sugar. Any residual sugar may cause fermentation to restart, so a sterile filter would be used to remove all of the yeast cells from the wine when it is bottled. Most home winemakers do not have sterile filtration equipment, so they use a different method to leave residual sugar in wine. Home winemakers add potassium sorbate to the wine. The sorbate stabilizes the wine by preventing the yeast cells from multiplying. All wines containing residual sugar are carefully watched for several weeks after bottling because the winemaker wants to be sure the wine is completely stable. Typical white wine finishing steps are shown below.
1. A bentonite fining is done to remove excess protein and "hot" stabilize the wine.
2. The wine is chilled to about 27 degrees and held at the low temperature for several days to remove excess tartrate crystals and "cold" stabilize the wine.
3. The wine is racked or filtered off the bentonite and tartrate lees while still cold.
4. After a few weeks, the wine might be fined with Sparkolloid to improve clarity.
5. After a few more weeks, the SO2 is raised to about 30 milligrams per liter, and the wine is racked, polish filtered and bottled.
Finishing Red Wines
The finishing process for full-bodied red wine is different from that described above. Following cold stabilization, most high quality red wines are barrel-aged for 12 to 24 months. During this lengthy aging period, the wine is tasted and tested periodically, and the free sulfur dioxide is maintained at a reasonable level (20 to 25 mg/l). The barrels are topped up each time the wine is tested. After the gross lees are removed, red wines are racked two or three times a year. If the wine seems too astringent, the winemaker might use a gelatin or egg-whites to reduce the tannin content. At the end of the aging period, the winemaker tastes and critically reviews the wine for any needed corrections, and the free sulfur dioxide level is rased to about 30 milligrams per liter. The wine is then filtered and bottled. High quality red wines are often aged in the bottle from six months to several years. During this time, the wine develops "bottle bouquet."
Experience Required
Many factors are involved in producing high quality wine, and considerable judgement must be exercised throughout the finishing process. From the time the grapes are crushed until the wine is bottled, many winemaking decisions must be made. The time wine spends in the barrels, the frequency and method of racking, how much tannin should be left in the wine, etc., etc. all contribute to the ultimate quality of the product. Making high quality wine requires experience and good judgment, and this kind of preparation is difficult to get by reading books.
SUMMARY
Wine enjoyment is strongly influenced by first impressions, and our first impression with any wine is visual. Consequently, clarity and stability are extremely important to both homemade and commercial wines.
Practically all wines can be clarified, stabilized and prepared for bottling using standard winemaking practices. These procedures include cooling the wine to cold temperatures, fining the wine with suitable materials and using appropriate filtration methods.
All white and blush wines require both hot and cold stabilization, and most commercial red wines are cold stabilized. Although they are perfectly clear, red wines can throw a noticeable deposit after bottling unless they have been stabilized by fining, filtering or bulk aged for an unusually long time.
Many factors are involved in producing high quality wine, and time, effort and good judgement are required.
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