Vintner's Corner

Vol. 12, No. 3 May - June, 1997

Bruce Zoecklein

Department of Food Science and Technology


Blacksburg, VA 24061-0418

Table of Contents

I. Wine Quality Control 1

II. Bottling Quality Control 2

III. Wine Corks 3

IV. Coming Events

Annual Meeting of the American Society for Enology and Viticulture, Eastern Section and International Riesling Symposium 5

Wine and Juice Production and Practical Monitoring Workshop 5

I. Wine Quality Control

The principle quality control difficulties of the wine industry are: lack of adequate record keeping, fruit quality, control of phenol extraction in whites and oxidative degradation. The key to adequate quality control is to monitor how each production activity affects wine palatability and to make adjustments accordingly. Complete and accurate record keeping is the corner-stone of a successful quality control program. Only when proper up-to-date accounts of wine production activities are kept can a full understanding of the parameters affecting wine quality occur.

Aging and Storage Quality Control

1. Sanitation: Each winery should have an established sanitation program and periodically monitor the effectiveness of that program. Such simple procedures as tasting barrel and tank rinse water can be a significant step in insuring quality. Alcohol is an excellent solvent. Therefore, any off character in the rinse water may be picked up in the wine.

2. Chemical Analysis. A procedure should be established for running a specific set of analyses according to a specific timetable. The analysis performed depends somewhat upon the philosophy of the winemaker. However, pH, free and total sulfur dioxide(by the aeration oxidation method, see Vol 5, No. 3, 1990 or Zoecklein et al., 1994, 1995), titratable acidity, reducing sugar, alcohol, protein, potassium bitartrate stability and paper chromatography for M/L are the very minimum analyses which the winery should be capable of performing. Additional supplemental analytical support can be provided from my laboratory.

It is essential that the vintner know how each processing step affects his product, both chemically and organoleptically. For example, reference samples should be held for comparison of color and body stripping due to filtration, cold stabilization, etc.

3. Oxygen Pickup. Most winemakers strive to retain as much of the 'grape' as possible in their wines. The loss of aroma components between fermentation and bottle release is a significant problem in this state. The colder the wine the greater is the solubility of molecular oxygen in the wine. When the wine is then allowed to warm, oxidation occurs. This is a principle disadvantage of conventional cold stabilization for potassium bitartrate stability. Such procedures often result in prolonged refrigeration of wines resulting in oxidative degradation. Each winemaker must know how processing and equipment affect O2 uptake. Free sulfur dioxide analysis is a good indication of O2 uptake since sulfurous acid is oxidized by the dissolved oxygen in wine. Therefore a rapid decline in the free SO2 level in a short period of time is indicative of O2 pickup. (See in Vol. 2, No. 2 March/April 1987). Such preventive steps as proper equipment, sulfur dioxide additions during wine movements, nitrogen blanketing, CO2 sparging flushing lines and receiving tanks all have their place in reducing the likelihood of excessive oxidation. (The use of 'Y' on the suction side of a positive displacement pump is an easy way to introduce SO2, gases, fining agents, etc., during racking).

There is no substitute for storing wines in full containers with the possible exception of barrels. Peterson (1976) showed that partial vacuums form in properly sealed barrels over time. Prior to his work many assumed that barrels should be topped regularly to prevent oxidation and biological growth. Topping too often can result in possible oxidation due to partial pressure lost within the barrel.

As stated, wine temperature is important because of its effect on oxygen solubility. Knowing storage temperatures and temperature fluctuations is a key to understanding the aging potential of a wine.

4. Pre-Bottling. A check list should be established to insure that important factors are not overlooked.

a. Chemical analysis. Has the wine met the proper analytical criteria for bottling?

b. Stability analysis. Has the wine met such stability criteria as protein, color, bitartrate and microbiological stability?

c. Sensory analysis. All bottling lots should be determined by a review panel, not solely by the winemaker. It is easy for those in the commercial wine industry to overestimate their own sensory abilities. The fact that winemakers can distinguish between ethyl acetate and ethyl mercaptans does not necessarily mean they are the best judge of what the buying public desires.

5. Materials Quality Control. Are all materials needed for bottling present and in the proper condition? (See section on wine corks.)

II. Bottling Quality Control

1. Sanitation Program. The winery should have a set sanitation program and know its effectiveness.

2. Biological and Oxidative Quality Control. Aside from packaging, the two most important considerations during bottling are biological and oxidative. Spoilage organisms which are present in the winery can easily find an adequate growth media in spilled wine, particularly if the wine is not removed properly. The major sources of contamination during bottling includes the following:

a. Filter pad drip trays. This is of increased importance due to the use of cellulosic pads which drip heavily. Trays must be drained often during bottling runs if wine is being filtered during bottling.

b. Fill bowls. Leaky spouts, wine blown from snifter valves, residue wine on bell rubbers, etc., can harbor wine contaminants. It may be desirable, particularly during long runs, to occasionally mist bell rubbers and filler stems with a 60-70% ethanol solution to inhibit microbial growth.

c. Corking machines. Corkers are a significant source of potential sanitation difficulties due to the likelihood of wine spillage. Corkers are a large source of contamination. These units should be completely dismantled and cleaned before and after each bottling. Ethanol misting of the corker jaws during bottling can be a significant asset in minimizing biological problems.

d. Work activity. Increased worker activity in the bottling area increases the spread of airborne wine microbes. It is desirable to limit the number of employees around the filling and corking area to as few as possible.

