[1]

Dr. Peterson, I share your passion for the scientific basis of wine making and all things relevant. With the understanding that free O2 is not necessary for RedOx reactions to occur, I have always thought of a properly sealed bottle of wine as existing in a state of equilibrium leaning towards a reductive balance because of a lack of reagents. I do need some clarification from you to understand the matter better: Show me a cork that breathes and I’ll show you a bottle of vinegar.


You say all wine has bacteria (presumably Acetobacter?).

– What about the use of sterile filtration in modern wine making?

– Additionally, does Acetobacter thrive in a solution which is 13% ethanol (or more)? Is there a concentration of ethanol that is bacteriostatic or even bacteriocidal to Acetobacter?



I would like to hear your thoughts on this matter as the wine turns to vinegar notion is typically thought of as a wine myth.

[2]

Thank You, doctor Richardson, for your article -- corks do not breath. I have held this position for the same reasons you state for many years. I believe the slow oxidation of phenolics and other compounds in the bottle comes from the absorbed O2 in the wine, not through the cork. I also believe - w/o proof - a cork becomes a time capsule for the slow release of O2. The O2 becomes trapped in the inhomogeneous pores of the cork after compressing for insertion in the bottle. With time this O2 transfers to the wine as a result of the negative pressure in the bottle and other Oxy/Red forces. I also package and store all our wine neck up to minimize TCA contamination and have had a TCA incidence of < .5%, whereas industry ranges seem to be 3-7% for neck down. I have lost sales brokers and customers for my belief. I’ve been called a maverick, but I feel strongly about this and continue to do what I feel right. Our 3000 case winery’s sales increase each year.

[3]

You do not use the word ullage, which is air space between the wine and bottom of the cork. I do remember from chemistry that matter can neither be created nor destroyed, so where does it go when ullage volume increases over time? It happens with barrels, too. Even if the wine vaporizes, it still left the liquid state and went somewhere. Is it all contained inside the cork, or in the case of oak barrels, impregnated in the wood? Doesn’t air replace this wine, or is it a perfect vacuum?

[4]

I have read numerous articles about permeation studies and having it at the right rate so as to age the wine properly. Screw caps generally have been either too tight or have too much permeation. Synthetics are too tight to begin with and then let too much air in later. Here is a link where they mention a permeation study. I really just do not buy your take on it. I have read about a great number of these types of studies. Maybe your research was done with old equipment a long time ago and is out of date.

[5]

Thank you Richard!

Please now please explain how SO2 does not protect wine from oxygen but merely masks it by binding up the aldehydes caused by oxidation until it is exhausted and then goes to work on steps 2, 3, and 4, and I’ll gladly post it in my tasting room next to my wines finished with screw caps.

[6]

Thanks for the education.

[7]

The content of this article apart, I found I had to repeatedly keep myself emotionally neutral in order to remain receptive to its message. I found the tone of the writing negative, defensive, aggressive -- why? I do not know the writer and do not really know the reason. Could it perhaps it be related to the historical fact that, as we all know, a lot of so called scientific truths sooner or later are blown out of the water by newer, better, other "scientific absolutes". By the way, I tend agree with most of the article content, although wine -- including its aging -- still mystifies me. Well, yes, I know that scientists tend to be irritated by this human weakness of being 'mystified' by something...

[8]

This is a great article. Thank You, Dr. Peterson.

The logical question if you understand this article is: why should I keep using cork if there are other closures that provide an equally tight or tighter seal and that won’t contaminate my wine with TCA? It is precisely for this reason that we have just bottled Long Island’s first wines sealed with a screw cap! It has been demonstrated that a typical stelvin type closure forms a seal as tight or tighter than a top grade 2 1/4 inch cork. There is a big misconception (to which I plead guilty to having believed myself) about a wine’s potential to become reductive and for that potential to be exacerbated by screw caps, due to the tightness of their seal, and for it to result in serious sulfide issues in the wine. The reality is that if the wine is bottled with any residual sulfides, or if it has the potential to become reductive and form those sulfides in the bottle, it will do so regardless of the closure. From a wine quality point of view, I can not see any reason to keep using cork.

