Fighting cork taint: are
screwcaps and plastic corks the answer?
Jamie Goode analyses the results
from an significant independent study on the effectiveness of wine
bottle closures. This scientific paper, published on July 12 2001, has
thrown up some surprising results, and the ongoing trial it describes
promises to answer the key question of whether alternatives to cork
are suitable for long-term ageing of wine.
What's the single most controversial issue in the wine world today?
Without doubt I'd argue that it's the good old cork taint debate.
Plenty has been written on this subject, but what have been lacking
are decent independent data. In fact the whole area has been so
sullied by commercial interests (cork manufacturer Amorim has been
busy offering hospitality to journalists, and I'd be surprised if
synthetic cork manufacturers haven't been doing the same), that it is
only solid results that will settle this issue once and for all. Yet
more anecdotal observations and strongly held opinions won't really
do. This is why I was delighted to receive through the post the first
results of the Australian Wine Research Institute (AWRI) Wine Bottle
Closure Trial. Published in the Australian Journal of Grape and
Wine Research, this highly significant study looks like it will
help answer the question of what really is the best way of sealing
wine bottles. To those unfamiliar with the scientific literature, this
41 page paper would be pretty heavy going, so in this feature I'll
attempt to unpack what the data really show, and assess their real
significance.
The key issues
The widespread existence of cork taint, caused by chloroanisole
compounds, is now universally acknowledged, although there is still
debate over the prevalence. Whether it is 2%, or 5% - or as some
claim, as high as 7% - it's a huge problem, and so wine producers have
been looking for alternative closures for some time. But are these
alternatives - whether they are screwtops, plastic corks or
agglomerate corks - suitable for long-term storage of wine? It's one
thing bottling an inexpensive quaffer with a plastic cork, but would a
manufacturer be wise in using the same closure on a wine intended for
10 years in the cellar? And even if the plastic cork would maintain a
good seal for this length of time, would the wine age differently than
if it were sealed with a real cork? These are the questions this study
is designed to address.
Experimental design and
statistical treatment
The key to any study is good experimental design. You have to plan the
experiment so that with the appropriate use of statistics, you can be
sure you are able to answer the questions you are trying to assess
(the results have to be shown to be 'statistically significant'). In
this sense, the study here seems to be quite carefully thought out,
and the statistics look pretty solid. The essential details of the
experimental design are as follows:
- 14 different closures were analysed (see different closure types
used in the study): one ROTE (stands for 'roll-on tamper evident';
a posh name for a screwcap, or 'Stelvin'), two 'technical corks'
(part cork, part synthetic substance), two different grades of
standard cork and nine different plastic corks
- The wine used was a 1999 Clare Valley Semillon that had been
fermented and stored in stainless steel tanks
- Care was taken to ensure consistency in bottling with the
different closures
- Two batches each of 300 bottles were sealed with each closure,
with the exception of the screwcap (which had a single bottling
run of 800)
- After bottling the wines were stored in a temperature controlled
facility, where storage was randomised, with each bottle given a
separate number
A range of tests were then performed at three monthly intervals,
with the variables studied including:
- Extraction force required
- Ease of reinsertion into the neck of the bottle
- Incidence of leakage
- Concentration of free and total sulphur dioxide (SO2)
- Concentration of ascorbic acid
- Browning measures
- Wine sensory analysis, using a panel of experienced tasters
working under strict guidelines
- Interrelationships between sensory data and wine composition
analysis
The most important
results and conclusions
First, it's important to bear in mind that these are the first results
to be published from this ongoing study. There is enough wine bottled
for the study to continue for a decade, and as such these results are
best viewed as preliminary. Having said that, even at this early stage
some conclusions can tentatively be drawn.
Importantly, this study showed that the wines that retained the
highest concentration of SO2 and ascorbic acid showed the
lowest degree of browning, and in the sensory analysis tasted the
freshest. It seems that SO2 concentration, which is
relatively easy to measure, can act as a good predictor of future
browning and a useful proxy for oxidation. It therefore offers a
convenient way of measuring the effectiveness of a stopper.
But how did the different stoppers perform? The curves plotting
free SO2 concentration against time show a pronounced dip
with all the stopper types over the first year, that then begins to
flatten out. The best performance is from the ROTE (screwcap), closely
followed up by the Altec and then the Twintop. The two standard corks
perform pretty well, as do the Auscork, NuKork, Nomacorc, Aegis,
Supremecorq and Integra: these all perform surprisingly similarly.
