Enology Notes #97 January 13, 2005
To: Regional Wine Producers
From: Bruce Zoecklein, Head, Wine/Enology-Grape Chemistry Group, Virginia Tech
As wines age in the bottle, the oxidation-reduction potential decreases until it reaches a minimum value, which is dependent on the wine and how well it is sealed, among other things. 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.
Wine Closures, continued. The idea persists that corks let oxygen into the bottle, thus aiding in wine development. However, Duclaux, in his Traite de Microbiologie (1898), asserted the role of oxygen is of no account in the bottle: “In bottles, so long as the cork is sound…the protection of the wine in relation to oxygen is absolute or near absolute.” Decades ago, the French enologist Emile Peynaud noted that the ideal cork makes a perfect closed seal.
Desirable reactions taking place in a bottle require little or no oxygen. Studies involving wines sealed in glass ampoules, completely free from air/oxygen, showed that they developed aging bouquet. The question is -do they develop the same way or as well as wines with some oxygen?
The question of how much oxygen is needed for proper aging goes to the core of the current debate regarding screwcap closures.
Typically, consumers assume that wines need to be bottled in cork to properly age; after all, wines need to breathe through the cork, correct? No.
It may not be adequate to simply say that wines do not need oxygen for aging. The correct oxygen balance at bottling is imperative to ensure the stability of wine throughout its life. Wines do not require an ongoing source of oxygen throughout bottle development, but they do require the correct oxygen balance before bottling. Minute quantities of oxygen before or perhaps during bottling may be optimum, but this is yet to be fully resolved.
It is known that different wine styles perform differently under different closures. Why? An example of oxygen penetration in closures is given below.
Screwcaps 0.0005 mg/L
Natural cork 0.0179 mg/L
Thus, oxygen ingress post-bottling is usually very limited, or near zero. The exceptions include some synthetic closures. Initially, oxygen transfer with synthetics was thought to be a large problem. It may be.
We are entering the age of designer closures: a specific closure may be selected for a specific wine type and style. Therefore, a closure that allows a regulated ingress of oxygen may have its place, with the proper wine set for a specific shelf life.
An important consideration is how much oxygen is desirable in the wine at the time of bottling. This is different for different wines, and may also be very dependent on the closure selected, as will be discussed.
W know that we impart some oxygen in the wine at the time of bottling. How much is desirable, and how much is excessive? More than 1.0 mg/L oxygen at bottling is not desirable for any wine. Lower concentrations are best for aromatic whites. For example, in a study on Rieslings, aromatic freshness was best with no more than 0.2 mg/L; 0.5 mg/L produced citrus to lime notes as dominant characters, and at 1.0 mg/L, the wine was dominated by aged, toast-like tones.
Red wines, due to their higher buffering capacity, can withstand higher oxygen concentrations at bottling, up to about 0.7 mg/L. Naturally, the oxygen concentration at bottling has an influence on the redox potential: the more the oxygen, the higher the potential.
Factors influencing oxygen levels at bottling include wine temperature, bottling equipment, and closure type.
Oxygen in the bottle as a result of cork closure can be highly variable. When the cork is compressed in the neck of the bottle, gas pressure in the cork cells can double, thus releasing oxygen trapped in the lenticels. In a 750-mL bottle, several tenths of a cubic centimeter of oxygen can be released during the first weeks of bottle aging. How much oxygen is released depends on several factors, including the relative moisture content of the cork. The higher the cork moisture, the less oxygen is released.
If Ribereau-Gayon et al. (1976) are correct in saying, “Oxygen is not the agent of normal bottle maturation,” why do some red wines bottled under screwcaps have what has been termed the Peter Pan factor? That is, they do not age. Does a screwcap retard the development of a wine for so long that it effectively hibernates?
Generally, screwcapped wines develop at a slower rate, as evidenced by sensory evaluation and the higher free sulfur dioxide level demonstrated in most studies. It has been estimated by winemakers that have used both, that the aging ratio between cork and screwcaps can be 4 to 1, or even greater.
Variations in oxygen content of the empty glass, vacuum pressure effects of the corker, and slight variations in filler spouts can result in significant bottle variations with regard to dissolved oxygen in the wine.
30 mm of headspace is the international standard for fill height. This gives about 9 mL volume. If this volume is occupied by air, it adds about 2 mg/L oxygen. As such, many choose to reduce this volume, or have this space occupied by inert gas, such as nitrogen or carbon dioxide. In some bottling systems, the variation in oxygen remaining in the bottle after gas purge is significant.
Brajkovich (2004) suggests that the role of oxygen involves the following:
- 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 synthetic, this absorption can be from within the closure.
- Oxygen absorption ends within several months if the closure makes a complete seal.
The choice of closure to be used must depend on several factors, including when you expect the wine to be on the market, be sold, and be consumed. Naturally, these considerations are crucial, as targeted aroma/flavor compounds have different stabilities in wines, and are impacted by the wine’s oxygen content.
Screwcaps eliminate bottle variation and cork taint (but not necessarily environmental taint), while enhancing the keeping qualities of wine. It seems an abstract criticism to suggest that the reduction in aging rate with screwcaps is a problem, but it has been voiced. Changing to other closures, such as screwcaps, does not simply represent a cosmetic change. It may require fundamental change in winemaking practices. For a review of some of the issues, see Stelzer, 2004 Practical Winery and Vineyard, July/August. More to follow.
Wine Filtration Workshop. The Wine/Enology-Grape Chemistry Group, in conjunction with Pall Corporation, will offer a one-day juice and wine filtration workshop, February 10, 2005, at Horton Cellars, from 10:00 am to 4:00 pm. Fee is $35. See Enology Notes #95 for details.
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 both programs is limited. For registration questions, contact Terry Rakestraw at 540-231-6805 or .
Annual Meeting of the American Society for Enology and Viticulture-Eastern Section. The annual meeting of the eastern section of the ASEV will be held July 13-15 in St. Louis, Missouri. The meeting will involve an industry tour, technical presentations and a varieties symposium highlighting Pinot Gris, Traminette, and Norton. A half day session will also be devoted to several cold hardy varieties including Frontenac and La Crescent.
Additional information to follow. Mark your calendars!
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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