I thought I would pass this along for all those interested:

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Enology Notes #97
January 13, 2005


To: Regional Wine Producers


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


Subjects: Wine Closures, continued; Upcoming Events: Wine Filtration
Workshop, February 10; Juice and Fining Workshop, February 28; Annual
meeting of the American Society for Enology and Viticulture.

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.


We 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!

All past Enology Notes newsjournals are posted on the Wine/Enology - Grape
Chemistry Group's website at: http://www.vtwines.info


To be added to (or removed from) the Enology Notes listserv, send an email
message to with the word ADD or REMOVE in the subject line.



Dr. Bruce Zoecklein

Professor and Enology Specialist

Head, Wine/Enology - Grape Chemistry Group

Department of Food Science and Technology

Virginia Tech, Blacksburg, VA 24061

Wine/Enology - Grape Chemistry Group website:

www.vtwines.info