[9]

Dr. Peterson:

Please provide references to recent scientific studies to substantiate your beliefs. Given many contradictory claims, readers appreciate being given tools to further investigate the issue and decide for themselves.

[10]

Thank you for your article regarding breathing corks fable. Then does it matter the type of closure that is used as long as it seals properly and will not negatively add a flavor to the wine, i.e.: a composite cork, a cork made of alternate materials, a screw type?

[11]

Wow. Such strong, definitive statements from the author. You might even be fooled into believing he knows what he is talking about. However, his statements are essentially incorrect and misleading. I have read almost every study on this topic, including all of the latest studies out of Australia and Europe. I have done this both as a winemaker and former plastics engineer responsible for the design of plastic wine corks. Here are the facts: The oxygen transmission (breathing) rates of various closures are the following:

-- Screw caps (tin liner): 0.0002-0.0008 ml oxygen per sealed bottle per day.

-- Screw caps (saranex liner): 0.001 ml O2 per day.

-- Natural corks: generally accepted as lying in between screw caps with tin and saranex liner. Rate of transmission depends on whether bottle is lying down or standing up (lying down is lower and better). Because cork is a natural product, it can have variability in density, and this can lead to big variation in oxygen transmission and what is known as random oxidation.

-- Plastic corks: 0.007-0.020 ml oxygen per day.



To the un-initiated, all of these numbers may seem very small. However, they are very measurable and very significant from the point of view of the wine: On the high end of oxygen transmission, you have plastic corks. These rates of oxygen ingress result in wines with shelf lives of no higher than 2-3 years before they become 'oxidative' in their flavour profile and are essentially considered over-developed and treading toward undrinkable. On the low end (corks and screw caps), this permeability results in wines that can live for decades before they finally pack it in and become over-developed. All of these closures breathe (let in oxygen). It is the rate of oxygen ingress that determines shelf life and the speed at which a wine will develop and take on aged flavour characteristics. Is oxygen necessary for proper ageing? Less than you think. Wines will still develop, but they just do it more slowly. This area of whether oxygen is necessary for development from a flavour profile perspective is up for debate. But there is no debate from a shelf-life perspective. Oxygen does indeed enhance the rate of tannin polymerization for a given period of time (in red wines). If you have a harsh, youthful wine that is loaded with tannin, then the oxygen ingress will soften the wine by making those tannins bigger and less astringent. Eventually, they will become so big that they become insoluble in the wine and drop out, producing what is know as bottle crust. As the oxygen permeates into the wine, it is consumed by the wine, and so the actual concentration of oxygen in the wine is very low. The wine is therefore considered in a reductive state. At some point, the wine reaches a level where its ability to absorb oxygen without detriment is surpassed and it begins its downward slide into oxidation. In terms of shelf life though, a winemaker must know their wine and their market. If you have people who will put your wines down for many years, you as a winemaker better provide a wine closure that will go the distance.


And those are the facts. All closures breathe. It is just that their rate of breathing is incredibly low.

[12]

I appreciate the conveyance and accuracy of the information in the article. I find the attitude a little hard to swallow. It could use a little air.

[13]

I wish we had some earth shattering facts & figures that would convince the public that some sort of perfect seal on a $50 bottle of wine is preferable. I spend a lot of time & money picking out the best quality cork for my bottles & normally if (when) I run into a corked bottle it is only slightly corked. From my perspective, this is actually much worse because the taint was only strong enough to destroy most or all of the fruit but not strong enough to give off that bad corky impression & unless I am sitting there sharing each bottle, a consumer trying my wine with their friends for the first time, will get the impression that my wine is mediocre! If I were to get lower quality corks I’d have many more corked bottles. For a very long time I’ve wanted to use a perfect closure that is perfect for the wine & the consumer. If it’s the romantic tradition people will miss (without a cork) I advise everyone to get a fancy Crystal decanter at the price of a couple of great wines & never lose great wine to the Russian Roulette of cork!

[14]

AMEN!

[15]

Hi, Dick – remember me? Good article about corks, which really are the perfect closures for wine. Just too bad that there can be so much variability, although that also has improved dramatically in the last decade or so. I'm semi-retired now, consulting for a few small labels, teaching an online class for UCD extension, and have a small winery of my own. Life is much simpler and way more rewarding away from the corporate wine world.

