Emile Peynaud, the father of modern oenology, once said: “Wine is eternal, only the cork can kill it.” This quote shows the importance of the choice of closure to preserve the wine. For which market is the wine intended, what will its selling price be, what is its ageing potential, where does it come from, is it a wine for everyday consumption or a top-of-the-range wine, what is the budget available? These are all criteria to be taken into account when selecting the most suitable product. The market today offers a choice whose limits are constantly expanding: the eternal natural or sealed cork, agglomerated or micro-agglomerated corks, synthetic corks, screw caps, glass cork; what are the differences? Here is a small overview of the different solutions to help you find your way around.
Cork in all its forms
Known since Antiquity, cork is extracted from the bark of the cork tree (Quercus suber is its scientific name), the cultivation of which is essentially concentrated around the Mediterranean basin. Portugal, in particular, accounts for 60% of world production. The cork stopper has been the closure system of choice for many centuries, for good reason: the elastic and mechanical properties of cork make it perfectly suitable for sealing a liquid. The alveoli of which it is made allow the passage of oxygen in micro-quantities, which allows the wine to breathe while preventing the passage of liquid.
Beyond these technical qualities, the magic of the natural cork stopper lies in the fact that each piece is unique. Cut from a block in the bark of the oak tree, the natural cork cannot be replicated identically. In fact, numerous studies have shown that there is a great deal of variability in permeability within the same batch. This means that corked bottles will develop in a heterogeneous way. There is something very romantic in this observation, but in an age when wines are increasingly standardised, this can also be a problem.
Nevertheless, if cork is increasingly being replaced by other solutions, it is not because of its irregularity, but because it is still the cause of a well-known defect: the eternal “cork taint”, identifiable by its smell of wet cardboard or dusty attic.
The compounds responsible are the haloanisoles1. The most widespread (70%), 2,4,6 Trichloroanisole or “TCA”, is formed by the action of moulds in the presence of compounds that are synthesised from chlorine, which can come from the bark of the cork, polluted by insecticides, from the air, or even from products used in the winery. These compounds can cause great damage. For example, 1 gram of TCA can contaminate several tens of millions of bottles.
Fortunately, technology is progressing, since 25 years ago, studies showed that 15% of wines sealed with cork were corked, whereas today the values are around 2 to 3%. However, this defect continues to cause a great deal of harm to producers, as the majority of consumers do not know how to identify it, either through ignorance or for more physiological reasons, so that they often attribute the defect to the wine, when it is actually the cork that is to blame.
Despite this, the natural cork stopper remains the solution of choice for great wines, combining mechanical, aesthetic and traditional qualities. It is also an expensive solution as a quality natural cork stopper can cost up to one euro each.
Natural corks with a large number of lenticels2 are subjected to a sealing operation. The pores are sealed with a mixture of glue and cork powder to guarantee better performance. They are less expensive than monobloc cork but also less suitable for storage.
Agglomerated corks are made from reconstituted cork scraps and bound with glue. They are more or less suitable for ageing according to their density and length, but some age badly and can release cork particles into the wine over time. Uninformed consumers may then think that the wine is “corked”. However, they have the advantage of being much less expensive than a one-piece cork stopper.
Don’t push it!
When wine consumption exploded in the 1970s, the traditional wine-producing countries were joined by new players on the market, which increased the demand for cork. Manufacturers, in response to this boom, increased and accelerated production, which resulted in an explosion of various organoleptic deviations caused by poor quality cork. Weary of this problem, some producers rejected the cork stopper and turned to alternative solutions, which have continued to improve and multiply ever since.
One of the most interesting innovations in technical closures is certainly the use of supercritical CO2 (subjected to a pressure and temperature above its critical point) in the treatment of cork. The properties of supercritical CO2, already used to extract caffeine in decaffeination processes, make it possible to clean the cork, whether in one piece or reduced to fine granules, of all traces of TCA or other substances responsible for deviations in wines. The cork can then be reconstituted with a binder.
The French company Diam, or the Portuguese company Amorim, leader in the international cork market, have developed and patented this technique to guarantee a cork with no cork taste. These solutions also offer different levels of permeability depending on the wine’s ageing potential, the index of which appears on the cork. However, the evolution of the binder over time is still poorly understood. Revolutionary, these corks are gaining market share but they remain expensive, ranging from 0.40 cents each to one euro depending on quality and performance.
The 1+1 is derived from Champagne cork manufacturing techniques. It is a cork consisting of a body made of agglomerated cork and a natural cork disc at each end. Again, there are different levels of quality. The price is of course lower than that of a natural cork.
