|Paul White replys to the AWRI|
|Friday, 26 January 2007 01:00|
Before getting into a detailed discussion of the issues at hand, I?d like to address AWRI?s complaint that Harper?s and Scientifically Speaking have not rigidly followed academic scientific standards. Harper?s is obviously not a peer reviewed scientific journal. It?s a magazine focused on the wine trade and interested consumers. Given that audience, its primary task is to translate complex issues in a way that makes sense to its readers so they can make informed, rational decisions about wine related issues. Dressing Scientifically Speaking with snappy titles and headings goes a long way towards engaging readers? attention, not to mention keeping them from dozing off mid-paragraph. AWRI appear to have misunderstood the point of this and seem to have been a touch overly sensitive concerning choice of titles and section headings.
Truth be told, as a wine writer I'd rather not be described by Harpers as the 'Bete Noir of Screwcaps,' although it wears easier than being labeled an "Ayatollah of Screwcap," but I'm not going to get all whiny about it just because that's how Harpers chose to describe me. Given the alternatives, I'll always side up with freedom of the press.
All this aside, where high academic standards are absolutely essential and expected is when scientists publish in academically oriented scientific journals. If those standards slip or are found to be wanting they should be fair game for the press to scrutinize.
Contrary to AWRI's complaints, nowhere is it claimed that Scientifically Speaking was a science review, nor have I ever claimed to be a scientist. As a journalist I have done my best to understand and explain some extremely complicated issues concerning post bottling chemistry that ultimately matters a great deal to wine drinkers and wine makers. My job, as an investigative journalist, is to advocate consumer interests, first and foremost, and poke holes in fuzzy logic, incorrect or shoddy analysis, and distortions of truth. On the basis of that I'll stand by everything I've written so far on wine enclosures.
Scientifically Speaking was my sixth article published concerning the ongoing sulphide reduction issues with screwcaps since first discovering the problem, mid-2003, during blind tasting surveys for my former New Zealand newspaper column. Throughout the course of my writings I've consistently argued the core issue has not been about screwcaps vs corks, but rather, whether a little bit of oxygen is neededpost bottlingso that wine naturally avoids sulphide reduction. I quickly learned that, to paraphrase Bill Clinton's campaign manager, "It's all about chemistry, stupid."
Historically, TCA has been an easy thing to understand and explain. Whereas, explaining the faultiness of sulphide reduction, its symptoms and treatment (or inability to treat absolutely) is infinitely more complicated.
If at any point I thought I had written about the science behind sulphide reduction under near anaerobic enclosures incorrectly I would have admitted my errors and stoppedI'm a wine writer first and foremost, not an enclosure writer by choice. If screwcap advocates had admitted they had previously not understood the underlying chemistry correctly and admitted to having problems, I probably would have stopped writing about this subject long ago. And if journalistsgloballyhad been doing their jobs properly from the start by challenging loads of dodgy screwcap PR and poorly thought out science I would probably have never started writing about this subject matter in the first place.
Let me also reiterate my position again. Contrary to unsubstantiated and often slanderous rumours spread by the screwcap lobby, I earn my living solely as an independent journalist and have never been in the employment of any enclosure company or industrial group.
The main sources I have followed have provided consistently useful information on the complex chemistry that underpins sulphide reduction, and have frequently been corroborated when checked against information and sources elsewhere - as will become apparent in the following reply. As far as AWRI's complaint that I have relied too heavily on Dr. Alan Limmer for information, I have found Limmer, who is a successful winemaker with a PhD degree in chemistry, somewhat more reliable as source of technical information than those that have driven screwcap advocates.
Once again, if I had become convinced that the scientific points espoused by the scientists I have followed and the scientific literature that drove them to their conclusions was either weak or debatable or just plain wrong, I would have stopped writing about the subject at hand long ago. So far, that hasn't been the case.
With all this in mind I reject the Australian Wine Research Institute's (AWRI) criticism (Harpers, 1 September 2006) that my article, 'Scientifically Speaking' in Harper's December 2005 closure supplement was factually incorrect, misleading or scientifically unsound.
Frankly I was surprised how closely AWRI appears to have identified itself with screwcap advocates and the strength of its defense of this enclosure in preference to others. Many of the points AWRI have taken issue with had not been directed at their institution in Scientifically Speaking, but rather had been aimed quite specifically at screwcap advocates and the screwcap lobby. It would seem in the interest of scientific objectivity that any scientific institutions involved in ongoing research into enclosures would want to maintain a high degree of distance from the subject matter of any ongoing studies.
On a separate note, Dr Alan Limmer also stands by all of his quotations. He does, however, happily concede one small point in hindsight: that AWRI had mentioned sulfide issues under screw caps in 2001 earlier. However, at the time of Limmers first discussion concerning the problem in his February 2002 newsletter he had been unaware of AWRI's papers, and his own conclusions had drawn instead on sulfide chemistry from international chemistry journals.
The major points AWRI raise in its letter to the editor centre around sulfide chemistry, variability of cork, involvement in Tyson Stelzer's book (1), blame aimed at winemaking/winemakers versus closure, and the incidence of sulfide reduction under screwcap. These issues and others will be dealt with in considerably more detail in the sections below.
Limmer's sulphide chemistry differences with AWRI
"However, we note here that there does not appear to be any marked difference between our position and that of Mr Alan Limmer regarding the currently accepted science behind what he calls SLO (sulfur-like odour) and as stated in Limmer (2005) "the chemistry is reasonably well known and readily available if you know where to look." AWRI letter to Harpers Sept. 1, 2006
According to Dr Limmer, understanding sulfide chemistry correctly and 'knowing where to look' go straight to the heart of the issues here. Dr. Alan Limmer, whose writing on post-bottling sulphide chemistry recently earned him election as a Fellow of the New Zealand Institute of Chemistry, gets down to the meat of the matter:
"While AWRI state they agree with my chemistry on this topic, I do not agree with much of theirs. The most marked difference is that I subscribe to the kinetics and reduction mechanisms as described in existing scientific literaturewhile AWRI seems to refute or ignore theseoffering no alternative explanations to replace these mechanisms, nor any reason for not subscribing to them. Unless their position has changed over the last few months, one can only assume the authors still adhere to the speculative suggestion put forward concerning the possibility of as yet unexplained 'sulfide equilibria' (Godden, et al p18/Stelzer p117) being central to their understanding of the chemistry. So far this phenomenon has not appeared in scientific literature and the only people I know of invoking these nonexistent equilibria as a mechanism for explaining sulfide chemistry are AWRI, most specifically, Peter Godden and the screwcap author, Tyson Stelzer, neither of whom are chemists. Indeed, it runs contrary to our understanding and experience of these compounds for the last 100 years. Should these 'equilibria' appear, 101 level chemistry text books will need rewriting."