[16]

Thanks Dr. Peterson for your excellent contribution to this debate. I discussed many of the same issues in my book "To Cork or Not To Cork," which came out last year. And I'm not surprised that APPELLATION AMERICA gets lots of feedback on the topic. Given your obvious expertise, what is your view of screwcaps as a wine bottle closure and their effect on the aging of wines?

[17]

Dr Peterson, first I repeat the request of Lise Ciolino [comment #9] - please provide references to your works proving your claims. As for me, sure - I am a screw cap fan, and bottle over 66% of our production this way. I am also a fan of science, and have had sulfide analyses performed at ETS labs over the five years we have used these closures. Screw caps slow the aging of wine without doubt (please visit and taste some wines) and increase the incidence of reduced sulfide compounds, in addition, we see less free SO2 consumed. I admit that increased reductive components, higher levels of free SO2, and more fresh flavors in the screw cap finished wines aren't proof of oxygen transmission, but they do indicate something is going on. As for corks, the best cork for me is the one that is closest to a screw cap - i.e. it doesn't let in oxygen. The trouble is, corks are so varied it isn't funny: I had a lively discussion this month about which bottle of 2004 Cabernet was better - the one that was more developed or the one that was more fresh - and these were identical, sterile filtered wines opened at the same time. What can I attribute this to?

[18]

They [corks] do “breath” and that’s a big problem. Bigger than TCA. And some “breath” more than others.

[19]

...a very interesting piece, but it raises the question: if natural cork is such a perfect seal, why then have many of those who have chosen to go the route of screw-top closures reported slight increases in off-odor/reduction upon first opening their wines ? I haven't heard a similar complaint from fellow natural cork users.

[20]

I will repeat the requests of others: Please cite references and more facts. It seems to me the author is trying to prove his point through bullying and harsh language, taking the pose of "I’m right and everyone else isn’t." Another typical APPELLATION AMERICA article with trite opinion and very little fact leading to GREATER misinformation of the wine consuming public.

[21]

Thanks for your intelligent article.



Wine makers I have dealt with claim that wine closed with plastic corks, for example, age differently than with cork closures. We know it's not the cork "breathing" so is there any truth to their perception?



When I retired, a fairly large number of cases of two different vintages of a cab blend came along, and have been stored properly. A few bottles have had "cork" taint as would be expected. The others, as the years have passed, are fairly uniform but a few are "good bottles" and a few are a bit less desirable. The vintages are 98 & 99, and in a great many cases dregs from last night's wine are really quite good. No oxidation has been observed as yet. They are closed in cork.



What explains the differences? Is it each bottle wandering chemically a bit?

[22]

Dr. Peterson, you have some good points here and we all know that there are simply too many myths when it comes to the whole closures debate. Something I would love to see is another article on TCA. People are simply not aware of current TCA controls in Natural Cork. Respectable manufacturers have this well below the threshold of the human senses. And regardless of what anyone says, that means all humans, including sommeliers and other industry professions. For more on what TCA really is, how it comes about and how its occurrence can be inhibited -- please, please visit Centro Tecnológico da Cortiça, then Click on Symbios – English. Perhaps then we can start addressing the next major issue in the closures debate. Carbon Footprints!!! Do we really need to be producing more synthetic materials and non-biodegradable wastes when cork forests actually counteract Carbon emissions and manufacturing has the lowest footprint of all closures?

[23]

Dr Peterson, I have noticed that it is easy to blame the cork for wine problems. The biggest problem is what lies below the cork - the head space. When the head space is eliminated, even milk can be packaged and sold without refrigeration. All bulk wine is stored without a head space, so why sell a bottle with it? There is a wine dispenser soon to be released that can store wine indefinitely and dispense at the same time. The temperature does not change one degree. Wine can now last longer than wine that is left in the bottle. If you like, I can send a picture.

[24]

I can't be the only one to have brought this up... Cork is a renewable resource. It provides jobs. How long does it take for plastic or the screw-tops to breakdown in our landfills and how many jobs do they take away from the Cork industry?

[25]

As a home winemaker & wine enthusiast who is quite well read on this subject I must also side with those who question your statements (and attitude). One thing that has not been asked - and with all due respect - you are a Doctor of which Philosophy??

