Enology Notes

Enology Notes #98 February 17, 2005

To: Regional Wine Producers

From: Bruce Zoecklein, Head, Wine/Enology-Grape Chemistry Group, Virginia Tech

Subjects: Wine Closures, continued; American Society for Enology and Viticulture–Eastern Section; Annual meeting of the American Society for Enology and Viticulture; Juice and Wine Fining Workshop

I. Wine Closures. As discussed in Enology Notes # 97, as wines age in the bottle, the oxidation-reduction potential decreases until it reaches a minimum value, which is largely dependent on the wine and how well it is sealed.

The nature and intensity of bottle bouquet is, in part, dependent on the redox potential. Thus, variations in closures and/or closure performance can impact the redox potential and the sensory characteristics of a wine. This issue continues the discussion of wine closures and the role of oxygen.

As discussed in Enology Notes # 97, the ullage space in bottles can be substantial. I reported that 30-mm headspace for screwcap closures was an international standard. Such a headspace could provide 9 mL of volume. However, Bruce Scott of Scott Labs reports that North American standard cork-finished bottles have fill heights ranging between 61 and 67 mm, with an average of about 64.

Bruce Scott notes that using a 45- or 49-mm cork in a bottle with a 64-mm fill height at 20°C provides an ullage space of 19 or 15 mm, respectively. The throats of bottles average approximately 0.3 mL per mm. This would result in a 5.7 or 4.5 mL volume at 20°C, using a 45-mm or 49-mm cork, respectively. If this volume is composed mainly of oxygen (air) it could impact wine development. Naturally, variation in the oxygen concentration in the ullage space can result in bottle variation.

Sulfur Dioxide. The free sulfur dioxide and optical density (OD) at 420 nm are perhaps the best predictive measures of how wines will hold up post-bottling, regardless of closure type. For example, there is a non-linear relationship between oxidized character development and the concentration of free sulfur dioxide. The cut-off appears to be about 13 mg/L free sulfur dioxide for white wines.

As wines develop in the bottle and retain a level of free sulfur dioxide exceeding 13 mg/L, the likelihood of developing oxidized aroma/flavors is minimized and the wine will usually remain sound. When the free sulfur dioxide level drops to less than about 13 mg/L, we can expect perceptible “developed” and/or oxidative aromas.

A level of 13 mg/L or more free sulfur dioxide should be considered an average concentration. Naturally, different wines have differing oxidative buffering capacities due to their redox potential, which is impacted by the total antioxidant concentration.

Antioxidants include phenols, ascorbic acid, glutathione, sulfur dioxide, etc. Regardless, small differences in post-bottling sulfur dioxide has been found to be strongly correlated to later sensory scores (Godden et al., 2001). Such differences can result from different oxygen levels as suggested by Brajkovich (2004):

• Wine absorbs oxygen at bottling, dependent on the bottling equipment.
• Wines continue to absorb oxygen from the ullage gas in the bottle.
• With corks and synthetics, this absorption can be from within the closure.
• Oxygen absorption ends within several months if the closure makes a complete seal.

The ratio of free to total sulfur dioxide may also be important. When there is a large difference in this ratio, the free is being bound. Several compounds in wine are active in binding free sulfur dioxide, including sugars, anthocyanins and acetaldehyde.

Acetaldehyde formation is a direct result of oxygen exposure. Differences in ratios of free to total sulfur dioxide have been observed in studies comparing screwcaps with natural and synthetic closures. Presumably, these differences are due to oxygen.

Monitoring the free sulfur dioxide concentration could be an effective means of reviewing both the impact of different closures, and how a wine bottled with a particular closure is holding up. This may allow for a periodic review of longevity so as to aid in wholesale and retail distribution as suggested by Godden et al. (2001).

A periodic evaluation of the free sulfur dioxide level from an adequate number of randomly selected bottles is an effective gauge for monitoring wine development. Again, if the free sulfur dioxide level in a white wine is maintained at 13 mg/L or above, oxidative aromas will be minimal.

Naturally, measuring sulfur dioxide concentration and the change in concentration over time to predict wine longevity implies a certain level of analytical accuracy and precision. Note that the Ripper titration method, and associated kits that may employ the same chemistries, may not be sufficiently accurate for such a review. I suggest using the Aeration Oxidation method (see Zoecklein et al., 1999), with reference standards.

The Wine/Enology-Grape Chemistry Group is currently evaluating the impact of accelerated aging, as a means of determining shelf life and differences among closures. This follows a study conducted by one of our students several years ago (Mansfield and Zoecklein, 2003). This will be discussed at the Wine Closure Roundtable to be held this spring (Check future Enology Notes and my website for the date, time, and location).