3. Wine Oxidation. Another potential problem during bottling is wine oxidation. It is not unusual for bottling to impart from 0.5 to greater than 2 mg of O2/L into the wine. Such addition can have a profound effect on wine quality and shelf life. It is therefore essential to know your bottling equipment and how it affects wine oxidation. Such production practices as sulfur dioxide additions just prior to bottling, nitrogen sparging, carbon dioxide or nitrogen flushing bottles prior to filling, vacuum corkers and fillers, etc., can be useful in limiting O2 problems.

The lost of free sulfur dioxide in wine is proportional to the dissolved oxygen content. Producers not using vacuum fillers, corkers or bottle gas flushing can have up to 5 mL of air in the head space of their bottled wine (750 mL). This amounts to approximately 1 mL (1.4 mg) oxygen. Four mg of sulfur dioxide are needed to neutralize the effects of 1 mg of oxygen. Using this relationship, an additional 5-6 mg of free sulfur dioxide is needed to reduce molecular oxygen in the head space. This represents a rather significant loss of free sulfur dioxide which could otherwise be available as an antimicrobial agent. If the extent of potential oxidation is high, wines should not be bottled cold due to the increased solubility of molecular oxygen. High levels of oxygen are particularly detrimental to wines which contain sorbic acid (potassium sorbate) due to the development of oxidative products which impart an unpleasant character to the wines.

4. Warehousing and Bottle Release. Bottled wines should be periodically tasted by a panel againstreference samples (held at < 40oF) to determine how the wine is developing. Too early a release date results in a less than fully developed bottle bouquet, too late may mean a large segment of the consumers could receive the wine after its quality began to diminish. Bottle aging is dependent upon the wine chemistry and the warehousing conditions. It is essential that the winemaker understands how each processing step affects wine chemistry and therefore wine shelf life.

5. Label Coding. Label coding is a means by which the winemaker can extend his quality control into the market place. By placing very small notches, one for day, month and year on the label, winery personnel can determine the bottling date and from there the complete history of the wine.

Label coding can be done by simply placing a stack of labels in a vice and using a saw to cut a small notch on each axis. Using a standard - usually a piece of plastic - the vintner can identify the bottling date. This can be highly important if the winery is forced to have several to many bottling runs of a particular wine lot. We have had several cases sheer biological or physical instability occurred with only one bottling date of a wine with several bottlings. Had these wineries coded their bottles they could have gone into the market place and simply removed only that particular bottling date effected. Instead they were forced to recall all bottling dates of that particular wine resulting in a major credibility problem - to say nothing of the direct economic loss.

Premium wine quality is the result of quality fruit and many processing steps. These steps, viewed individually, may be insignificant. However, collectively they make the difference between standard and outstanding wines. It is the responsibility of the winemaker to understand how production parameters affect wine quality and to make adjustments accordingly.

III. Wine Corks

In Virginia we have experienced both a rash of leaking bottles and some sensory problems as a result of cork closures. The following is a brief review of the work of Lee, et al. (1984) on the nature of off flavors and aromas believed to be derived from corks.

About half of the world's corks are grown in Portugal, principally in southern Portugal. After the trees reach maturity (25 years), about every 9 years the bark is stripped from cork oaks. The first stripping is known as virgin cork and is generally unsuited for wine closures. After cutting, the cork may spend a year in the forest before being transported for processing. As might be expected, a host of organisms, i.e., molds, bacteria, and yeast, have been isolated from cork wood prior to processing.

During processing, the cork slabs are first graded and then boiled in water for about one hour. Boiling would be expected to kill all microorganisms. Some microbes do survive, however, perhaps being located in the lenticel tissue where incomplete penetration of the water occurs. Additionally, the cork slabs may become contaminated simply due to the high titer of microorganisms in the air within processing plants (Lacey 1973). High levels of mold spores - particularly Penicillium glabrum have been found in cork following boiling. The boiling step, although designed to rid the cork of contamination may indeed promote microbial growth due to the increased water activity of the cork following boiling.

Following boiling, the cork slabs are stacked to flatten them and to equilibrate the moisture within the slabs. Naturally, mold grows on the slabs, particularly those on the outside of the stack. The slabs are then cut into strips and corks punched out. It is safe to say that the mold content has little or no direct effect on the sensory quality of the corks at this time - otherwise processing changes would have been instigated years ago.