Limmer again: "A chemist knows these equilibria do not exist, so would look for another answer. In this case, a redox balance. Which is different in terms of behaviour to equilibria and each have to follow specific, but different chemistry rules (See explanation in endnote 1 below). I believe it is important to understand the mechanism for this problem precisely, in order to address the issue at the root cause. Without this mechanistic understanding, things become guesswork." (my bolding)
Frustrated early on by what appeared to be a lack of scientific rigor by screwcap proponents concerning known sulphide chemistry mechanisms, in addition to the article explaining these mechanisms for the origin of this problem (ref 6), Limmer produced a 5,000 word article, drawn from existing scientific literature, which specifically focused on ways to address the problem of post bottling reduction ('Suggestions for dealing with post bottling sulfides, Australian & NZ Grapegrower and Winemaker, Dec 2005; reworked as 'The chemistry of and possible ways of mitigation of post bottling Sulfides,' NZ Winegrower, Vol 8 No4 , 2005).
Clearly there has always been a large body of scientific literature to draw on for a more obvious explanation, provided of course "you know where to look." It's difficult to understand why this literature is neither cited or, more tellingly, cited and decisively refuted in any of AWRI's writings on the subject, including attempts at explanation by Godden et al (p17/18, 05) and Skouroumounis et al (p376, col 1, p 2, 05).
Limmer continues, "The authors (Godden et al 2005) talk of the probability of thiol production as a cause of post bottling 'reduction,' while existing literature tells us it must be the cause. This work has included very detailed investigation of the behaviour of these compounds in wine (eg. Rauhut). Further, the reduction of disulfides to thiols requires a reducing agent, just as the oxidation of thiols to disulfides requires an oxidant (O2). Scientific literature, cited over 50 years ago and more precisely analysed approximately 20 years ago (Bobet, R.A, A.C. Noble, and R.B.Boulton.1990.), identifies this reductant as SO3=, a component of SO2. And yet, Godden et al, have not identified any possible reductant while citing possible disulfide reduction as a cause of post bottling 'reduction'. They dismiss the link found in their own trials between SO2 and post-bottling sulfides as "coincidental and not causal" (Godden et al 2005, p15). Their explanation focuses instead on the mysterious 'sulfide equilibria' mentioned above." (my bolding)
The authors also appear to have fundamental problems with not only identifying compounds, but understanding their properties and behaviours.
"Additionally, whilst the compounds responsible for reductive aroma are chiefly assumed to be those containing chemically reduced forms of sulfur, particularly the thiols ( a large group of compounds containing chemically reduced forms of sulfur, including mercaptans),......" (Godden 05 paper, P13/14.)
Limmer comments, "this further suggests some problems understanding chemistry long described in scientific literature. The literature tells us the thiol group does not INCLUDE mercaptans - thiols and mercaptans are simply different names for the same thing, they are one and the same. As chemical groups are classified by their reaction schemes, to separate out mercaptans as a sub-group of thiol implies they have different chemistry, which they don't. Their chemistry is one and the same. This confusion seems to repeat itself in Stelzers book, Taming the Screw."
It would seem, according to what Limmer presents above, that after four-plus years of experience dealing with post-bottling sulfide issues in AWRI's closure studies published between 2001 and 2005, Godden et al (2) appear to have ignored the most important aspects of sulfide chemistry, i.e. the nature of thiols and the sulfide reductant, SO32- , which are well described in scientific literature. Instead they have come up with a new explanation based on sulfide equilibriaconsidered non-existent by chemists and which they have not supported by scientific references.
Which begs the question, given what appears to be an unusual, if not wayward, understanding of sulphide chemistry that is fundamental to the problem, how valid is AWRI claim that they have done more than anyone else to solve the post-bottling reduction problem within the industry?
"We do not believe that any other organisation or individual can claim to have been as proactive as the AWRI in raising awareness of this issue, and the success of the strategies in Australia suggests that the science behind the hypotheses on which the strategies are based is sound." AWRI letter to Harpers, Sept. 1, 2006.
Practical aspects of this issue are dealt with in Section 5 below.
1227 fold rate of variation for cork
"One of the most spurious claims is that corks have a thousandfold variability rate [of oxygen ingress]based on misinterpretation of a MOCON study using dry corks as a model" [with reference to the Australian Closure Fund paper], [Page 28, column 1, paragraph 2].
Without entering into a discussion of the Australian Closure Fund paper here, Peter Godden's Screwcap Symposium presentation and Godden et al. 2005 each include data on the oxygen permeation of twelve Reference 2 corks from the AWRI's original closure trial which show a 1227-fold range. (Note that this is not the "MOCON study using dry corks as a model" referred to above.) The author of the Scientifically Speaking article has overlooked the existence of these data in his discussion of the variability in oxygen permeability of cork, which appears to be a serious omission in an article that purports to include scientific discussion of the oxygen permeation of cork." AWRI letter to Harpers, Sept. 1, 2006
It's baffling why AWRI has chosen to defend its MOCON-test derived 1227-fold rate of variation for the oxygen permeability of cork. Even more baffling, AWRI appear to have consciously chosen this highly spurious figure over other data (sensory, retained SO2 and OD 420) generated in their own closure studies which indicate cork variability lies within a considerably lower range, approximately comparable to screwcaps.
The issue here is that MOCON testing can't be done at 100% relative humidityto emulate the conditions in a wine bottleso MOCON results are unreliable predictors of corks' performance as a wine enclosure.
Alan Limmer explains why, "The ACF paper quotes a 1000 fold MOCON variation, the AWRI quotes MOCON derived 1227 fold. The same argument applies for both. The only meaningful assessment of corks' performance needs to be done at 100% relative humidity - to emulate the conditions in a wine bottle. MOCON tests have been unable to do this, and are known to be very sensitive to the relative humidity (RH) they are performed at, decreasing in OTR [oxygen transfer rate] and variability as the RH approaches 100%."
One of the world's leading experts on MOCON testing, Jim Peck (Senior Research Scientist, G3 Enterprises Closure Division), reinforces this: "Allow me to be the first in line to say that I don't believe that upright MOCON OTR testing whether dry or humidified is indicative of performance of a cork in situ (laid down)." (personal email comm., 9/15/2006)
Although Peck performed AWRI's MOCON tests, AWRI obviously disagree.