The screw cap
Developed for wine from the 1950s onwards, the screw cap really entered the international market in the 2000s, when it was democratised by the Australians and New Zealanders. In Europe, it was the Austrians and the Swiss, major producers of white wines, who were the first to turn to this closure solution. According to a recent study, and against all expectations, the screw cap now represents 37% of the market (Diéval 2020), against 30% for cork-based closures.
However, it is still very much frowned upon in many markets, where it is associated with low-end wines, intended for quick consumption. It is criticised for not allowing the gas exchange necessary for the evolution of great wines, and even for putting the wines in conditions of asphyxiation, which can cause the appearance of nauseating odours in the wines, cauliflower or blocked sink pipes. Studies have shown that the OTR (Oxygen Transmission Rate) of a screw cap is significantly lower than that of a natural cork. It is therefore ideal for sealing white wines, which are more sensitive to oxygen, and is ideal for aromatic grape varieties, for which it protects the varietal character. New Zealand Sauvignon Blanc is a perfect example.
However, manufacturers now offer different seals that allow gas exchange to be adjusted to the millimetre, thus allowing this system to be adapted to all markets. Furthermore, it should be noted that the evolution of the wine’s aromas towards what is known as the “reduction bouquet” requires little oxygen. The oxygen in the wine and the oxygen in the headspace (between the cork and the liquid) is sufficient to allow a favourable evolution towards the so-called tertiary aromas.
In addition to guaranteeing zero cork taint or other deviations linked to the stopper, the screw cap allows for a very homogeneous evolution of the wines, and therefore the assurance that all the wines in the same batch will age in the same way over time, which cannot be guaranteed with a natural cork.
Another advantage, a minor one, is that the wine can be stored horizontally, vertically and even diagonally, if the owner feels like it, which is not recommended with cork, which must remain in contact with the liquid in order not to dry out. The simplicity of its use for the consumer is another asset in its favour.
Admittedly, it is not very romantic. On the one hand, let’s remember that, although it can be recycled, its manufacture is not ecologically neutral, and on the other hand, it is still more exhilarating to take a cork out of its glass case and to hear the “pop” that it causes. Is corkscrew not the man’s best friend?
Synthetic corks are generally made of polyethylene foam. They take the shape of the cork and its principle, to satisfy the consumer’s desire to handle the corkscrew, and they protect the wine from deviations such as TCA. However, studies have shown that with a synthetic cork, the rate of oxygen transfer (OTR) is rapid and significant. They are therefore generally suitable for fast-moving wines and their low price also places them in this category. Another disadvantage is that while corks retain liquids but also gases, synthetic corks do not have this property, which means that there can sometimes be aroma transfer. This is known as “flavour scalping”.
However, the synthetic closure industry is also evolving, and manufacturers are stepping up their innovation. Nomacorc, the market leader, now offers synthetic closures made from polymers derived from natural products, such as sugar cane, which have a neutral carbon footprint and are 100% recyclable. The best ones also offer good performance in terms of oxygen supply and therefore allow wines to be preserved in very good conditions.
The glass stopper or other curiosities
The Vinolok stopper consists of a glass stopper and a seal. It is easy to open and close and, according to a study by the Australian Wine Research Institute (AWRI), its oxygen transfer rate (OTR) is similar to that of a natural cork. In addition, there is no risk of cork contamination or off-taste and it is infinitely recyclable. Its design is attractive, especially to rosé producers, who are using it more and more, which has the effect of increasing the price of these wines even more, as this system has a cost (from 50 cents a piece). 30 million units will have been sold in France by 2021, so this is a system that has the wind in its sails!
Not that simple!
In 2017, Oxford University conducted a study involving 140 participants who were asked to taste two wines, one sealed with a screw cap and the other with a cork. Unaware that they were actually tasting the same wine, the participants judged the corked sample to be superior in quality to the screw-cap sample. On average across countries, 90% of consumers associate cork with higher quality wines, and are willing to pay more for a cork closure. One should not underestimate the “magical thinking” in wine tasting, i.e. the influence of context and image! The greatest challenge for the industry is still to convince the consumer, especially in conservative European markets such as France. Let’s leave it to Philippe Guigal of Domaine E. Guigal (Ampuis), to close this article with a quote that illustrates the above: “Wine is not only a question of technique, it is also a question of emotion, and behind the emotion, there is the human element. It is quite possible that in my dreams I will see a beautiful cork, a high quality cork. I must say that I have rarely dreamt of screw caps. And if it happened to me, it would not be a dream. “