Most corks after grinding and shaving are bleached by dipping in a solution of chlorine (approximately 35%). This is followed by a rinse with a 0.6% oxalic acid solution, followed by a water rinse. The corks are then sun dried, surface dried or subjected to vacuum drying. The water content in the corks is then about 6-8% which inhibits most microbial growth.

The bleaching step is considered by some to be the major source of musty cork off odor and flavor (Tanner, et al., 1981, Buser, et al., 1982). During bleaching, chlorophenols are formed from the direct chlorination of the cork lignin. Chlorophenols may then be methylated (CH3 group added) to form 2, 4, 6 trichloroanisole (2, 4, 6-TCA) (Buser et al., 1982). 2, 4, 6-TCA has a musty odor and is one of the most potent odorants known. The odor threshold needed to give the typical musty cork odor in wine is only 30 ng/L (Buser et al., 1982). A nanogram (ng) is equivalent to 1 x 10-9 grams. Tanner and Zanier (1983) report finding up to 130 ng/L of 2, 4, 6-TCA in corked wines and up to 40 ng in the cork itself.

Molds may also play a role in the formation of 'corky' wines. Curtis, et al., 1974, reports that some molds can easily methylate chlorophenols to form chloroanisoles. Whether or not off aromas from cork microflora can be prevented by simply using nonbleached corks is still unresolved. It is safe to say that there is increased interest in using either non-bleached corks or corks which have been bleached using hydrogen peroxide instead of chlorine compounds - such products are now available.

Lefebvre, et al. (1983) isolated a host of organisms from raw cork, new cork and corks extracted from bottled wines. They found a significant group of organisms which were able to produce compounds which they described as: phenolic, earthy, moldy, rancid, potato and pharmaceutical. This underscores the need for cork sterilization at the factory and the maintenance of sterility at the winery.

The sorting process combined with storage at the factory allows for high levels of microbial growth. Many believe that this time between possible bleaching and disinfection and use is where the real problem arises. During this time, recontamination of the corks can occur, particularly if the water activity of the cork is high. Water activity is the moisture content available for microbial growth or enzymatic activity. It is essentially a measure of the free water. Naturally, as the moisture content of a substance is lowered so is the water activity.

As a generalization, if corks are transported at 20oC, then they should be maintained below 8% moisture. At this temperature and moisture content the water activity is generally low enough to inhibit mold growth. Fortunately, this moisture content is consistent with the operation of most corkers.

To help ensure that corks are free of microorganisms that may affect wine quality winemakers either treat corks with sulfur dioxide before bottling and/or purchase treated corks from suppliers. Generally suppliers treat corks with gaseous SO2 or with ionizing radiation. Davis et al. (1982) demonstrated that SO2 treatment achieves nearly 100% inactivation of molds present in corks. Bacterial counts were not as effectively reduced, possibly because of resistance of spores from organisms such as Bacillus. Sulfur dioxide treatments satisfy the major prerequisite of elimination of mold from corks, however, direct mold growth in corks stoppered into wine may not be the sole cause of cork related off odors and flavors. As stated, mold growth during processing or shipping may produce metabolites which are both responsible for tainted wines and which are unaffected by SO2 treatments.

To help minimize the likelihood of cork problems - know what you are buying. Are the corks bleached? What is the moisture content when shipped? How are the corks sterilized? Do they remain sterile in your cellar?

IV. Coming Events

Annual Meeting of American Society for Enology and Viticulture and International Riesling Symposium

The 22nd Annual Meeting will be held at the Radisson Hotel in Corning, New York on July 9 - 11, 1997.

The International Riesling Symposium will involve:

Keynote speakers include:

Dr. Monika Christmann, Geisenhiem, Germany

Dr. Joachim Schmid, Geisenhiem, Germany

Dr. Ernst Ruhl, Geisenhiem, Germany

Dr. Andrew Reynolds, Agriculture Canada

Dr. Stan Howell, Michigan State University

Enk Olsen, Chateau Ste Michelle, Washington

Dr. Robert Pool, Cornell University

Dr. Thomas Henick-Kling, Cornell University

Peter Bell, Fox Run Vineyards, New York

Scott Osborn, Fox Run Vineyards, New York

Robert Summers, Cave Spring Cellars, Ontario, Canada

Dr. Sibylle Krieger, Condimenta, Stuttgart, Germany

The Technical Program of the annual meeting will involve:

Wine and Juice Production and Practical Monitoring Workshop

This course will be a regional meeting of the American Society for Enology & Viticulture-Eastern Section held on June 7 - 9, 1997 at the Holiday Inn Washington Dulles, 1000 Sully Road (Route 28), Dulles, Virginia 20166.

The program is presented and sponsored by the Enology-Grape Chemistry Group, Department of Food Science and Technology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia and Viticulture and Enology Research Center, California State University-Fresno, Fresno, California. See Vol. 12, No. 2, 1997 for details.