In an earlier email correspondence to Alan Limmer Peck stated, "I have developed a "wet cork" Mocon method but have not run a large number of tests. As you might expect, the OTR results are much lower and much more consistent than the upright humidified test. In fact, the OTRs are very much in the region you would predict from the wine analyses in your paper." (personal email comm. to Limmer, July 01, 2006) Peck refers to Dr. Limmer's 'The permeability of closures', The Australian and New Zealand Grape Grower and Winemaker, Annual Technical issue June/July 2006 (3), where Limmer uses AWRI trial data to show cork variability at no more than 3 times that of ROTE. (my bolding, please also note ROTE is the technical name for screwcap)
Another email from Peck details these figures, "My range of wet OTR's has been from non-detect (<0.0005) to 0.005 ccO2/day. Obviously, if the OTR variation of laid down corks were 1,000 fold, there would be a huge problem with oxidation in premium wines which we are not seeing. If one assumes that the best wetted cork has an OTR of 0.0001 ccO2/day (similar to screwcaps' tin/saran), then the 1,000 fold greater would be 0.1 cc/day, at which rate even the heartiest red is going to be done in within a year. (personal email comm., 9/18/2006)
AWRI's letter to Harpers criticized the results from a study by the University of Bordeaux (4) which were cited in my article, because the sample size was small and because the trial never set out to determine this parameter. Limmer explains, "Lopes used the same sample size (12) as AWRI did for their MOCON study. Whatever the trial's reason, it reported the statistical variance of the results for the corks tested - as it should. So, the data is available, and contradicts AWRI's MOCON derived 1227-fold variation, and gets us somewhat closer to the truth while possibly not being the final word on this matter."
Even without this data there is a simple logic here that AWRI appear to have dismissed. Limmer explains:
"Synthetic stoppers, we know, have a MOCON oxygen transfer rate (OTR) of around 0.01 cc/day. But wine develops fast under synthetics, which have the highest rate of oxidation - higher than cork, by several orders of magnitude. This result has been demonstrated in the AWRI trials, where the monitoring of the synthetics was discontinued at 36 months, unlike for cork. This result has also been demonstrated in the Lopes trials, where the synthetics reached the limit of the technique to determine O2 ingress, within a year, unlike cork. This result is also the common experience of any number of in-house trials, and winemaker's general experience. There is no one I know of who would claim synthetics have a lower OTR than cork. Therefore corks' actual OTR has to be several orders of magnitude LESS than the synthetics MOCON value (0.01cc/day) - ie 0.00X cc/day. This is just common sense. AWRI's MOCON results had them up to 12 times higher than synthetics (0.12 cc/day). Clearly this is nonsense. According to the published results, no corks exceeded the development of the synthetics in the AWRI trial, and the samples for the MOCON test were taken from the AWRI trial wines."
Limmer continues, "At the end of 63 months, the OD 420 (colour intensity) data in the AWRI trial shows the worst statistical variance for all the parameters measured for natural cork (Ref 2) to be a little less than 3 times more than screwcap [refuting their more spurious 1227 MOCON figure.]" To quote concerns AWRI voiced in its letter to Harpers back to them, this "appears to be a serious omission in an article that purports to include scientific discussion of the oxygen permeation of cork."
Finally, simple common sense tells us that if we tasted a dozen bottles of Chateau Latour (the 12 cork sample used by AWRI)--following AWRI's analysis--we would then effectively find a 1227 fold difference within the lot and wildly different wine characters to the point where some/most/all would be unrecognizable as Chateau Latour. Times that by hundreds of thousands of bottles tasted over centuries and nothing of any sense could or would ever have been written about Chateau Latour.
It should be patently clear that the MOCON data is highly problematic in a rational discussion on the variability of cork in situ. AWRI complain that Peter Godden's 1227 figure wasn't referenced in Scientifically Speaking. It quite simply wasn't referenced because it is just as invalid and irrelevant as the previously discredited 1000-fold variation in the paper sponsored by the Australian Closure Fund (5).
Of far greater concern here is that AWRI has allowed this notion to persist, effectively condemning cork as being wildly oxidative. At the same time they appear to have ignored sensory and other data from their own closure trial and other studies which clearly demonstrate cork variability lies closer to the range found in screwcaps than the MOCON results. Limmer warned against accepting the ACF (Australian Closure Fund) 1000 fold conclusion approximately 20 months ago in "Do Corks breathe? Or the origin of SLO" (Australian and New Zealand Grapegrower and winemaker Annual technical Issue June/July 2005). Other writers have also taken ACF to task for its highly spurious analysis.
Unfortunately the AWRI 1227-fold figure, like the ACF figure, has entered the realm of urban legend and has beencontinues to be banteredaround by wine writers globally as if accurate, factual and trustable information.
AWRI appear to be of the (mistaken) belief that their 1227 fold data is somehow more valid than the previously discredited ACF's 1000 fold data. This belief appears to hinge on their corks being described as 'wet' at the point of MOCON analysis and therefore performed more like natural cork in bottle. There are large problems with this belief.
Please note the 'wet' corks AWRI describe here are significantly different from the 'wet' corks tested by Peck in his quotes above. As noted, Peck has tested corks recently using a new method which simulates wet corks in situ more closely. This he says demonstrated cork performance along the lines Limmer had suggested, where cork variability and OTR is close to that of screwcaps. In fact, AWRI's use of the term 'wet' corks does not equate to Peck's usage of the term, nor do their results correspond to this. The AWRI wet corks were equilibrated at less than 100% RH prior to analysis - and give quite different OTR results tested this way - with much higher variability.
Here's where AWRI's defense of MOCON becomes somewhat confusing. AWRI submitted two versions of their web response to Harpers. The first of these was eventually withdrawn for a revised version. In the first version, their 1227 position was defended with the following statement "...(Note that this is not the "MOCON study using dry corks as a model" referred to above). These data indicate 1227-fold variability in oxygen permeation between those twelve corks. These data were obtained from MOCON measurements using wet corks that had been in contact with the wine in the bottles until immediately before testing. These data were also published in" (my bolding)
AWRI's statement (in bold above) is not accurate because their 'wet' corks were not in contact with the wine 'immediately' before testing, which is their rationale for simulating wet corks in bottle. Prior to analysis by MOCON testing, corks must be equilibrated for several weeks in the instrument, at less than 100% RH. Therefore the corks didn't go directly from bottle to test and after equilibration would have been little (if at all) different from the previous ACF 'dry' (humidified) MOCON tests. In other words, the corks may have started out wet but were at an equilibrated relative humidity less than 100% by the time they were tested, so AWRI's results were equally invalid.
The above bolded quote becomes especially significance in terms of AWRI's defense in that Limmer's article on closure permeability (The 'permeability' of closures - Australia and New Zealand Grapegrower and Winemaker Annual Technical Issue 2006) was printed in Australia around the time the first response was sent to Harpers. This article explained in detail why the MOCON data was in error (see previous paragraph). It also explained the significance of the moisture uptake by a cork during its time in a bottle, and the effect on the performance of cork as a closure, and why MOCON was unable currently to replicate this effect, and hence the disparity in the actual performance of the cork compared to the MOCON data.
The article also explained why the results for storage orientation of wine, as seen in the AWRI trial, and the Lopes results, are very similar. In other words why the results of O2 ingress are not significantly different whether the bottles are stored inverted or upright. Indeed, if these MOCON data were applied to one situation, eg stored upright, and not another, eg stored horizontal, we would have difficulty explaining why two trial results (Lopes and AWRI) found no significant difference in the performance of cork either way. All this made it clear why the MOCON results do not apply to either situation.
