defence of screwcaps
to recent news pieces on screwcap taint
Jamie Goode, 19th January 2007
The last few days have seen a number of reports in the
national press about wine ‘faults’ caused by screwcaps, not only
in the UK (Times, Daily Mail, Daily Telegraph, BBC News Site), but
also in New Zealand and Australia.
I feel strongly that there is a need to address these
reports because, in my opinion, they represent the science of closures
badly, are filled with inaccuracies and misunderstandings, and do not
serve the wine industry well: they have the potential for misleading
consumers about an important issue.
The reports are centred on the results from the faults
clinic at the International Wine Challenge. It is puzzling that this
story should suddenly be deemed ‘news’: the results were actually
released last September (see here),
and were given some press coverage then.
My concern is that while the issue of screwcap reduction is an
important one that needs to be addressed by the industry, the reality
is rather different (and rather more complex) than the one portrayed
by these news reports.
Significantly, the pieces fail to recognize that
screwcaps with liners allowing different oxygen transmission rates
exist, and that winemakers can work with their wines to bring them to
a point at bottling where reduction is much less likely, even if they
use the tin-lined screwcaps that are currently the most popular sort
and which allow very little oxygen transmission. Also, when reduction
does occur, it is not the same as cork taint, which irredeemably ruins
a wine, but is rather a more subtle effect that almost all consumers
would fail to spot.
I’ll address the specific points raised in two of the
news pieces below.
contrast to traditional corks, the caps stop the wines breathing,
leaving them at risk of the chemical process of sulphidisation.”
‘sulphidisation’ is meaningless here. And the term ‘breathing’
is misleading: the normal level of oxygen transmission by a cork is
tiny, albeit significant. ‘Breathing’ suggests that air can
diffuse/permeate freely through the cork: this would result in
the top is taken off, wine lovers are confronted with the reek of
sulphur—likened to burning rubber or rotten eggs—rather than an
attractive bouquet.” Daily Mail
problem with screwcap reduction, when it occurs, is the presence of
disulfides and thiols (this is another name for mercaptans). Typical
descriptors for these are ‘burnt match’, ‘struck flint’,
‘rubber’ – not rotten eggs (see the table below). The latter is
a descriptor for hydrogen sulphide. Hydrogen sulphide is caused by
reduction: a lack of oxygen in fermentation, or an absence of other
nitrogen sources means that yeasts use the amino acid cysteine as a
nitrogen source, producing hydrogen sulphide. Hydrogen sulphide is
then reduced to disulfides, which in turn can be reduced to mercaptans.
In the reduced environment of a wine sealed by screwcap you’d be
unlikely to see eggy hydrogen sulphide, unless the winemaker was
negligent to bottle an already reduced wine this way.
producers switched to screw caps about five years ago because of
increasing concerns over cork. It can be affected by a mould -
trichloroanisol - leaving as many as 4.4 per cent of wine bottles with
an unpleasant smell and taste.” Daily Mail
is not a mould. It’s a compound produced by microbes that live in
the lenticels of cork bark.
problem with the screw caps was identified at the annual International
Wine Challenge (IWC) event at which tens of thousands of wines from
all over the world are examined. A test of 9,000 bottles found 2.2 per
cent suffered from sulphidisation and other problems linked to the
wine not being able to absorb oxygen, or ‘breathe’.” Daily Mail
last autumn on 9,000 screwcap wines by the International Wine
Challenge found that 2.2 per cent of bottles were affected by
sulphidisation because the contents were not allowed to breathe.”
No, there were
10 000 bottles opened overall, and of these, 2.2% of those sealed with
screwcaps (a much smaller number) were found to have sulfide issues.
This was sensory analysis only. No bottles were subjected to chemical
testing. The dreaded ‘sulphidisation’ term was not used anywhere.
