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The wine fault known
as 'random oxidation' costs the wine industry dearly but the
chemistry underlying the process is, as yet, unclear and solutions
to the problem are not well appreciated.
Recent research that is beginning
to reveal the factors that contribute to random oxidation
may help winemakers avoid the problem.
Random oxidation describes the
phenomenon where a bottle of white wine turns brownish, losing
flavour and bouquet six to 18 months after bottling. The acknowledged
incidence ranges from one in 100 to one in 25 bottles.
Many winemakers blame random
oxidation on the closure, citing variations in cork quality.
To complicate matters, the fault is often confused with 2,4,6-trichloroanisole
(TCA) contamination.
Winemakers typically add sulfur
dioxide to wine to prevent oxidation. However, if the sulfur
dioxide content drops below a certain concentration, browning
may commence suddenly.
Ascorbic acid, a well-known antioxidant,
is sometimes also added to wine to 'brighten' the fruit quality
and scavenge oxygen.
According to Professor Geoff
Scollary, director of the National Wine and Grape Industry
Centre (NWGIC) at Charles Sturt University in Australia, solving
the riddle of random oxidation may depend on the interaction
between ascorbic acid and sulfur dioxide.
Ascorbic acid itself acts as
an antioxidant, but it breaks down into chemicals that can
promote oxidation. One of these is hydrogen peroxide but NWGIC
researchers have shown that there is another, as-yet-unknown,
breakdown product of ascorbic acid that also promotes oxidation.
Scollary's team has established
that when ascorbic acid and sulfur dioxide are present in
similar quantities, the onset of oxidation is delayed, but
not prevented, providing a possible explanation for the six
to 12 months lag before oxidation begins.
Contrary to the advice in some
winemaking texts, his team found that the addition of ascorbic
acid required more sulfur dioxide to prevent oxidation than
would be required if no ascorbic acid were present.
The use of ascorbic acid is a
controversial practice and researchers recommend its addition
at bottling time only. However, some wineries continue to
add ascorbic acid at the crusher.
Research also indicates that
the wine cork may not be as significant a factor in random
oxidation as many believe.
The search for chemical residues
within corks that might oxidise wine has proved largely fruitless,
while experiments on the oxygen permeability of cork stoppers
have been inconclusive.
Retired wine chemist John Casey
says the oxygen and oxidants that initiate random oxidation
are likely to be introduced during wine transfer and during
bottle filling and corking. Small amounts may also diffuse
from the cork.
"Excluding air during wine
transfer and bottling and ensuring adequate levels of sulfur
dioxide are the way to solve this problem," he says.
Vacuum corkers remove air from
the headspace, but they do not work consistently well, making
it possible for some bottles, but not others, to be sealed
with significant amounts of oxygen. Other sources of oxygen
pick-up include small leaks in feed lines and stoppages or
slowdowns in bottling runs.
"There is a widespread view
that the permeability of corks is the cause of post-bottling
oxidation, but the evidence just doesn't stack up," Casey
said.
The full text of this article, including references,
is available at www.corkfacts.com/banrmenu/webxmain.htm
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