The Fizzics of Fizzy Corks - (Article with Questions and Answers)

by John Casey

(Introduction - There are two ways to read this article. The first, which is located here, is a straight forward copy in its entirety. When I read the article, I found it very interesting and shot the author back a series of questions, which he was kind enough to answer. This version contains the article in black text, but interspersed though it, I have highlighted certain portions and then asked questions below them, in this shade of blue. John's responses look like this. Its easy to follow once you get into it. Happy reading - and thinking! )

I am a self-appointed expert on ‘fizzics’ and have written a number of technical articles about various types of closure systems and the physical chemistry of sparkling wines. A colleague thought I might be interested in your item about stoppers for sparklies, and I offer the following comments. Sorry about the length, but it is one of my favourite, harmless obsessions, if you can call inducing catatonia in others, harmless.

There is a straightforward explanation for the champagne stopper phenomena noted in your (article/item). In brief, they are due to the decline of resilience brought about by prolonged strain, and to differences in the degree of stress imposed on the corks. They are probably not caused by variations in the physical properties of the corks. Agglomerate cork stoppers are a manufactured product and are fairly uniform in their physical properties. There may be differences between manufacturers, and also differences of 1-2 mm in the diameter and length of the stoppers selected by winemakers.

TORB: Does this mean that you are saying that there is no variation in the physical properties of corks within a batch?

John: Probably no significant differences.

TORB: Or does it mean that different suppliers may perform differently in the same bottle?

John: I haven’t checked it out. There are a number of different methods of producing agglomerate cork, and I assume their products differ somewhat.

My guess is that the observed shortening of the stopper can be attributed to a bore that is narrowed or constricted by fractions of a millimetre. This resulted in a lesser depth of insertion, a greater degree of longitudinal compression of the upper part of the cork, and a greater degree of radial compression of the stem of the cork. The greater compression of both the head (axial) and the stem (radial) of the cork increased the rate of decline in resilience and in the long term, caused a reduction in both the final overall height and the sealing pressure. (Further explanation below)

TORB: Is this a “guess” or is it based on any scientific basis?

John: It is based on the observation of its occurrence and a consideration of the opposing forces involved in the insertion of a champagne cork.

 Surely the cork is plunged in with enough pressure to go in all the way? It’s a very tight squeeze, and when the bore offers additional resistance, there is greater axial compression and a consequent reduction in the depth of penetration...

Inherent variability of the stoppers is not a factorr and the major variables are as follows.

TORB: What evidence is there of this statement please?

John: There is a hierarchy of factors involved, and I would put variability in the corks at the bottom, i.e., possible but unlikely. I grant you that it’s an opinion.

Controlled variables:   Stopper diameter and length, ‘leg length’ of muselet, nominal insertion depth, nominal gas content of the wine and nominal bore and finish dimensions for different bottle styles.

Uncontrolled variables:  Actual dimensions and profile of bottle bore, actual insertion depth, actual gas content of wine and temperatures during storage and distribution.

TORB: As per a previous question, surely the insertion depth is the same in each bottle.

John: Nominally yes, but not when there are variations in bore dimensions and profiles. Bore diameters can ‘flare’ or constrict tin the region and just above the insertion depth

TORB: (At the time of bottling) how does the gas content vary between bottles in the same batch.

John: In ‘méthode champenoise’ there is a variable gas loss at disgorging because some bottles are quiescent, and others ‘fob’ to varying degrees. There is also some variations in bottle pressures before disgorging. In the transfer process, there are different gas losses between bottles during and after filling. Bottle pressure measurement is a subject in itself; I have written a short article about it.

Because a cork consists of some 80% air (by volume), it provides a pneumatic seal. After compression in the neck of a bottle, the elevated pressure of the air in the cork cells prevents the ingress of atmospheric oxygen, and at the same time, this pressure causes the air to permeate very slowly out of the cork to the atmosphere; the greater the degree of compression of the cork, the faster the rate of permeation and the decline in resilience. Thus the rate of decline in resilience of sparkling wine corks is faster than that of corks for still wines.

TORB: But surely the corks are made to different specifications to try and overcome this problem.

John: To minimise the loss. The containment of gases and liquids, (overcoming fugacity) is the province of a range of specialised industries. Footballs and tyres deflate in time, and tap washers, brake fluid seals, head gaskets, stuffing boxes, caulking, toilet flushing seals and little Dutch boys, all have to replaced at regular intervals of, say, one to ten years.

