For most of the history of fine wine there was only one way to seal wine into a bottle and prevent air from entering. That was the cork. In the last 30 years alternatives have appeared including crown caps, plastic corks, technical corks and screwcaps.
Over recent years Te Mata Estate has been experimenting with different wine closures. Across our range of wines we use a combination of natural corks, screwcaps and DIAMS each for different reasons. This article explains the closures we use and some of the reasons why.
Natural corks are made from the bark of the cork oak. About every 9 years sections of bark are pealed off the tree, dried for a few months then processed into corks. Corks have the property of being virtually impervious to liquids and gasses and when compressed into a bottle neck rebound and seal against the glass better than any man made product. Corks permit tiny quantities of air to reach the wine during bottle ageing. This process works well and we know that a good white wine will mature in flavour and improve over 3-5 years and this extends to 10 or more years for a good red wine. Being a natural product, however, corks show some variability and there are two issues associated with this.
The first is cork taint. This is referred to as corkiness or the wine is referred to as being corked. It occurs in a very low percentage of bottles (somewhere around one to five bottles in a hundred) and appears as a slight musty or dusty character in the wine and seems to obscure the fruit. Cork taint is caused by naturally occurring fungus coming in contact with chlorine molecules and growing on the surface of the cork either on the tree or during storage before processing. The fungus produces a compound called trichloro-anisole which has a detectable odour at extremely low concentrations.
The cork industry has done a lot lately to minimise corkiness. The main advances have been the elimination of chlorine from the forests and factories, the selection of cork from higher up the tree where cork taint has been found to be less of a problem and finally the washing process is now done in stainless steel baths and the water is filtered. In our recent experience cork taint in our wines is currently around the one percent mark.
The second issue with cork is the appearance of random oxidation in some white wines. Most white wine is consumed within a year or so of being bottled but some styles improve with bottle age. Two examples of this style are barrel fermented chardonnay and sauvignon blanc. In the mid 90s the technical wine literature began talking about random oxidation in white wines. Some white wines with a cork closure were showing bottle to bottle variation in aged character after a couple of years. This phenomenon was put down to corks permitting variable amounts of air to reach the wine. It is easy to imagine some corks being more or less porous than an average cork in a batch. Corks didn’t suddenly get more variable in the 90s so random oxidation must have always occurred in some batches more than others. It is just that the wine industry and the consumer has become more aware of the issue. Random oxidation can be addressed to various degrees by using the best quality corks, bottles with a narrower bore, larger diameter corks or higher levels of sulphur dioxide, however, for white wines there are alternative closures that solve the problem. Random oxidation does not seem to be an issue for red wines no doubt due to the much higher content of phenolic molecules (tannins and colour) that react with oxygen.
Natural Corks at Te Mata Estate
We still prefer to use natural cork with Coleraine, Awatea and Bullnose. We know from our experience of the worlds finest red wines and from our own experience over 25 years that most corks by far do a great job of sealing the bottle and allowing the wine to age gracefully over five to twenty years or more depending on the vintage and style of the wine. We are reluctant to dismiss all that experience for the sake of 1 or 2 bottles in a hundred that may show corkiness. We think that there is a risk, however small, that alternative closures in red wines may not allow the wine to develop in the way we are used to and if that did eventuate every bottle would be in the same condition. Of course it is annoying if you strike a corky bottle at an inconvenient time but the bottle is happily replaced by the winery.
We have always used the highest quality corks which these days cost over a dollar for Awatea and Bullnose corks and two dollars for a Coleraine cork. The processing of corks has become more professional and scientifically based in recent years. This has lead to cork quality being higher than it has ever been along with an increase in price.
All cork batches that we use at Te Mata are tested in a laboratory in Portugal for corkiness and moisture content before being dispatched. The agent in New Zealand retests the cork and we always demand a sample so that we can test it in our own laboratory before giving final approval. We are satisfied that the corks that we are being supplied are better than they have ever been.
We cannot deny that we like the idea of sealing wine with corks. The concept of fermented grape juice packaged in a glass bottle made from sand and sealed with a natural cork is a nice one but be assured that we would not be using cork if we did not think it was the best closure for our three prestige red wines.
Technical corks is the name given to corks formed from cork particles or granules rather than being cut from the bark in one piece. These corks became quite popular about 15 years ago. They were cheaper than and sealed as well as natural corks but the corkiness issue was not addressed. More recently technical corks have a natural cork disc on their ends. This disc usually underwent a washing process to reduce any cork taint. We experimented with 2 manufacturers of technical cork quite a few years ago but found no improvement in performance over good natural corks.
Recently we have been very impressed with a technical cork called the DIAM. You will notice when you open a bottle of Elston or Cape Crest from the 2005 and more recent vintages that they are sealed with the DIAM cork.
DIAMS are made using a high tech procedure where any flavour and odour is extracted from the cork granules using super critical carbon dioxide a process used in the perfume industry. The cork particles are then bound together to form a very smooth sided cork closure each of which is absolutely consistent and guaranteed to be free of cork taint.
We are pleased to be using DIAMS with Elston and Cape Crest as they overcome the corkiness and random oxidation issues. You might well be asking at this stage why we do not use screw caps for these wines.
We have done trials with Elston and Cape Crest comparing DIAMS with Screw Caps and we have a preference for the wines sealed with DIAMS. The Australian Wine Research Institute has long running trials comparing corks with screwcaps and now with DIAMS. We find the results to date quite interesting. It seems that the screwcap and the DIAM seal the wine tighter than an average cork keeping the wine fresher after a few years but that the DIAM lets in a tiny amount of air which the screwcap does not. In most screwcap trials to date the tasting panel detects what they call a flint/rubber note after a year or so in bottle. This note seems to be related to the screw cap completely sealing the bottle. It is just a slight aroma but it gains in intensity over further years and the DIAM trial does not show this note. The consistency and freshness of fruit in screwcap wines far outweighs any slight flint rubber aroma in the first year or so but we recommend cellaring Elston and Cape Crest for up to 5years.
Screwcaps have been around for some time but only really took off this decade. The technology is simple and is designed to completely seal a wine bottle by very tightly holding a plastic wad down over the top of the bottle with an aluminium cap.
We have been very happy with the performance of screwcaps since we introduced them on the Woodthorpe wines in 2004. We all enjoy their convenience, there is no chance of corkiness and every bottle is identical. We are yet to be convinced that screwcaps are the ideal closure for long term cellaring particularly of red wines.
Things move rapidly these days and I am sure that there will be new developments in the world of wine bottle closures and that no doubt we will adopt some of them but for now we are offering the best closure for each wine style that we make consistent with our experience and knowledge to date.