This was found on rec.crafts.winemaking. I am in no way qualified to
determine the merits of the posted material, but am posting it for the
benefit of those on alt.food.wine. Jim
Quoted material follows:

> 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