Enology Notes

Enology Notes #99, March 8, 2005

To: Regional Wine Producers

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

Subjects: Closure Review (continued): Flavor Scalping; Upcoming Events: Wine Closure Roundtable Meeting, American Society for Enology and Viticulture, Eastern Section meeting

1.Closure Review: Flavor Scalping. The Wine/Enology-Grape Chemistry Group at Virginia Tech is involved in a study evaluating the impact of closures on wine volatiles.

Wine volatile compounds can be affected by closures in at least two ways. As discussed in previous editions, differences in oxygen and oxidative degradation can occur, due to bottle closure type. Additionally, most closure materials have the ability to adsorb, or scalp, some aroma/flavor volatiles from the wine. This continues the Enology Notes series on wine closures.

In our studies, identical wines were bottled using at least two different closure types. At various stages post-bottling, we quantified 27 headspace volatile compounds in each wine, using solid-phase microextraction, gas chromatography-mass spectrometry (as described by Whiton and Zoecklein, 2000).

The aroma components of grapes and wines consist of hundreds of individual constituents, representing a number of chemical functional groups. However, the most important fermentation-derived volatiles can be grouped into three main chemical classes.

Ethyl esters of medium-chain fatty acids (ethyl butyrate, hexanoate, octanoate, decanoate, and dodecanoate) represent compounds which are fruity, wine-like, and usually contribute positively to wine odor.

Acetate esters, such as isoamyl and hexyl acetate, are responsible for the tropical fruit and banana-like notes. A third group of chemical compounds we evaluated were the higher alcohols, which included isobutanol, isoamyl alcohol, and hexanol. These are rather unpleasant when isolated and characterized alone, but may provide complexity to wines if their collective concentration is below 300 mg/L.

The analysis of headspace volatiles is of interest in helping to categorize differences among wines. However, there are few individual compounds whose concentrations can be directly correlated to sensory responses.

Therefore, sensory analysis was also conducted using a paired comparison test procedure (as described by Duncan, 1999), in which three sets of wines were presented. Sixty participants reviewed the wines blindly and indicated which wine was preferred in each set.

Examples of our results are as follows, comparing natural corks, screwcaps and synthetic closures. All volatile data shown are significantly different at the 5% confidence level.

Wine 1. Cabernet Franc, 1994. Natural cork vs. synthetic closure. Generally, as indicated in Fig. 1, there was a higher concentration of alcohols present in the synthetic closure wines, and a lower concentration of esters. Specifically, the synthetics had a lower concentration of ethyl hexanoate, ethyl octanoate, and ethyl decanoate.

Figure 1.

For the sensory evaluation, 65% preferred the natural cork, while 35% preferred the synthetic.

Wine 2. Red blend consisting of 38% Merlot, 25% Petit Verdot, 25% Tannat, and 12% Malbec, bottled in natural cork vs. synthetic closure (different from wine 1). The natural cork had a higher concentration of ethyl hexanoate, octanoate and decanoate, as well as two higher alcohols, isobutanol and isoamyl alcohol.

Figure 2.

For the evaluation, 60% preferred the natural cork, 40% the synthetic.

Wine 3. Red blend consisting of 32% Mourvedre, 27% Carignan, 25% Syrah, 5% Tinta Cao, 5% Touriga Nacional, 3% Tannat, 1% Cabernet Franc, 1% Malbec and 1% Nebbiolo, natural cork vs. synthetic. The analysis of the headspace volatiles indicated a reduction in the ester concentration in the synthetic closure wines, as indicated by Fig 3.

Figure 3.

This wine was not reviewed sensorially, due to time constraints.

Wine 4. Chardonnay, 2001. Natural cork vs. synthetic. The wines did not differ very much in the concentration of headspace volatiles measured. There were higher concentrations of ethyl octanoate and ethyl decanoate in the natural cork wines.

Figure 4.

The sensory preference indicated that 45% of the evaluators preferred the natural cork, while 55% preferred the synthetic.

Conclusions. Wine volatile compounds can be impacted by closures as a result of oxidative changes. Additionally, volatile compounds can be adsorbed, or scalped, onto the closure itself. Regardless of the mechanism(s) involved, it appears that non-polar, or non-water soluble, volatile compounds are in lower concentration with synthetic closures, thus demonstrating significant reductions in the concentration of certain esters.

This study is another example of both the complexity of wine and the subjective nature of wine preference evaluations. While in many cases the concentrations of key volatiles were different, evaluators’ responses frequently did not demonstrate dramatic preferences.

2. Upcoming Events:

Wine Closure Roundtable Meeting. A wine closure roundtable meeting is scheduled for April 25 at Veritas Winery. The primary purpose of this meeting will be to evaluation wines bottled using screwcaps, synthetic closures and natural cork. Wines will include products I brought back from New Zealand and wines involved in our wine closure research trials. Pre-registration will be required. Further details to follow. Mark your calendars!

Annual Meeting of the American Society for Enology and Viticulture, 2005. The 2005 annual meeting of the American Society for Enology and Viticulture–Eastern Section will be held July 13-16 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/.

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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