Random oxidation - not so random However, recent research is beginning to elucidate the factors that contribute to random oxidation
and consequently may help winemakers avoid the problem.
Random oxidation (also known as sporadic post-bottling oxidation) describes the phenomenon where,
for no apparent reason, a particular bottle of wine, usually white, begins to turn brown and lose
flavour and bouquet six to 18 months after bottling. Estimates of its incidence range from one in
100 to one in 25 bottles. The problem is often missed during production or distribution because
the browning typically does not start until the wine has reached the retail market.
Many winemakers attribute random oxidation to the closure, blaming variations in cork quality.
To complicate matters, the fault is often confused with TCA contamination. Both faults are responsible
for a flattening of the fruit quality and, at low levels, even experienced wine noses cannot easily
distinguish between them.
Winemakers typically add sulfur dioxide (SO2) to wine to prevent oxidation. SO2 is a traditional
and reasonably effective antioxidant, but its role is sacrificial: it is consumed in the process of
reacting with any oxidants present in the wine at the time of bottling. When the SO2 content drops below
a certain concentration, browning may commence suddenly, resulting in rapid loss of the wine's qualities.
Ascorbic acid, a well-known antioxidant in biology, is sometimes also added to wine to 'brighten' the fruit
quality or to scavenge oxygen dissolved in the wine.
According to Professor Geoff Scollary, director of the National Wine and Grape Industry Centre (NWGIC)
at Charles Sturt University in Australia, solving random oxidation may depend on understanding the interaction
between ascorbic acid and sulfur dioxide.
Although ascorbic acid itself acts as an antioxidant, it breaks down into chemicals that actually promote oxidation.
One of these chemicals is hydrogen peroxide but NWGIC researchers have shown that there is another,
as-yet-unknown breakdown product of ascorbic acid that also promotes oxidation.
Using an experimental wine model, Scollary's team has established that when both ascorbic acid and SO2
are present in similar quantities, the onset of oxidation is delayed, but not prevented, providing a possible
explanation for the 6 to 12 months delay before the onset of random oxidation.
Contrary to the advice offered by some winemaking texts, his team found that the addition of ascorbic acid
required more SO2 to prevent oxidation than would be required if no ascorbic acid were present.
Scollary said that to fully protect against browning in the wine model, SO2 molecules had to outnumber ascorbic acid molecules by three to one,
even for the relatively short duration of the experiment (14 days).
The use of ascorbic acid has long being a controversial practice and researchers have recommended its addition
at bottling time only. However, some wineries continue to add ascorbic acid at the crusher, a practice likely
to increase its pro-oxidant potential.
Further, it appears the closure may not be as significant a factor in random oxidation as many believe.
The search to identify chemical residues within corks that might oxidise the wine has proved largely fruitless,
while experiments on the oxygen permeability of cork stoppers have been inconclusive.
Oxygen and other gases can permeate cork but, overwhelmingly, experience shows that cork stoppers can be an
effective oxygen barrier for wine bottles for periods of many years.
In the NWGIC's wine model, allowing the wine to be exposed to air after the browning commences does not appear
to affect the final level of browning that develops. This, says Scollary, "suggests that oxygen ingress is required only
to initiate the browning," but not to keep it going.
Retired Australian wine chemist John Casey says the oxygen and oxidants that initiate random oxidation
are likely to be introduced during transfer of the wine and during the filling and corking operations.
Small amounts may also diffuse from the cork.
"In winemaking, some contact with air is inevitable," he says. "Oxygen is unavoidably incorporated in the wine during
bottling and also it diffuses from the cork after it has been compressed in the neck of the bottle.
Excluding air during wine transfer and bottling and ensuring adequate levels of SO2 are the way to solve this problem."
Vacuum corkers, for example, remove air from the headspace between the cork and the wine,
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 possible sources of oxygen pick-up include small leaks in feed lines and stoppages or slowdowns in bottling runs.
"These sorts of malfunctions are often intermittent, which is why oxidation may occur in one bottle but not the next.
But when winemakers see this, they think the only significant difference between the two bottles are the corks.
"There is a widespread view that the permeability of corks to oxygen is the cause of post-bottling oxidation,
but the evidence just doesn't stack up," he says.
Casey believes some wineries fail to take account of the need to compensate for the SO2 that is consumed as a result
of the bottling method used. He estimates that bottling without measures to reduce air contact might result in up to 30
milligrams of SO2 per litre being lost - a surprisingly large amount, given that white wine generally requires about 20
milligrams per litre for reliable long-term protection from oxidation. A winery that uses inert gas flushing and vacuum
bottling might only need to add an extra 10 to 20 milligrams of SO2 per litre to compensate for bottling-related losses.
It all depends on how effectively and consistently contact with air is prevented. Casey believes the winemaker should seek
to achieve consistency, where a constant amount of oxygen and oxidants goes into each bottle of wine, rather than trying to
eliminate oxygen, which can lead to variable oxygen levels.
With the trend towards minimising SO2 levels in the final product, wineries should take extra care in determining the amount
required for a particular combination of wine and bottling process. When SO2 is added to a wine, some of it becomes bound to other
wine components. The bound SO2 plays a crucial role in tying up carbonyl compounds that, if released, would lead to adverse colour
and odour development. The rest, known as free SO2, is what prevents oxidation and inhibits bacterial growth.
If free SO2 drops to very low levels as it is used up in preventing oxidation, bound SO2 is converted back into free SO2.
The right amount of SO2 for a particular wine can be judged from experience or by stepwise addition. Wineries are advised to wait
for several hours or even a couple of days before attempting to measure free SO2 because it may take this long for the bound and free
forms to reach equilibrium.
Casey says manipulating the level of free SO2 in a wine requires a good understanding of the variable relationship between free and total SO2.
For example, it may require an extra 20 milligrams of SO2 per litre to raise the free SO2 from 5 to 15 milligrams per litre,
but only 10 milligrams per litre to raise it from 20 to 30 milligrams per litre.
Winemakers may baulk at the idea of adding up to an extra 30 milligrams of SO2 per litre to a wine before bottling when the wine is intended
to have a final level of only around 20 milligrams per litre. But the result may be worth it.
Wines with low pH values (less than around 3) may be adversely affected by free SO2 levels above 35 to 40 milligrams per litre,
but post-bottling decline would reduce this problem.
Amorim welcomes comment on the issues raised in this article. Please contact us at
feedback@corkfacts.com
--ENDS FEB 2003--
References
For so serious a problem, the wine fault known as random oxidation
is poorly understood. 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.
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