Slightly less effective are the Tage and the ECORC, and the only
significantly poorer performer seems to be the poor old Betacorque
(which showed dismally). With regard to browning, it's even harder to
separate the various stoppers, with only the Betacorque performing
less well than the others. The ROTE seems to perform the best again,
but not significantly so.
The sensory analysis threw up some surprising results. The ROTE
closure produced a rubber-like flavour/aroma in the wine after 18 months. This
is unexpected and alarming, considering that the most of the Clare
Valley Riesling producers decided to switch to screwcaps for the 2000
vintage, in a well publicised move. Considering that there is a long
track record of bottling Riesling using screwcaps, and many library
reference samples are available, it's an odd result. The authors of
this paper suggest that it could be a consequence of the lack of
oxygen, and that leaving a slightly higher headspace may have
alleviated this anomalous result. Apart from this, no plastic-type
taint was associated with any of the synthetic corks. [Note added
later: Peter Godden, lead author of this trial, communicated the
following by e-mail:
"We are very confident
that the 'rubber-like' character is not a
taint, but is an unwelcome
modification due to chemically reduced
sulfur, as a result of lack of
oxygen. However, it is certainly an
important character in ROTE-closed
wine, and we have highlighted its
existence to avoid mass-bottling of
wine
under extremely anaerobic conditions which
might then develop a similar
character somewhere in the future.
However, you can see from the other
sensory data that it has not detracted
from the fruit characters and
intensity of aroma of the wine to a great
extent so far, although the
intensity seems to have increased at 21
and 24 month post-bottling
testing."]
As many professionals have long suspected, the Altec cork tainted
all the bottles with TCA from an early stage, despite providing a very
good seal. Surely the results of this study should spell the death
knell for the use of this stopper by producers who care at all about
the quality of their wines. Two of the 14 one-plus-one closures were
affected, and for both of the normal corks, four of the 14 bottles
were tainted with TCA. These results were confirmed with GC/MS (gas
chromatography/mass spectrometry; an analytical technique).
I was reassured by the fact that the performance of the synthetic
closures was very similar to that of the normal corks, but without the
taint. What this study doesn't show is that plastic stoppers are
unsuited to long-term ageing of wines. We'll have to wait for a few
more years' data yet, but I suspect that the best performing synthetic
corks will be just as good as real corks, but without the variability
in performance. This would be an exciting result for all those who've
gone to the cellar only to find that their irreplaceable aged bottle
has been ruined by cork taint.
Of the synthetics, the best performers in the sensory analysis seem
to be the Auscork, Aegis, NuKork, Integra, Supremecorq and Nomacorc.
The poorer performers are the ECORC and Tage, and of course the poor
old Betacorque. Thus both the extruded and moulded plastic corks show
a mixed performance.
An important question to consider: do we really want a perfect
seal? Assuming that people generally like the way that wines change
with age when they are sealed with corks, my assumption would be that
all we are asking from the perfect stopper is that it should seal as
well as a cork and no better, without the risk of taint from TCA. The
fact that screwcaps provide a better seal than corks may actually work
against them. Indeed, in the discussion of this paper the possibility
that the rubber-like aromas were the result of them providing an
almost hermetic seal: the report suggests an extended period of
anaerobic storage may not be desirable, and this 'reduced' aroma may
be avoided by allowing a slightly greater headspace or by treating to
eliminate sulfide compounds before bottling.
Rather teasingly, this paper mentions that a separate study using
measurements of oxygen permeability will be reported elsewhere. These
will be of great interest with regard to the role the stopper plays in
the normal ageing of wine. I'm also looking forward to seeing the next
set of results from this well planned and executed ongoing study.
See also: different closure types
used in the study
Reference for the study discussed here: Godden P, Francis L,
Field J et al 2001 Wine bottle closures: physical characteristics and
effect on composition and sensory properties of a Semillon wine. 1.
Performance up to 20 months post-bottling. Australian Journal of Grape
and Wine Research 7:64-105
Conflict of interests: None. I have never received
hospitality or remuneration from any cork or synthetic stopper
manufacturer, and I have no axe to grind or hidden agenda on this
topic. Acknowledgements: Thanks to Val Rechner of the AWRI for sending
me a copy of this report.
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