Coincidentally (?), shortly after Limmer's publication, AWRI replaced their first version with a final versions that was eventually published in Harpers on Sept. 1st (p27, third column). One significant textual change removed the justification for terming the MOCON results as having used 'wet corks' ("These data were obtained from MOCON measurements using wet corks that had been in contact with the wine in the bottles until immediately before testing." see section above). This suggests that AWRI understood the error of their previous special claim of 'wet cork' testing as being more accurate than ACF's 1000 fold MOCON data. After this qualification had been taken away, it is hard to understand what justification AWRI (and others) have for defending their '1227 fold' position to the exclusion of other data.
Here then is a recap of the issues. AWRI appear to have faced a choice between two sets of data concerning the variability of cork. One set of data showed cork to be no more than 3 times more variable than screwcaps which is consistent with other peer reviewed scientific research. Without refutation or an explanation for why this data appears to have been ignored, AWRI instead chose to follow MOCON data that showed corks to be wildly oxidative with a 1227 fold rate of variation, which by all other accounting is highly spurious.
To re-emphasize the point made above. The perpetuation of AWRI's 1227 figure is seriously misleading. A prime example of how this affects the global wine community can be seen in a very recent presentation in South Africa, where Tyson Stelzer continued to quote the 1227 fold variation data, with absolutely no reference to any other data relating to the variability of cork as a closure. The audience would not know from this presentation, such data contradicting the MOCON conclusions even exists.
AWRI input into Tyson Stelzer's screwcap manual, Taming the Screw.
"The implicit assertion... that the AWRI reviewed or was responsible for the final text of Taming the Screw is incorrect....Consequently, Peter Godden and to a lesser extent other members of AWRI staff reviewed portions of [an earlier] manuscript and the resulting comments were verbally relayed to Mr Stelzer by Peter Godden. The acknowledgement of Peter's input on page 291 of the book accurately reflects the manner in which the comments were relayed to Mr Stelzer. The AWRI was not given or asked to review any further versions of the manuscript. It is apparent when comparing the comments made by AWRI staff on the version of the manuscript received on 23 February 2005 with the final version, that all of the errors that we are aware of in the final version did exist in the earlier version, and that they were all noted by AWRI staff members and were all relayed to Mr Stelzer." AWRI letter to Harpers, Sept. 1, 2006 (my bolding)
There seems to be two issues here. The first concerns AWRI staff involvement in "reviewing" Stezer's book, the second deals with the identification of "errors" concerning sulphide chemistry in that book.
AWRI's stated involvement with Tyson Stelzer's book, Taming the Screw, does not exactly equate with the impression espoused by Stelzer through his book acknowledgements and email correspondence. AWRI claim Peter Godden and other staff were not responsible for reviewing the final text. It was never stated in 'Scientifically Speaking' that they were. What was stated was that the book appears to have "not been screened for accuracy pre-publication by a qualified chemist," and that Peter Godden--who is not a chemist--had "reviewed" it before publication. This conclusion was based on both acknowledgements in the book and a series of emails to Alan Limmer in which Stelzer specifically stated that AWRI's Peter Godden was one of two reviewers and that Mr. Godden "had input before the final version went to print." Additional quotes from Stelzer suggest Godden had a close and long standing advisory role with Stelzer both before and after the book's publication or, at the very least, that Stelzer considered this to be the case.
The following email correspondence between Alan Limmer, myself and Tyson Stelzer (personal email comm., 29/6/2005) explains the position, as described by Stelzer.
TS: "The book was edited by Michael Brajkovich, John Forrest and Jeffrey Grosset and reviewed by Peter Godden. All editors had significant input into the sulphides and other issues at many stages of the writing process, and Peter had input before the final version went to print."
Further solidifying a close working relationship with Godden, who is specifically listed as reviewer of the book, Stelzer makes the following acknowledgments on page 291:
TS: " I remain grateful for late nights and long phone conversations' [with Peter Godden and 'Special thanks to those who assisted the contributing editors in reviewing the manuscript: Peter Godden, Manager Industry services, AWRI...'
AWRI appears to be distancing itself from Stelzer's incorrect sulfide chemistry, suggesting it had noted errorsverballythat were not incorporated into the published version. This may or may not be the case as we are not given any indication of the specifics of these errors and which of these AWRI considers to have been consciously ignored by Stelzer. However, when Limmer took Stelzer's book to task over the accuracy of its sulphide chemistry (discussed in section 1 above), Stelzer replied that he wanted to consult with Godden and others before responding.
Stelzer's initial response cites Godden as a source in defence of the book's errant chemistry and a later response indicates AWRI's senior research chemist, Dr Leigh Francis, reviewed the contentious sulfides chapter of the final text (post-publication, Oct 2005) saying it "generally looks fine." Although AWRI offer no indication of what errors they relayed to Stelzer 'verbally' pre-publication we can only conclude that these appear not to be the same ones noted by Limmer. This conclusion is based on Stelzer's citation of Godden's views of sulphide chemistry in his defense against Limmer's challenge.
Here is a detailed sequence illustrating the above summation. Dr. Limmer originally challenged nonsensical chemistry put forward in the Sulphide Chapter of Taming the Screw in an email (personal email comm., June 28 2005) to Stelzer. This noted the following errors in chemistry espoused in his text-- all of which relate back to errant sulphide chemistry discussed in section 1 above:
H2S can be reduced to mercaptans (p. 116)
Thiols can be copper treated but mercaptans are more difficult. (p. 116/117)
There is no SO32- in wine (p. 101)
DMS and DMDS are examples of mercaptans (p. 116)
Since sulfide compounds exist in an equilibrium balance, the removal of hydrogen sulfide will activate changes which may prompt more aroma-active compounds to develop. (p117).
Stelzer responds (personal email comm., June 29, 2005) with "I have put your letter to the contributing editors and Peter Godden, and will respond to each of the issues that you raise accordingly." Following on from this, Stelzer's emailed response to Limmer (personal comm., July 6, 2005) states the following divisions over the contended chemistry:
"On every point which you list, our sources are in disagreement with your
Mercaptans and copper fining - Gibson cf Limmer etc.
Equilibrium shifts - Godden cf Limmer.
SO3 in wine - Ribereau-Gayon et al cf Boulton et al.
SO2 - AWRI cf Bobet et al.
Cork permeability - AWRI and winemakers experience cf ASEV".
An important point here is to note that Stelzer and his advisors still appear to not recognize or refute the significance of the existing literature that Limmer has put to them concerning the points directly above. Those points are not Limmer's opinion, they are based on accepted chemistry as described in current scientific literature, the same cannot be said of Stelzer's sources.
