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In defence of screwcaps
Responding 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.

“In contrast to traditional corks, the caps stop the wines breathing, leaving them at risk of the chemical process of sulphidisation.” Daily Mail

The term ‘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 oxidation

“When 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

The 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.

“Many 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

2,4,6-trichloroanisole is not a mould. It’s a compound produced by microbes that live in the lenticels of cork bark.

“The 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

“Tests 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.” The Times

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 serious issue.’

“When naturally occurring sulphides degrade in wine, they produce the compound thiol, which gives sulphur its smell.” Daily Mail

Wrong. Sulphur smells of sulphur. Thiols smell quite different (see table below).

“Wine 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

“They 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

“Leading 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.  

“Screw 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

“Screwcaps 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

Both quite 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


Sensory impact


Hydrogen sulfide

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 formation.

Mercaptans (also known as thiols)

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.

ethyl mercaptan



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

dimethyl sulfide

Cooked vegetables, cooked corn, canned tomato at high levels; blackcurrant drink concentrate at lower levels. Quince, truffle.


diethyl sulfide 



carbon disulfide

Sweet, ethereal, slightly green, sulfidy


dimethyl disulfide 

Vegetal, cabbage, onion-like at high levels


diethyl disulfide   

garlic, burnt rubber


4-mercapto-4-methylpentan-2-one (4MMP), 3-mercaptohexan-1-ol (3MH), 3-mercaptohexyl acetate (3MHA)

Tropical fruit/passion 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.


Smoky/gunflint aromas

Can be positive in the right context and at the right levels


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