A ‘champagne’ cork, (75% air), is compressed in two directions. The business end is compressed radially from 30 or 31 mm to a nominal 17.5 mm, and its volume is reduced by at least two-thirds. The pressure of the air in the cork cells is then of the order of 900 kPa. The upper section of the cork is compressed longitudinally between the muselet and the rim of the bottle by the brute force application of the muselet. The extent of this compression depends on the depth of insertion, the effective ‘leg length’ of the muselet and the external dimensions of the ‘crown/cork’ or ‘cork’ bottle finish. Minor variations in the profile and dimensions of the bore can affect the depth of insertion of the stopper, and its sealing pressure.

Another variable is the amount of gas in the wine. In my indirect experience with ‘méthode champenoise’, bottle pressures before disgorging were about 800 kPa, and about 600 kPa after disgorging and liqueuring, (other companies may have different specifications). There were no further formal checks on bottle pressures, but after ten or more years, gas contents were noticeably lower. The need for retention of adequate carbon dioxide in the wine for a decade or more must be a consideration in decisions about initial bottle pressures and stopper dimensions.

TORB: What experience do you actually have; are you in the industry? (Either cork, wine, scientist etc.) 

John: I worked as a chemist/technical trouble-shooter for McWilliam’s Wines for thirty two years at their Head Office in Sydney. Just after I started, ‘champagne’ making was moved to Yenda, but samples of all batches of tirage and expédition wines were sent for checking in Sydney. Bodega Sparkling, a bulk fermented wine was produced in Sydney largely under my technical control. For a time in the 1970s, it must have been the biggest selling sparkling wine; at one stage we were pumping out about a million bottles a month.

Like corks for still wines, champagne corks vent when the internal pressure exceeds the sealing pressure of the cork, and the seal is restored after the drop in internal pressure. On several occasions, I have seen ‘complaint’ bottles of sparkling wine that had leaked twenty or thirty mL of wine but were still retaining 400-500 kPa of gas without any leakage. The most plausible explanation being that they had been stored at >40°C, the cork vents liquid, (one had leaked/vented on the back seat of a parked car), the pressure is reduced, and after returning to normal temperatures, there appeared to be an inexplicable loss of liquid from a sealed bottle of unspoiled sparkling wine.

TORB: The two bits highlighted seem to be a contradiction; I don’t understand this at all.

John: Yes, that was the conundrum. The insufficiency of wine in these bottles was clearly due to leakage/venting because of corrosion of the muselet and the ‘hood’, and wine staining and residues on the bottle and label. Yet the cork seal appeared intact and the pressures within the normal range. So clearly, the seal pressure had been exceeded for short time, (presumably by elevated temperature), and then the seal had been re-established after liquid loss and returning to normal temperature.

The resilience of the wad or gasket in metal caps also declines over time, but there is not much quantitative information about their performance; long shelf-life is not a feature of other products. The nominal shelf-life of bottled beer is 9-12 months, but this is largely due to oxidative decline. My own impression is that very old bottles and cans of beer tend to be a bit flat.

TORB: If you mean crown caps, Champagne can spend many years on lees with caps and they do not suffer. Ed Carr even talks about the importance of time of lees.

John: I have written an article called “Méthode champenoise or mythe champenoise.” which offers a rational explanation of champagne ageing. I haven’t looked thoroughly at gas loss from tirage wines, although I had noted that bottle pressures were always less than the theoretical amount of gas produced by the secondary fermentation.

Stelvin wads from old bottles are heavily indented, and the rate of loss of SO₂ in wine under Stelvin for several decades is about the same as that for cork. The performance of both closures depends on the nature and dimensions of the original materials and the quality of application.

I’m inclined to agree with Ed Carr’s comment about the amelioration of the flavour that occurs with some wines under cork. It also seems possible that the current vogue for ageing finished sparkling wines could be due to the lack of exposure to cork before disgorging; that is, because of the use of crown seals for tirage wines over the last few decades or so. The widespread belief that ‘ageing’ of wines under cork is due to the ingress of small amounts of oxygen is incorrect and not supported by any evidence. The circumstantial evidence suggests that it is caused by cork’s affinity for volatile, hydrophilic compounds in the wine. “Breathing corks” was consigned to the scientific rubbish heap about seventy years ago but strangely, it still has a strong hold on the hearts and minds of otherwise sensible people in the wine industry.

TORB: Assuming you mean sparkling wines, can you please explain why this is the case.