Eventually, three months later and presumably after further checks with his advisors, a further response from Stelzer (personal com., Oct 5, 05) conceded that there had been a single chemistry error in the book. Admitting "H2S can be reduced to mercaptans" was incorrect, Stelzer then suggested Limmer's other points were open to debate. Concluding this email with a quote from Dr. Leigh Francis, senior research chemist with the Australian Wine Research Institute, accepting the sulphides chapter "generally looks fine."
Limmer's reaction, "It is hard to understand how a senior AWRI chemist could be in agreement with my chemistry as they state in their letter to Harpers (Sept. 1, 2006), while also concluding Steltzer's chapter looks fine. I told Stelzer it was fundamental to understanding the problem, and seriously flawed."
Subsequent to Limmer taking Stelzer to task, Stelzer wrote a glowing self-review of his own book which featured prominently in New Zealand Winegrowers magazine. Limmer elaborates, "There he included quotes from AWRI and others which basically told readers how good and well received the book was and acknowledged only TWO errors: one on CO2 levels in wine, wrong units quoted, and the H2S bit. So, apparently, in spite of my critical correspondence he was still not convinced he had the chemistry wrong."
Given that AWRI had ample opportunity for post publication comment, especially after Limmer's critique of the chemistry, one would have expected AWRI to have taken the opportunity to reiterate to Stelzer the errors they had previously noted. If these errors were in the sulfide section, it would appear they did not subsequently draw Stelzer's attention to them.
Furthermore, it also seems very odd that AWRI did not actively set the record straight through the post-publication book reviewing process. One would reasonably expect irritation on their part that Stelzer had not incorporated the errors they noted, thus misrepresenting their scientific input and misleading unsuspecting winemakers about the implications in following Stelzer's error-ridden sulphide chemistry.
In total, all of the above suggests that both before and after the publication of his book, Stelzer remained heavily dependent on Mr. Godden and AWRI staff for advice on sulphide chemistry. And that the only error that any concerned appear to have acknowledged is that "H2S can't be reduced to mercaptans."
AWRI believe the 'overwhelming majority' of wines closed with screwcaps do not show post-bottling reduction, and therefore... claims to the contrary, should be treated with skepticism."
Offering no statistics to back this assertion, it seems a bold statement given AWRI's own clinical evidence to the contrary. So far every clinically based scientific trial which has placed the same wine (whether copper fined before bottling or not) under screwcap, cork and/or synthetic, has consistently shown a significantly higher incidence (nearing absolute) of sulphide reduction under screwcaps' near anaerobic conditions compared to the negligible levels found under all other oxygen permeable enclosures, cork, synthetic, etc. Clinically based enclosure trials, in fact, offer the only sure means to measure post-bottling sulphide reduction. On that count screwcaps fail consistently.
The kinetics of sulphide chemistry tell us this is not only possible, but probable, whereas AWRI tell usignoring their own trial based evidencethat while this consistently happens in their clinical studies, it is not a significant problem in the real world. So the question that begs for an answer from AWRI is where does 'overwhelming majority' begin and end? TCA rates have been deemed unacceptably high above 3% and the whole point of the closure trials originally, it would seem, was to find a less faulty, if not totally fault free, closure. Given the screwcap lobby have consistently professed faultless performance, how close does that jibe with 'overwhelming majority?'
In lieu of AWRI not defining "high incidence" or "overwhelming," lets turn to a recent statistic from a screwcap sympathetic wine writer in Australia, Campbell Mattinson, who descibes himself as 'not a fan of cork.' His commentary in www.winefrontmonthly.com.au (March-April 2006) is brutally frank:
"This is the article I never wanted to write - about screwcaps. It's a negative article. It's a major concern. I never wanted to write it because I've always wanted to like screwcaps...until I noticed, and heard of, significant variation between newly-released bottes - variation that is not related to storage conditions."
Going on to describe bottle variation encountered in Adelaide Hills sauvignon blanc last winter "that was outrageously different from one bottle to the next" and similar issues with "two bottles of the same Hunter Valley shiraz tasted dramatically different, both of them sealed under screwcap. Both were from the same case. If anyone thinks that screwcaps sound the death knell to bottle variation within a case - think again. It is, without a shadow of a doubt, not true."
Mattinson concludes, "Two things: reduction is not peculiar to screwcapsealed wines, and I would be the first to admit that I see reduced flavours and aromas on cork-sealed wines - quite often actually. I sometimes even enjoy it. The above points relate to the fact that reduction is more common in screwcapped wines, and when it takes hold - seems to be become more pronounced. Screwcaps are less forgiving. I have never seen widespread reduction issues in a clutch of wines like I did in the New Zealand wines presented on the opposite page at any tasting where cork is the predominant seal (though of course, cork taint and associated issues is then the problem; I am not a fan of cork).
"The main point: excessive reduction in a wine is a winemaking issue, not a closure issue. I know this. I've been told it 357 times - at least. Right What I want to know then is: screwcaps have been in increasingly high volume use ever since Orlando bottled its 1998 Richmond Grove riesling under it. It's now 2006. How long, en masse, is it going to take for winemakers to get it right? When is the "winemaking issue" going to end? Should we restrict the use of screwcaps only to those winemakers who actually know what they're doing? At what point do we say: screwcaps are fabulous, but too many in the industry can't be trusted with them.
Do we need to introduce a Screwcap License system?"
One could, but it's unlikely to help, given that the 'kinetics and mechanisms of sulphide chemistry' inevitably make it impossible to guaranteeregardless of how much copper sulphate is thrown at a winethat it will not eventually suffer from post-bottling sulphide issues.
In the same issue Mattinson reviews twenty-eight New Zealand pinot noirs under screwcap, of which a third showed reduced sulphide characters. He makes this comment, "...The idea that an everyday wine consumer found cause to pour wines down the sink because of a problem caused, or locked in, by the screwcap seal should frighten a lot of folks. The level of reduction exhibited in a number of high-priced pinot noirs at the New Zealand pinot noir tasting (right) should frighten them even more if not enrage them."
In light of this, do AWRI consider a 30% sulphide reduction rate to be of high incidence? More troubling, what about the bottle variation described above from an enclosure that has been guaranteed to be variation free from its introduction into the market?
Another Australian wine commentator, Eric Einstein (www.torb.com.au), expressed surprise at both the bottle variation and reduction he encountered in wines bottled under screwcap at Wine Australia 2006, emphasizing that some were from "well-respected and experienced winemakers." Although Einstein does not qualify this with statistics he is not alone and there is little evidence that sulphide reduction is in decline.
The chief judge's comments for the 2005 Hawkes Bay A & P Wine Show noted an unacceptably high number of screw capped wines with sulphide problems. This followed the previous year's Air New Zealand Wine Show head judge's criticism of producers "for far too many sulphide problems found in wines bottled under screwcap." The UK's Tim Atkin, writing in OLN (November 26, 2004), put reduction levels in a pre-release tasting of NZ whites at about 20%. Recently I heard that a major Australian producer is about to pull his rieslings out of screwcaps because of ongoing problems.