‘In a number of cases the
IWC chairmen validated a link between screw cap use and a unfavourable
vegetal/rubber flavoured compound—presumed to be a complexed
sulfide’, reported Sam Harrop, who headed up the IWC faults clinic,
when I quizzed him on this. ‘At first glance a percentage of 4.9% of
total faults may not seem high, but when examined in the context of
total screw cap figures, a more worrying rate of 2.2% [of all
screwcapped wines] emerges. In the context of the 2006 IWC cork taint
figure of 2.8% [of all natural cork-sealed wines], this fault type is
significant and should be given more attention by wineries using
screwcap.’ However, Harrop was keen to emphasize that he wasn’t
equating the two, as some of the newspaper reports did: ‘While the
IWC figures for screwcaps are a concern, there is no question in my
mind that the continued incidence of cork taint is still a more
naturally occurring sulphides degrade in wine, they produce the
compound thiol, which gives sulphur its smell.” Daily Mail
smells of sulphur. Thiols smell quite different (see table below).
expert Martin Isark said consumers find it much easier to identify the
sulphidisation caused by screw caps than problems with cork.
Consequently, they are more likely to return these bottles to stores.
‘The everyday wine shopper would have no problem identifying a wine
smelling like a stink bomb,’ he said.” Daily Mail
thought that the problem of corked wine had been solved by introducing
the screwtop. But now red wine producers are grappling with an even
worse issue: the whiff of rotten eggs.” The Times
wine stores and supermarkets have been told to expect returns from
unhappy customers.” The Times
No. This is
wrong, in my opinion. He’s confusing hydrogen sulfide with the other
sulfur compounds. I’ve tasted a number of wines with what I suspect
to be screwcap reduction, and it’s not as noticeable as cork taint.
I doubt any consumers will spot it unless they are coached. Certainly,
the 2.2% of screwcapped wines picked out at the IWC weren’t
suffering from eggy/drainage hydrogen sulphide smells.
caps have tight-fitting seals which prevent the air from getting in.
Some wineries are working on designs with more room around the head of
the bottle to allow air in.” Daily Mail
have tight-fitting seals which prevent air from getting in. Some
wineries are working on designs with more room around the head of the
bottle to allow air in.” The Times
similar sentences, which makes me think that the journalists were
working from a press release sent in, or reworked the same original
article. Who was the author? Anyway, it’s wrong again. The author
fails to distinguish that there are different sorts of screwcap liner.
Those with a tin layer in the liner (the majority) allow very little,
but some, gas transmission. Those with a saranex-only (white-looking)
liner allow a good deal more. Screwcap manufacturers are looking at
engineering liners with intermediate gas transmission, but no one
would think of allowing ‘more room around the head of the bottle’
because this would cause the wine to oxidize fast!
TABLE 1 Some of the volatile sulfur compounds in wine
Rotten eggs, sewage
This is the main baddy,
made by yeasts when they use one of the sulfur-containing amino
acids as a nitrogen source. Stress also encourages its
Mercaptans (also known as
This is a large group of
very smelly sulfur compounds. Terms such as cabbagey, rubbery,
struck flint or burnt rubber are used as descriptors.
If hydrogen sulfide
isn’t removed quickly, it can result in mercaptan production.
This is a big worry for winemakers.
burnt match, sulfidy, earthy
Often negative, but can be
positive in the right wine environment at certain levels.
methyl mercaptan (methanethiol)
rotten cabbage, cooked cabbage, burnt
rubber, stagnant water
One of the compounds
implicated in screwcap reduction
Cooked vegetables, cooked
corn, canned tomato at high levels; blackcurrant drink
concentrate at lower levels. Quince, truffle.
Sweet, ethereal, slightly green, sulfidy
Vegetal, cabbage, onion-like at high
garlic, burnt rubber
3-mercaptohexan-1-ol (3MH), 3-mercaptohexyl acetate (3MHA)
fruit at low levels; cat’s urine at higher levels.
Common in Sauvignon Blanc
but also found in red wines where they can contribute to the
blackcurrant fruit aroma. An example of sulfur flavours that can
be positive in the right environment.
Can be positive in the
right context and at the right levels
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