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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. Temperature stability is an issue, and commercial wines are specifically treated to make them stable. 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 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

Most new wines contain 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 unsightly, so the wine industry considers excess protein removal essential 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 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.

Bentonite additions rang from one to ten pounds per thousand gallons of wine. 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 should settle for a week or so before racking. Tannin in red wine reacts with protein and the protein precipitates out during fermentation. Little protein remains at bottling time, 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 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. Wines containing residual sugar are carefully watched for several weeks after bottling to make sure the wine is completely stable. Typical white wine finishing steps are shown in Figure 6.

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 of 20 to 30 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 raised 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 judgment 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 judgment are required.

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.

Figure 6. Typical finishing processes for dry white table wines.

Chapter 16

WINE