I first bumped into seriously faulty screwcapped wine--unexpectedly-- back in mid-2003 through a series of varietal surveys for my newspaper column. Always tasted blind in the presence of two other wine judges, these have consistently shown rates of reduced characters consistently hovering above 20%, sometimes considerably higher, depending on grape types and age of wines tasted. The last was a group of 2005 NZ sauvignons blind tasted four months ago, same story.
All this has been going on for a while now. As the 2003 Australian Alternative Varietals Wine Show's international guest judge, after checking with behind the scenes statisticians that 16 and 50% reduction/oxidation rates under screwcaps depending on variety. The same year, UK-based wine writer Andrew Jefford relayed similar concerns about reduced wines encountered while judging in Australia.
Most telling for me are figures generated through a blind tasted 'intro to wine' course I've taught weekly over last four years. I regularly record group wine tasting notes dictated back to me from dozen or more novice students tasting a dozen wines blind. Deconstructing back from their descriptors (burned rubber, rotten egg, flint, stunted fruit, palate bitterness) reduced sulphide characters under screwcaps continue to hover around 20% rate, where these characters were rarely encountered under other enclosures. That objective assessment tells me that novice wine tasters and everyday consumers can observe these characters and don't like them.
Experience suggests that statistics from wine shows could be underestimating the incidence of sulphide reduction in wine, especially where screwcaps are concerned. In most cases wine shows focus on new release wines, where the vast majority of wines tasted are recently bottled, therefore under the 12-18 month time frame where sulphide reduction rears its ugly head under near anaerobic conditions. A freshly bottled screwcapped wine is likely to have been recently sterilized with copper sulphate fining giving the illusion of permanent cleanliness. Whereas over time in bottle the redox potential begins shifting back into reductive mode and a year or so on a once seemingly clean 'medal winning' wine could look very ugly. The great irony here is that a slightly sulphidic wine will only get worse in anaerobic conditions, whereas the oxygen steadily seeping through cork or synthetic (or screwcaps with permeable foils when they are developed) over a few months can and will naturally clean up a wine over time.
The nature of how wine shows cull out faulty wines also suggests hidden sulphidic wine failure rates. Experience suggests that many wine judges (and wine writers) simply cannot identify it for the fault that it is. They may rightly reject the wine as poor quality (smelling bad, stunted fruit, back palate bitterness, etc.) and score it low, but are unable to put a name to it like they would if it was a more easily identified fault like cork taint or oxidation. Subsequently, a wine suffering from sulphide reduction is more likely to get hung with a generic 'dirty' or 'grubby' descriptive and simply tossed out of contention. Replacement bottles are rarely called for, so the true nature of the fault doesn't get entered under any fault category.
I ran into this most recently while judging the Mondial du Bruxelles where, of the five judges on my table, only myself and the panel chair, a trained winemaker from the University of Bordeaux, identified reduced post-bottling sulphide as a fault. That's not to say the rest of the panel judges had not scored the faulty wines low as simply of poor quality and tossed the same wines out of contention without an afterthought. As is almost always the case, when I drew the fault to the attention of the panel chair, he agreed it had sulphide problems andtaking the traditional assumption that it suffered from dirty winemaking therefore was unlikely to change with a second pouring. In this case at least a half dozen sulphide faults did not enter the stats, where all the oxidation and TCA faults encountered did.
Further to this, all humans seem to have differing sensitivities and blind spots for TCA, brettanomyces and sulphide reduction. Personally it took me years of experience to discern the mustiness of bret from that of TCA, whereas previously I had lumped both together as 'corkiness.' First I learned to pick these up on the nose, but over time I could see lower 'pre-aromatic stage' levels impacting the palate. Sulphides, which emerge over time gradually, behave similarly and can be just as tricky to detect. A few years back I only picked up the most obvious fully bloomed 'stinky' phase, now I'm better able to feel secondary symptoms on the palate as bitterness, coarseness or dryness at the finish and/or blunted fruit.
What can't be known easily and what isn't discussed is the possibility (likelihood?) that many of the medal winning wines under screwcap might (would?) have been even better under a closure with oxygen ingress. In Scientifically Speaking I suggested an easy before and after comparison test that consumers could do themselves. This traditional, time tested, method fills two glasses of the same wine and then drops a penny into one. Copper cleans up post-bottling sulphides, if present, allowing a side by side comparison where negative aspects of sulphide reduction are easily smelled, tasted and felt.
Oddly, AWRI criticised me for using this technique, but it is effectively the same method they originally used in their own closure trials to determine that screwcaps were suffering sulphide reduction problems.
It is also surprising that AWRI disagree with the following quote from Scientifically Speaking: Negative characters will appear on the palate well before they show up as aromatics" [Page 27, column 2, paragraph 1]. To this they reply: "This has not been the experience of AWRI sensory panels with our closure trials for the reductive character, or for other attributes."
Unsupported by specific evidence or explanation it is difficult to respond with certainty to this other than to say that the limitation of AWRI's closure trial analysis of sulphide reduction is that their technique appears to focus on aromatic detection versus palate detection. Aromatic detection catches reduction only when sulphide reduction is in its final full-blown aromatic phase. Previous to this there is a gradual buildup of reductive changes to fruit and palate character, the so called 'screwcap palate.'
Chemist-winemaker, Dr. Alan Limmer, "has frequently seen wines described with reference to their particular acidity, and or mineral tones when the presence of sulfides is apparent. A copper clean-up in the glass (as long as the dinner guests don't object) will more often than not alter this aspect of the palate dramatically. The sulfides impact from an organoleptic perspective, towards the end of the palate imparting a 'mineral' or bitter /hard/astringent aspect. This has the appearance of shortening or closing up the palate, so the wine does not display a fine fresh long finish, but ends abruptly, and somewhat harshly. These effects vary depending on the wine and the sulfide levels." In Scientifically Speaking I also quoted similar impressions from senior New Zealand wine judge and winemaker, Kate Radburn.
Anyone who has observed wine writers in Australasia over the last couple of years cannot help but note the increased occurrence of euphemisms like 'minerality' or 'struck flint' in wine descriptives, many applied to wines that were formerly described exclusively in much fruitier terms. It's hard to believe so much Sancerre-like terroir has suddenly cropped up over the last few years. This instant terroir wouldn't have anything to do with the advent of screwcap bottling in Australasia?
In their letter to Harpers AWRI cite an AWRI Advanced Wine Assessment Course where participants consistently identified a higher incidence of reductive characters in wines sealed with cork closures compared to screwcaps.