It is likely that critical loss of antioxidant levels relate, at least in part, to oxygen and oxygen permeability via closures. This will relate to the development of oxidized aroma and flavors, and the variation among wines bottled with different closure types. More to follow.

II. The American Society for Enology and Viticulture–Eastern Section. All wineries, regardless of size, operate within the global marketplace. This marketplace has been subject to a steadily increasing disparity between wine production and wine consumption.

This disparity (currently estimated at 50 million hectoliters) has resulted from a dramatic decline in consumption in traditional European wine-producing countries, and increased production in ever-increasing “new world” regions and subregions.

What does this imbalance in supply and demand mean to the eastern US wine industry? It is safe to say that market forces will promote additional competition, and some producers that do not adjust will not survive. Our challenge is not unique: like all regions, we need to retain current customers and persuade potential new ones. How do we do this?

To increase consumption of our region’s wines, we need wines with desirable sensory characteristics, products that present no risk, perceived or real, to the health of individuals and the environment, and distinctive wines that are of a competitive price per quality, while remaining profitable.

There is no question that the development and application of new technologies can assist the industry in meeting these challenges. Winemakers in our part of the world must become more sophisticated in applying new technologies, when appropriate, while maintaining the cultural and traditional nature of our industry.

Through the exchange of scientific research findings, student scholarships, and practically oriented industry symposia, the American Society for Enology and Viticulture–Eastern Section has been working to aid the industry in meeting today’s challenges.

A significant portion of the Society’s revenues go toward providing scholarships to viticulture and enology students working throughout the region. In virtually every state in the Midwest and East, ASEV-ES students are engaged in practical research studies that could impact our industry. Most of these students are training to become our industry’s future grape growers and winemakers.

In recent years, the Society has sponsored symposia on a broad range of practical vineyard and winery issues, including a cork symposium, and programs on varietal wine production, sparkling wines, wine taints, and grapes, wines and the environment. The purpose of these programs has been to provide the best technically oriented information available, to allow our industry to reach its full potential.

The American Society for Enology and Viticulture–Eastern Section is the professional society for your industry. Your support of the Society helps to assure that all of us meet the challenges of today and the future. If you are not a member, I urge you to become one today. Membership is very inexpensive.

III. Annual Meeting of the American Society for Enology and Viticulture, 2005. I am pleased to announce that the 2005 annual meeting of the American Society for Enology and Viticulture–Eastern Section will be held July 13-15 in St. Louis, Missouri, at the Millennium Hotel. This spectacular facility, on the banks of the Mississippi, is within walking distance of the Arch and downtown cultural attractions.

The meeting will involve technical presentations, the Viticulture Consortium East research summit, wine industry trade show, annual banquet, local wine industry tour, and a symposium.

This year’s symposium will involve viticulture and enology discussions, and sensory evaluations, on the Cutting Edge Varieties: Norton, Pinot Gris, Traminette, and the cold-hardy Frontenac and La Crescent. Speakers from the academic community, commercial growers, and winemakers will present practical information and extensive sensory evaluations. For information, see the website at www.nysaes.cornell.edu/fst/asev/.

IV. Juice and Wine Fining Workshop. The Wine/Enology-Grape Chemistry Group, in conjunction with Scott Laboratory, will offer an afternoon advanced workshop on juice and wine fining, February 28, 2005, at White Hall Vineyards, from 12:30 to 4:30 pm. Fee is $30. See Enology Notes # 95 for details.

Enrollment for the program is limited. For registration and questions, contact Terry Rakestraw at 540-231-6805 or .

References

Brajkovich, M. 2004. Bottle maturation. In: New Zealand Screwcap Wine Seal Initiative. First New Zealand Screwcap Symposium, Marlborough, New Zealand, November 10-13.

Godden, P., Franccis, L., Field, J., Gishen, M., Coulter, A., Valente, P., Hoj, and E. Robinson. Wine bottle closure. 2001. Aust. J. Grape and Wine Research.

Mansfield, A.K.and B. W. Zoecklein. 2003. Effect of fermentation, post-fermentation and post-bottling heat treatment on Cabernet Sauvignon glycoconjugates. Am. J. Enol. Vitic. 54: 99-104.

Zoecklein, B., Fugelsang, K.C. and B. H. Gump. 1999. Wine Analysis and Production. Kluwer Academics, New York, New York. pp 621.

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Dr. Bruce Zoecklein
Professor and Enology Specialist Head Enology-Grape Chemistry Group
Department of Food Science and Technology, Virginia Tech
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: http://www.vtwines.info/
Phone: (540) 231-5325
Fax: (540) 231-9293
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