Clarification and Stabilization
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.
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.
Once established, bad bacterial hazes can be difficult to overcome. The infected wine can be pasteurized, or the wine can be passed through a "sterile" membrane filter. Unfortunately, few home winemakers have the sophisticated and expensive equipment needed to apply either of these treatments. Consequently, maintaining strict hygienic winemaking conditions and keeping the free sulfur dioxide in the wine at about 30 milligrams per liter is very important.
Grapes contain small quantities of protein, and 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 until the particles are too large to remain suspended in the wine. This growth process is very slow at normal cellar temperatures, and at temperatures above 100 degrees, protein molecules form large particles in a short time. Winemakers call protein haze "hot instability" because warm storage conditions trigger the phenomena. Excess protein seldom causes stability problems in red wines. 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 reacts with tartaric acid and forms a material called potassium bitartrate, and grapes always contain some of this material. Cooks usually refer to potassium bitartrate as "cream of tarter," but most winemakers call this material "tartrate." By the end of fermentation, new wines are often over saturated with potassium bitartrate. The tartrate precipitates out of the wine, but the tartrate crystals can precipitate for many months. Potassium bitartrate often causes serious long term stability problems, so 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 compounds are present in wine in small amounts, and many phenolic compounds polymerize just like protein molecules. The phenolic molecules combine and slowly grow larger, but the phenolic molecules remain suspended for a long time. Consequently, haze and bottle deposits often occur several months after red wines are bottled unless the excess phenolic material is removed. Any of the protein fining material such as gelatin, casein, egg-white or isinglass can remove phenolic materials from wine, and most red wines are lightly fined with one of these materials several weeks before bottling time.
Polysaccharides are very large molecules consisting of many simple sugar molecules 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, home winemakers use pectic enzymes to remove pectin.
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. In recent years, the prevalent use of stainless steel and plastic materials has virtually eliminated metal haze problems.
Large quantities of carbon dioxide gas are produced during primary fermentation and considerable turbulence is produced as the bubbles rise to the surface. When fermentation is finished, bubbles are no longer produced, and the wine becomes still. Gravity then 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 so small they never completely settle out of the wine.
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 siphoning 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.
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.
Most white and blush wines will be nearly 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. After a few months, red wines normally come clear without any fining or filtration treatments because the tannin in red wine acts as a fining agent. Clarity is seldom a problem in red wine. But, long term bottle stability is always an issue because red wines produce significant bottle deposits unless they are filtered or lightly fined with a protein material.
Wine may be perfectly clear when bottled, but it may develop a haze unless the wine is completely stable. Sometimes a winemaker bottles a wine without doing stability tests because the wine had been brilliantly clear for several months. But, a few weeks after bottling, the wine develops a bad haze or drops an ugly sediment in the bottles. Bottling unstable wine can be a discouraging occurrence for any winemaker.
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 small wineries use a simple method to cold 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 helps other unwanted materials settle out of the wine, and sometimes suspended pectin and gums can be removed by chilling the wine.
When white or blush wines are stored under warm conditions, protein instability can cause hazes to form. Fortunately, excess protein is not difficult to remove from most wines. The standard treatment 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 is reasonably clear. Nevertheless, many winemakers find it more convenient to stabilize their wines by removing the protein earlier in the winemaking process. Tannin in red wine reacts with protein and causes the protein to precipitate out of the wine during fermentation, so protein hazes are seldom a problem in red wines.
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. 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.
Many factors are involved in producing high quality wine, and considerable judgment must be exercised throughout the 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 this kind of background is difficult to get by reading books.
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.
White and blush wines need to be fermented at cool temperatures, and they require both hot and cold stabilization. In addition, white and blush wines are more susceptible to oxidation, so many winemakers feel these wines are more difficult to produce than red wines.
Although they may be perfectly clear when bottled, red wines can throw a noticeable deposit after bottling. Consequently, most red wines need to be stabilized by fining or filtering.