Alan Limmer comments, "while it is entirely feasible that corks produce post-bottling sulphide reduction, the evidence to date suggests this is not a regular occurrence. There has been minimal sign of this in the sensory data from AWRI's closure trial from a pool in excess of a hundred wines under cork subjected to sensory assay. Further, the evidence above should be questioned in this respect. Taking wines off the shelf and conducting sensory assessments of their sulfide content tells us nothing of the original state of the sulfide profile at bottling. The wines may well have been bottled with residual H2S and thiols. In which case, to assess them at some point later does not tell us whether the thiols are diminishing through oxidation, or increasing through post-bottling 'reduction'. The only way to determine this aspect is to compare the same wine under various closures, as per the AWRI closure trial. Which to date has not demonstrated this behaviour of cork."
One has to question then what's the point being made here. Firstly, the AWRI citation isn't a clinical survey with controlled conditions tightly following scientific methodology. The samples were collected randomly, we don't know how recently bottled they were or their individual sulphidic history, and the scoring seems to have been more along the lines of a show of hands by non-professionals who have not previously participated in AWRI's enclosure trials. Many scientists would have a hard time taking any of this seriously. At face value it would seem to be an argument put forward to downplay the most damning aspect of screwcaps, while implying that corks suffer more from the malady. All of which flies in the face of AWRI's own clinical research trials.
Practical applications based on AWRI's misunderstood sulphide chemistry mechanisms
From the points made in Section 1 and Section 3 above it would appear that AWRI do not agree with Dr. Limmer's explanation of sulphide chemistry based on known scientific literature, but rather have chosen to champion a new theory inspired by Australian winemakers that is based on non-existent "sulphide equilibria." Accepting this is the case, then how valid is AWRI's claim that they have done more than anyone to fix the post-bottling reduction problem within the industry?
Let us re-examine AWRI's guidelines for the preparation of wine for screwcaps. The following winemaking advice is taken from Godden et al, p18:
"A theory postulated to the authors by some Australian winemakers is that there is a finite 'sulfide (thiol and thiol precursor) pool' at the end of fermentation, and a relatively large copper addition at this stage will potentially remove a large proportion of this 'pool'. Over time, because the compounds responsible for reductive aroma are probably in complex equilibria, it is possible that the concentration of aroma active compounds will again increase to a concentration above the sensory threshold. Consequently, additional small copper additions might be necessary during wine maturation to again lower the concentration of these compounds to below the sensory threshold. By the time the wine is bottled, the aim is to ensure that the concentration of these compounds has been lowered to a point where further equilibrium shifts will not cause the concentration to again rise above the sensory threshold after bottling.
Conversely, copper fining close to bottling is not considered ideal, especially if only temporary removal of reductive characters is achieved, and if repeated fining increases the copper concentration in the wine, thereby increasing the risk of later copper instability. It should also be noted that with Sauvignon Blanc, Chardonnay, and other varieties, compounds containing chemically reduced forms of sulfur are important in varietal expression. Greater care with the timing and magnitude of copper additions should, therefore, be exercised when working with these varieties, and fining trials should be conducted." [my bolding]
A few months prior to this publication Stelzer seems to have paraphrased this advice in his defense to Dr. Limmer of the nonsensical sulphide chemistry espoused in Taming the Screw (personal email comm. Stelzer to Limmer, July 5 2005):
"Peter Godden has worked extensively with reduction issues through his role at the AWRI in recent years. At the screw cap symposium last year, he stated:
"Rather than large amounts of copper fining late in the winemaking process, copper fining should take place while yeast lees are still there. The theory is that if you have a sulphide "pool", maximum concentration at the end of fermentation, then adding copper then, to get rid of that pool, would be a smart thing to do, because yeast have a strong affinity to adsorb copper. This has been advocated by a lot of our leading winemakers for some time, and they are not having reduction problems.
"These compounds are in equilibrium, so when they are removed they may change, and more aroma active compounds may develop and come back above the sensory threshold. It may therefore be necessary to make a few sequential very small copper additions to bring it back below sensory threshold and the point is that by the time the wine goes into bottle you want to make sure that you've cut the concentration sufficiently that further equilibrium shifts will not come back above the sensory threshold in bottle."
AWRI's and Stelzer's technique appears to boil down to three issues: timing of copper addition, repeated additions, and how much.
Let's look at each of AWRI's practical solutions in more detail. In terms of timing, where Godden et al 2005 and Stelzer's Taming of the Screw suggests copper fining while on yeast lees to minimize the amount of residual copper, this advice would seem to be both ineffectual and unnecessary.
Limmer explains, "It is true that yeast (as with other proteinaceous matter) have an affinity for copper, but it makes no difference to the efficacy of the treatment to fine while in contact with yeast. If anything it complicates the process, in terms of determining the required amount of copper which may be eventually be needed to minimise thiols and H2S. Depending on the quantity of these in any given wine, there is a finite amount of copper required per wine to do this. Having yeast compete with the thiols for the copper at the end of fermentation simply obscures how much copper is actually needed, because more copper is required - to satisfy the yeast's appetite for it. There is no advantage to adding copper in the presence of yeast. Any copper consumed by the yeast is unavailable to remove thiols and H2S.
Limmer continues on the subject of repeated doses, "AWRI also advise successive additions of copper (Godden et al). Again, this seems a confused approach. There is a finite amount of H2S and thiol to be removed, and there is no reason why a single correct dose of copper cannot do this completely or why successive doses would be any more effective in this regard. Logically, it makes most sense to delay fining until close to bottling when the final pool of sulphides is at its largest natural accumulation - contrary to AWRI advice, which is to do it early. Remember - it is the thiols we are trying to remove here, and the chemistry in the literature tells us they are accumulating over time in the wine, post ferment. All of which may explain why AWRI call for successive doses. But following the chemistry of Copper interactions with Sulphides, and simplicity of winemaking, it makes more sense to make one correct addition near bottling."
Limmer elaborates, "The AWRI state the thiol concentration and H2S will be at their maximum just after ferment - hence their suggestion for an early Cu addition. This is contrary to our understanding of post bottling sulfides and the mechanisms subscribed in my articles. These mechanisms explain the closure trial observations of the accumulation of thiols post bottling. Where these processes are capable of initiating straight after ferment - they take time to go to completion (approx 18 months). Consequently we can conclude the maximum quantity of thiols present will be some time after ferment (many months) not immediately post ferment. Hence delaying addition of copper would seem to make better sense than the advice to fine early."
The third issue is how much copper to add? Determining the exact thiol make-up of each wine is difficult. Each variety has its own patterns and these differ in individual wines from different vintages and points of origin. The sulfide profile of a wine has also been shown to be highly dependant on the specific yeast strain used. What may work one vintage won't necessarily another. The closest one can get is to use detailed sulfide profile analyses, and only after knowing this the best one can hope for according to Limmer is 'an educated guess'.
One of the most worrying issues in the extract from AWRI 2005 publication and Harpers letter of 2006 is that the authors appear to have started not from a complete understanding of the underlying principals of sulphide chemistry and worked out solutions from that position, but rather are stillat a relatively late stage in their investigation--following "theories" built around "sulfide equilibria" derived from "some Australian winemakers" which appear to be based on trial and error or guesswork, not known chemistry and scientific literature.
