Making the most of cork

António Velez Marques, Helena Pereira

António Velez Marques, Helena Pereira, Rui Reis, Susana Silva, inventors of a cork-expansion method

A team of researchers at the Portuguese Amorim Group, the world's leading producer and distributor of cork products, has developed and patented an ingenious method to dramatically expand cork's volume without using any harmful foreign substances. The invention promises to not only help the centuries-old cork industry thrive in the new millennium but also supports environmentally friendly, sustainable production and ensures wine aficionados continue to enjoy the beloved cork stopper for years to come.  

The waiting game 

CorkThe satisfying pop that comes from uncorking a good bottle of merlot or chardonnay is music to the ears of any wine connoisseur. Fine wine takes time, and so does the cork used to seal its bottle. In fact, the cork used to make wine bottle stoppers comes from the bark of a cork oak tree, which is only able to produce a yield every nine to twelve years.

Producers in Portugal and Spain, the two countries that generate the lion's share of the world's cork, have long known the important ingredients that go into growing a perfect harvest: a warm, dry climate with regular rainfalls, and a healthy dose of patience.

Cork producers must first wait 25 years until a young oak has matured into a cork-producing tree, and then about a decade between harvests to allow the oak to regenerate its bark.

A versatile product

The wait is worthwhile. A cork tree lives about 200 years, and its unique bark is an important component in a variety of products. Cork, of course, floats on water; it is quite elastic, partially permeable to air - the property that makes it ideal as a wine stopper - but it also makes an excellent, non-toxic insulation material because it lets through little heat or cold.

Cork is found in products as diverse as shoe soles, cricket balls and - when treated with adhesives - floor panels. It can even orbit the planet in the form of spacecraft heat shields.

Undoubtedly, cork's best-known use lies in the roughly 16 billion cork bottle stoppers produced every year and sold to the wine industry worldwide. About 80% of all wine bottles are equipped with cork stoppers, and they offer several important advantages over their plastic and screw-cap rivals. Not only do cork stoppers allow oxygen to interact with wine for proper aging, their use supports an industry with a track record of environmentally responsible and sustainable production.

However, the rather long interval between harvests and the limited number of cork oak trees have put limits to the industry. Further pressure has come from plastic stoppers and screw caps, which are cheaper to produce and have gouged out a 20% chunk of cork's once unquestioned dominance of the wine market.

Pump up the volume

CorkAmid this ‘battle for the bottle', methods that can increase cork yields and further improve the bark's expansive properties has become increasingly important. Here, even little improvements can translate into big returns. 

Since the early 1990s, a team of Portuguese scientists has been working on solving this issue. The team is comprised of: António Velez Marques (professor at the Polytechnic Institute in Lisbon), Helena Pereira (professor at the Technical University of Lisbon), Rui Reis, (CEO of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine) and Susana Silva Estima Martins (head of the R&D department at Corticeira Amorim).

In 1993, this research team developed a patented method of expanding cork in an autoclave - a sterilising machine. Using this method, cork was exposed to high-pressure saturated steam to increase its volume. However, this procedure also involved the use of solvents such as methanol, ethanol, formic acid, dichloromethane or chloroform.

Traces of these toxic solvents would remain in the cork after the process, and the residual toxicity, along with the sheer logistical feat of using an autoclave, prevented this method from taking off in the industry. 

Just put it in the microwave

The team continued to search for easier expansion methods that didn't require toxic chemicals or leave undesirable residues. Through these efforts, they discovered that cork swells when exposed to microwave radiation. Microwaves had already been used to clean, sterilise or decontaminate cork, but its use to expand the material's volume had not been researched.

Through a series of tests, the team developed a new expansion process that uses only water and microwave radiation to increase cork's volume by 40 to 85%. The process not only increases corks elasticity and buoyancy - the very characteristics that make it a sought-after material - it also allows significantly more products, in this case wine stoppers, to be produced from the same yield.

A boon for wine drinkers

The method patented by the Portuguese team is suitable for nearly all types of cork: virgin cork taken directly from trees, cork that has been ground into a powder or cut into planks, and even the small cork shavings left over from plank and stopper production. This means that nearly every application for which cork is used benefits from greater yields and more competitive pricing.

Especially important for wine drinkers, the process reduces cork's already low density and makes sure the material has the perfect level of permeation (the wine's interaction with air) to age the finest bottles of vino.

From Portugal to worldwide locations

The innovative new expansion method was patented in 2011 by Corticeira Amorim. The company produces about 4 billion cork stoppers each year that are exported to more than 100 countries.

Amorim's 3,357 employees generated sales of €295 million in 2011. The company holds 25% share of the worldwide cork stopper market, 55% of the composite cork market, 65% of the cork floor market and 80% of the cork insulation business.


How it works

Cork has a cellular matrix similar to a honeycomb. The cells have flexible, curved walls that surround pockets filled with air. If these curved cell walls are straightened, the cork material increases in volume. For this straightening to occur, the pressure of the air inside the cells must be raised, and the cork's cell walls softened.

The cork-expansion method invented by the Portuguese team of scientists aims for the maximum net volume increase in the shortest amount of time while employing a minimum amount of energy. However, because cork is insulating, heating up the inside of the material in order to increase air pressure is no easy feat.

This problem is solved through the exposure of cork to microwave radiation, which causes heat to penetrate the cork quickly and thoroughly, the same way regular kitchen microwaves heat up a meal. Because microwaves need moisture to warm materials and cork contains very little water, it has to be moistened - either cooked in water or steamed - before radiation is applied. 

The microwaves rapidly heat the water in the cork, which in turn heats up the cell walls and increases the pressure inside the cells. At the same time, it pushes the water out of the cork. The combined result of adding water and then exposing the cork to microwave radiation causes the water to dissipate and the cork material to expand by up to 85%.

Cork production and the environment

The expansion method developed by the Portuguese team is a rare example of technical innovation contributing to the protection of an age-old traditional industry. The technology supports the survival of what is recognised by numerous wildlife and environmental groups as an important sustainable and eco-friendly resource.

The cork harvest employs tens of thousands of workers each summer, so the cultivation of cork forests is an important sector for the producer countries. And because cork oaks are not cut down during harvest, the forests and their ecosystems are preserved, contributing to a healthy environment, protecting valuable habitat for numerous animals and helping combat desertification.

Cork oaks consume three to five times more carbon dioxide than normal while they are regenerating bark, thus contributing to the reduction of the greenhouse gases that cause climate change.

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