[26]

In the context of the article the author and the readers might want to take a look at Amorin research which, I am told, is available online. Source of the research notwithstanding, it looked at a number of driven closures to establish the source and rate of oxygen ingress in the bottle, and arrived at the conclusion that both closures (plastic and cork) contributed oxygen directly initially due to compression, after which cork allowed a near constant, if somewhat variable (cork being a natural product) oxygen ingress over a long time through the seal. So although it does prove the point that the cork does not breath, this stopper does contribute micro burst of oxygen right after bottling, and steady and low oxygen supply via the seal with the neck, possibly due to the suction-cup like nature of cut cork cells, and that does play a role in the development of the wine in the bottle. Those are facts, not notions, unless one can find flaw with the research methodology. I could not.

[27]

Dr. Peterson,

1.) For many years, it was thought that oxygen didn't move through the staves of a barrel. Then Australian research showed that it did. Given that corks are also made from material form an oak tree, perhaps more research is needed to study the possibility that oxygen also moves through corks at some very small, but significant, rate.

2.) Recent reports of issues with wines under screwcaps indicate that wine organoleptic problems under those closures are more common than those under cork closures, with problems associated with reduced aromas predominating with screwcap closures. Perhaps this provides clues that there actually is some oxygen flow through corks and that it actually does help eliminate reduced sensory expression of wines closed with them.

3.) Isn't it really all about partial pressures of vapors across semi-permeable membranes and the flow that results there from? And aren't corks semi-permeable, albeit only slightly? Isn't a bottle with a cork closure really like a little barrel, only with a very small area available for oxygen flow?

[28]

While I have respect for Richard Peterson's education and experience, his writing is quite pretentious. Calling other people idiots might make him think he is smarter than others but all it does is lower my respect for him as an individual. I'm sure this kind of sensationalistic and antagonistic writing gets more publicity. However, it does more harm to the industry and is not the kind of publicity I want to be associated with.

[29]

Thank you for saying that!!   I get sick of hearing it
too.   We use Stelvin closures, and protect our wines from the mold spores and bacterias of cork, as well as the plastic taste of the artificial ones.



The myths about wine corks are many and mostly untrue, and lead the customers to believe a lot of things that really don't make sense.



Thanks for making a point to do something about at least ONE of the myths.

[30]

I have found the science that the cork does breathe to be irrefutable... I would like you to retract your article... in light of this empirical information and study... The Chemistry of Post–bottling Sulfides in Wine. see Page 1: Fig. 1 [Oxygen transmission rate for cork (measured by MOCON)]. This science proves corks do breath and that air transfers via cork... end of discussion... The Cork Does Breath...

[31]

While I agree with the cork facts. You seem to say that there are no risks of quick oxidation, while leaving out the fact that very old bottles of wine, after opening, the flavors can volatize within the hour.

[32]

Dr. Peterson, thank you for stating so clearly what many of us in the wine business have observed for years but haven't been able to articulate as well. Since alternative closures were introduced in response to the unacceptable rate of TCA spoilage in cork finished wine, we have come to appreciate the convenience of screw caps, as well. Someone aptly said that screw capped wine when opened will taste as the winemaker intended it to taste. This makes one wish all wines were bottled under screw cap. And begs the question, why use cork at all?

[33]

I am very interesting in this kind of article because Portugal is the biggest producer of cork in the world. One thing is sure: Until today I had not read any article about "wine closures" and their negative and positive points where I felt that the writer was completely neutral in their article. The young people are not sleeping, and they are the ones that are doing more for defending the Earth from pollution. And you may know how positive the cork forests are for it. Let’s be honest!

[34]

Bravo! Some evidence presented instead of the usual uninformed opinion. Now, what about breathing barrels and the fad of micro-oxygenation in tanks? Any evidence there?

[35]

Dr. Peterson,

I accept your argument that cork closures do not transmit oxygen. However, how do you explain that older wines seem more oxidized than their younger counterparts? Is this the result of an anaerobic reaction taking place inside the bottle that appears to the taster to be an oxidative effect?