I am sorry that AWRI has been deeply offended by my strongly worded article in Harper's Closures supplement December 2005, but this issue is extremely important for both winemakers and the public as well. Scientific research into screwcaps and reduction issuesnot forgetting oxidation, TCA and other closure driven faultsmust be seen to be ruthlessly objective and backed by impeccable wine science, anything less than that is bound to have extremely serious consequences for the wine industry globally.
It is disturbing for AWRI to call into question the integrity and ethics of other scientists, such as Dr. Lopes. In what appears to be a roundabout way to tag Lopes's research as tainted, AWRI state the following in their letter to Harpers:
"We also note that the authors of the paper rightly, following scientific protocol, declare that they received funding from a cork company for their study, and that the same company supplied all the cork-based closures examined. However, these factors are not noted in the Scientifically Speaking article."
Firstly, it is true that I did not mention that the Lopes study was funded by Amorim because it is not uncommon for universities or research institutes throughout the world to have their research funded by related industry. So long as scientific 'peer review' is strictly adhered toas it was in the case of Lopes's doctoral dissertation at Bordeaux Universitythen there should be no conflict of interest going on here. With that established and given conflicts had been stated elsewhere, there is no need for a consumer oriented feature to encumber readers with overly detailed, irrelevant information. Word counts rule journalism, where academia can waffle on as much as it wants (Please note, Dr. White knows this from personal experience.)
By the same token, I also did not mention that some AWRI's studies appear to be heavily sponsored by closure companies and the Australian wine industry. And, in my previous articles, I also failed to mention the fact that Peter Godden enthusiastically uses screwcaps privately in his own winery, Arrivo (WBM, Dec. 2005, p26), which I have no recollection of having seen declared as a 'conflict of interest' in any of AWRI's publications he has authored.
One can only assume that the above quote was an attempt to imply that Lopes's PHD thesis research while at Bordeaux University was anything other than completely unbiased, high quality scientific research. The key difference between Lopes and others above is that Lopes's research and publication has gone through rigorous scientific peer review by multiple scientific institutions and universities based in several countries. I cannot believe that AWRI would call Bordeaux University's reputation into question like this. On the other hand, nearly all (as far as I can tell) of AWRI's publications on closures were released through non-peer reviewed in-house bulletins or were published in local, non-academic Australasian wine trade publications.
On another note, I also find it unnecessarily insulting that Dr. Alan Limmer is consciously referred to as 'Mr. Limmer' throughout AWRI's letter to Harpers. Given Dr. Limmer's higher degree is in chemistry and that his work on sulphide chemistry published in peer reviewed journals has earned him election as a Fellow of the NZ Institute of Chemistry, this appears petty, vindictive, and an unusual way for academics to refer to others with strong academic credentials. Just because AWRI have chosen not to acknowledge Dr. Limmer's doctorate doesn't make it go away.
ENDNOTE 1. Alan Limmer's evidence for why sulfide equilibria do not exist.
1) It does not exist in any chemistry texts, or science papers on sulfides, or more particularly, sulfides in wine. Equilibria reactions describe specific forms of chemical behaviour and allow calculations and predictions to be made about those components. To date we have been able to explain all aspects of sulfide chemistry regarding wine, and non- wine sulfide reactions without reference to equilibria.
2) If an equilibria reaction exists for a compound this means you get various versions of the compound co-existing in solution - in very precisely predetermined ratios. A good example would be the SO2 in wine, which exists simultaneously in various forms, including molecular SO2, HSO3- (bisulphite ion) and SO3= (sulphite ion). We are able to calculate these individual quantities very precisely for a given situation. We cannot have one equlibria form without the other. The sulfides in wine behave as discrete compounds.
3) Common sense can also tell us these sulfide equilibria do not exist. We can form a specific mixture of them, eg as a standard analytical solution for quantitative assay of these components , as is commonly done. The mixture says precisely as it is formulated. There is no interchange between these species.
1)T. Stelzer, Taming the screw, a manual for winemaking with screw caps, 2005, Australian Wine Press.
2)P. Godden et al, 'Towards offering wine to the consumer in optimal condition - the wine, the closure and other packaging variables', Aust. & NZ Wine Industry Journal, June-August 2005
3)A. Limmer, 'The 'permeability' of closures', Aust. Grapegrower and Winemaker Annual Technical Issue, 2006
4)Nondestructive Colorimetric Method To Determine the Oxygen Diffusion Rate through Closures Used in Winemaking. P Lopes, C. Saucier, and Y. Glories . Journal of Agricultural and Food Chemistry. 2005, 53, 6967-6973
5)A. Hart & A. Kleinig, 'The role of oxygen in the aging of bottled wine', http://www.winepressclub.com.au/pastEvents.cfm, February 2005
6) Do Corks breathe? or the origin of SLO; Aust. Grapegrower and Winemaker Annual Technical Issue, 2005
Other references noted in section 1:
Suggestions for dealing with post bottling sulfides, Australian & NZ Grapegrower and Winemaker, Dec 2005; reworked as 'The chemistry of and possible ways of mitigation of post bottling Sulfides,' NZ Winegrower, Vol 8 No4 , 2005.
Godden et al (p17/18, 05) and Skouroumounis et al (p376, col 1, p 2, 05).
(Bobet, R.A, A.C. Noble, and R.B.Boulton.1990.),
Bobet, R.A, A.C. Noble, and R.B.Boulton. 1990. Kinetics of ethanethiol and diethyl disulfide interconversion in wine-like solutions.. J. Agric. Food Chem 38, p 449-451
The impact of closure types and storage conditions on the composition, colour and flavour properties of a Riesling and a wooded Chardonnay wine during five years storage. G.K. Skouroumounis, M.J. Kwiatkowski, I.L. Francis, H. Oakey, D.C.Capone, B. Duncan, M.A. Sefton and E.J.Waters. Australian Journal of Grape and Wine Research. 11, 378-384, 2005
Rauhut D; Volatile Sulfur compounds - impact on 'reduced sulfur' flavor defects and 'atypical ageing' in wine. 31st Annual New York Wine Industry Workshop. VINDIDEA.NET.INTERNET TECHNICAL JOURNAL, 2005, #3
Rauhut D, Kurbel H, Die Entstehung von H2S aus Netzschwefel -Rckstnden whrend der grung und dessen Einflu auf die Bildung von bckserverursachenden schwefelhaltigen Metaboliten in Wein. Wein -Wissenschaft 49; 1, 27-36. 1994
Rauhut D, Kurbel H, Dittrich H, and Grossman M. Properties and Differences of Commercial Yeast strains with Respect to their Formation of Sulfur Compounds. Vitic. Enol. Sci. 51 (3), 187 - 192 (1996)
The views in the above article do not necessarily reflect those of Harpers.