Hey, it's Only a Piece of Bark!

Cork’s first recorded use as a stopper was by the Egyptians thousands of years ago

I see there is an upcoming trend favoring the return of the natural cork closure; what is the deal with all of the different closures utilized in today’s wine bottles? Of all of the winemaking decisions that the winemaker has to make, probably the most important decision is which type of closure to use. What are their differences? What are their advantages? It can be quite confusing. There are several different types of wine closures and all of them, as you may guess, have some good points and bad points. The following are the basic types of closures:

Natural Cork is harvested from the Portuguese oak cork tree. The properties of cork that make it suitable for closures in bottles has been known for centuries even though it is only recent science that finds that cork has 35 million cells per cubic centimeter which makes it light, elastic and impermeable. The trees must be at least 25 years old before starting to harvest and the bark from each tree is stripped once every nine years. This ensures that the tree has plenty of time to regenerate between strippings. Once stripped, the bark cork planks are laid out to dry. After drying, the cork planks are boiled. This might sound strange to boil the planks after drying but the dry-boil-dry process removes impurities that can cause cork taint and also kills bugs and standardizes the final moisture content. This also allows the cork maximum compressibility as well as breathability and provides a leak-proof seal for wine bottles. The corks are graded visually for imperfections and then cylinders are punched out to be smoothed and sized for wine bottles. Cork companies have re-engineered this process over the past several years in direct response to the T.C.A. (Trichloroanisol) cork taint crisis. It is the bottom few inches of the tree where the bark touches the earth which contains the most pre-cursors of T.C.A. In the past this would have been processed but now a team of workers carefully slices each piece of bark to remove it. There are several laboratory tests on the cork as it makes it way through the manufacturing process of which none of these were applied just a few years ago. The key test is gas chromatography. This can detect contaminants like T.C.A. at an extremely low concentration of one part in two trillion. On the subject of oxygen transmission, there is documented research in conjunction with the University of Bordeaux. It suggests that bottles sealed with cork do transmit oxygen to wine but there is no incoming external oxygen. 90% of cork’s structure is air so cork itself is responsible for the transmission of the oxygen to the wine.

There was a high profile wine faults seminar a few years back sponsored by a major cork manufacturer. They invited 91 well known wine writers and members of the trade and gave them wine spiked with T.C.A. and asked what fault, if any, it demonstrated. Only 2 people identified it correctly.

Technical Cork is made from a lower grade cork than the natural version and is combined with the leftover bits and pieces from trimming the natural corks. To make a technical cork, many cork granules are pressed together with a food safe binding agent. Then they are either injected into stopper molds or are extruded like pasta. In some technical corks, thin cork discs are glued to the bottom and top of each stopper.

Synthetic Cork is manufactured to look like natural cork, at least in size and shape if not in texture. Synthetic corks are made from plastic that is either injected into molds or extruded. Unlike natural or technical corks, synthetic cork eliminates the possibility of cork taint. While synthetic closures allow incoming oxygen they fail to maintain their seal over time and may react with the acids present in the wine. They can also be very difficult to remove from bottles (not to mention resealing) and they are not biodegradable like natural cork so they have negative environmental impact.

Cork stoppers arrived in Portugal around 1700. Some 70 years later they were used in cylindrical bottles In Oporto allowing the wine to mature slowly in a glass receptacle for the first time.

Screw Caps, also known as Stelvin Closures, seal very tightly and are easy to reseal. Sealing tightly is a two-edged sword. On one hand they don’t leak and, on the other hand, there is no chance for the slow oxidation that can occur with natural cork. As a result, Stelvin closures are basically impermeable and run a risk of reduction* and are not generally recommended for wines meant to age longer than a few years. Screw caps also suffer from a perception problem because wine with this type of closure does not exude romance or class. Another additional issue is, that if wines are shipped over long distances, this closure can hide the truth regarding heat damage. These closures are fine for wines which will be consumed early but are not conducive to aging and, because of these issues, several wineries are returning to natural cork.

The Zork closure is the brainchild of an Australian with the same name who offers closures for still and sparkling wines. The Zork has an outer strip of plastic wound a couple of times around the neck of the bottle. You peel this part off just like opening a container of milk. The cap itself is lined with a foil barrier that does allow for slow oxygen transfer which makes the Zork a viable closure for wines meant to be aged. There is also a plastic stopper portion that extends into the neck of the bottle, like a cork, which even makes a popping sound.

The Vino-Seal is an elegant glass stopper that has a plastic, non-reactive O-ring which provides an airtight seal for the wine. They are pretty cool although not a lot of wineries are using them because of the expense.

So which one do you think is best? You know that I am going to tell you what I think! I like natural cork because it is natural, biodegradable and it really is the best all around closure with no real negative environmental effect. It also has the tradition and romance of wine history in its favor and, with proper tree management, there is no danger of reducing the forest or having any significant impact on the animal life. Technical cork would be the next best because it is made from cork and those screwy Stelvin closures are fine for most white and red wines that will not benefit from long term aging.

* Reduction refers to a winemaking process that minimizes exposure to oxygen. Caps are so restrictive to allowing any oxidation (excessive reduction) that the environment promotes the creation of volatile sulpur compounds which emit unpleasant aromas.