John: Corks are like Dr Who’s Tardis, larger on the inside than the outside. Because of their cellular structure, they have an extraordinarily large internal surface area, at least 2-3 square metres. This gives them significant capacity to adsorb volatile compounds from their environment (e.g. TCA) or from the wine. Wood also has this capacity, but not as strong as cork. This adsorption modifies the taste of the wine over a period of time. You may have noticed a reduction in the ‘raw’ or ‘vegetal’ character of some wines after a number of years under cork.

Despite being permeable to gases, corks provide an impenetrable barrier to atmospheric oxygen because the concentration/pressure of oxygen in the compressed cells is much greater than that in the atmosphere. However, carbon dioxide is water soluble and, theoretically, it can permeate through a compressed cork. Although there is significant loss of carbon dioxide from sparkling wines, there is no post-bottling oxidation, except for wines sealed with plastic stoppers.

Differences between natural corks for still wine are nullified when they are compressed to 50-60% of their volume, (picture yourself squashed into a 40 litre drum!). Their sealing pressure depends mainly on their diameter and that of the bottle bore. The difference in appearance and performance of natural corks extracted from bottles of wine is not due to inherent differences between the corks, but to whether or not they have been subjected to high hydraulic pressure after insertion. Winemakers would like you to believe that any problems with ‘oxidation’ or so-called “leakage” are caused by inferior corks, but this is just a ‘cop-out’. The fault lies with the generation of headspace pressure during or after corking; this results in a softening of the cork cells, and also provides the motive force for the expulsion of wine past the cork. It’s a bit like carpenters complaining about bad nails that bend, go in crookedly or take more than the usual number of blows to drive home.

TORB: First highlight – How do you explain the difference in bottles from the same case?

John: Wines are corked on multi-head corking machines, and bottling managers are more interested in daily production quotas than close monitoring of the application of adequate vacuum to every bottle. If these remaining grey hairs could only speak! I have measured a range of pressures on bottles from a single carton.

TORB: Second highlight – How does this happen? Surely the bottling line process is consistent.

John: Most, but not all of the time. However, the main problem is the operation of the corking machine. Headspace pressure in bottled wine is like hypertension in humans, a silent, symptomless killer. I have written lots of articles, PowerPoints and other exhortations about it.

The erratic behaviour of your ‘leaking’ bottle is difficult to explain. The sealing pressure of the cork and the gas content of the wine both decrease over the years. During this time, there is a dynamic, quasi-equilibrium between the gas pressure in the cork, the sealing pressure, the gas content of the wine and the gas pressure in the bottle. When the bottle was removed from storage, it is possible that the movement disturbed the equilibrium, and the internal pressure exceeded the sealing pressure shortly before the wine was cooled. II was told by a hands-on ‘champagne’ maker that the way to test old sparkling wines is to shake the bottles close to your ear. Bottles with little or no pressure make a ’gurgling’ noise. If the gurgling noise persists, the bottle is flat, but if the gurgling stops, there is still adequate gas in the wine. I have never had the need to apply or test this.

TORB: How do you explain multiple bottles and vintages of Rockfords Black that were all shipped in cool weather and went into an air-conditioned cellar, and some of them leak before they come out of the cellar?  

John: This was speculation on my part. I would have to see the body(ies) before making definite pronouncements. Obviously “more research is needed” as they say at the AWRI.  I will donate my labour if someone donates the material.  Although the effective life of the expedition cork is limited, there shouldn’t be any widespread leakage after five years. Maybe Rockford know or could guess the reason. It also seems odd that the bottles were kept for five year before release; maybe a medal-winning wine? The traditional approach has been to age on lees, (6 months minimum) and release for sale shortly after disgorging. McWilliam’s had a medal-winning ‘champagne’ which after eighteen years, became an unreliable entry in the wine shows, even after using the ‘shake and listen for the gurgle’ technique to select the bottles. The bottles were emptied into a tank, and the aroma of aged ‘champagne’ filled the building. By my estimate, about 5% were dead flat about 40- 50% had adequate gas, and the remainder were very slightly ‘spritzig’ but seemed to be all the more delicious with minimum gas. I don’t recall seeing any bottles that had lost liquid.
uite like old white sparkling wines with low gas content, but the sweetness of sparkling reds can be a little cloying when there is very little gas left in the wine.

TORB: I look forward to your response, this is interesting.  

John: I am gratified by your interest, and I sincerely hope that your eyes are not glazing over. Did you ever see Michael Palin in “Ripping Yarns”(I think) and the episode of the Eric Oldthwaite gang, both members of which were obsessed with the study of shovel handles and weather maps. I claim to be the sole member of The Institute of Fizzics.

Copyright © Ric Einstein 2006