[36]

Dr. Peterson's conclusions seem to be in direct opposition to the latest research on closure permeability. The article linked in comment #4 should be read along with Dr. Peterson's article. I think the modern research trumps the older one. It is common for scientists to hang their hat on outdated research and questionable methodology, particularly if they were directly involved. If we were to rely on every test and scientific observation as the final answer we would have stopped scientific research centuries ago. At one time we had "proof" that it was fruit flies that caused fermentation.



Dr. Peterson's comment about the wine turning to vinegar may be simply tongue in cheek as he must know that wine doesn't automatically turn to vinegar, that wine normally oxidizes rather than turning to vinegar and that vinegar is caused by bacteria that can easily be omitted from the wine prior to bottling, if indeed it exists in large enough numbers to cause vinegar conversion. Surely Dr. Peterson is familiar with sterile filtration.

[37]

Why would we age our wine in barrels if not for the micro oxygenation that occurs with oak versus stainless steel? If you say that the best way to age a wine is in an anaerobic state, then why expose any oxygen at all to it with barrels/micro oxygenation?

[38]

Peterson's latest diatribe is no more convincing than before, and the repeated use of the terms "absolute" (truth or untruth) and "idiot" still does nothing to strengthen his argument.

[39]

Ok Dick,

So what is your recommendation for closure? Are you arguing for Noma or Stelvin? Or do you still think that cork is a viable closure?

[40]

I was technical director for one of the local wine labs for a number of years, and conducted proprietary wine aging studies for synthetic stopper producers. I also bought a vertical of those Plumpjack Cab 2-packs (cork finish, screw cap) and tasted them over time.


My 2-cents worth: if you like how your wine does under screw caps, good on ya – finish them that way. I'm sticking with bark cork. As I published in my own blog a while back, I believe the extractables from bark cork are a beneficial influence on wine quality – especially for red wines the way I make them. IMO it is these extractables that alter the redox equilibrium of the wine in the bottle (not molecular O2 trapped in the cork matrix – though that might explain the performance deficiencies of some synthetic stoppers).


RE: increased ullage [comment #3] in the bottle as wines get very old – AFAIK as the cork becomes saturated a vacuum is created that causes the wine to outgas, filling the space. Should the vacuum become too great and the cork too dry, the seal WILL fail and oxygen will get into the headspace – with vinegar being a possible result. This is why Bordeaux's 1st growths offer topping and recorking service to owners of older bottles.


RE: Arthur's question [comment #1] about Acetobacter – sterile filtration would remove all bacteria from the wine, but if the bottles are not thoroughly washed before filling bacteria may be present inside the glass. There is no alcohol level that is protective against Acetobacter – this bug's evolved role is to convert ethanol to acetic acid in the presence of oxygen.


RE: you "show your references" trolls and doubters – pshaw! – get thee to a library. Dr. Peterson's research is peer-reviewed, published and extensively cited.

[41]

I have to ask, do these cork studies reflect which kind of capsule is used to top the bottle? Plastic, aluminum, wax?? Or were the tops of the corks exposed? Does the capsule make a difference because I can't see plastic capsules being permeable in any way, unless it's cheap and flawed.

Also, I've had the unfortunate situation occur, where my "perfectly sealed" Stelvin wasn't compressed around the neck properly on the bottling line, and the closure slipped right off. So, as it seems nothing's perfect.

[42]

Dr. Peterson, thank you. I agree with your assertions about the difference between reductive ageing and oxidation. I will have to admit, though, that certain corks have proven to develop more "porosity" than hoped for. However - a little barrel room horse sense will show anybody that wine seeps through very tight oak grains. I cannot see why air wouldn't seep similarly through cork pores if they were indeed pores. There's another misconception. Is it true that these empty spaces in cork would be better consider tiny crevices rather than xylem & phloem, or what we know of, say, the epidermal pores in out own skin? I think of cork "pores", therefore, as a series of pockets/dead ends, where no air is really making the whole trip from end to end. Also, would you mind discussing bottle bouquet? I think your opinion would be helpful to many.

[43]

I must say that Lise Ciolino's comments are spot on... while I believe in science, I also believe in scientific method and thus, habeas corpus anyone?

Love the article nonetheless,

best

Joel

[44]

Oops, went through more comments, notes taken... my bad