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SPECIAL REPORT: Cork closures

Removing a cork from a bottle of malt whisky only takes a moment, though its journey to the bottle is a long one, and the age of the tree that supplied the cork generally exceeds that of the malt whisky it seals.

The cork tree is indigenous to the Mediterranean, with the greatest concentration of cork tree forests in southern Portugal.

Hugo Mesquita, sales and marketing director, Top Series Division, Amorim says: “The roots of the tree go deep into the ground to gain water, and the leaves are a very good sunshade. This is exceptionally important, enabling the trees to cope with very hot summers."

Apart from some pruning, cork trees don’t require much maintenance.

But they do require patience, as a Portuguese saying reveals: “if you want to make a business for your grandchildren, plant cork trees.” This is because it usually takes around 25 years before the bark from a cork tree can be harvested for the first time. However, this first harvest, known as virgin cork, is too rigid to use for cork stoppers and instead provides flooring or sound insulation.

It takes the tree nine years to ‘regenerate’ another layer of bark which can be harvested, with this second harvest also used as flooring or sound insulation.

Only cork from the third harvest (another nine years later) has the degree of flexibility required to produce stoppers. This means waiting an average of 43 years after planting the trees. At least the average life-cycle of a tree is 220-250 years, enabling around 15-17 harvests that can be used to make stoppers, which is the most lucrative use of cork.

The harvest takes place between May-August when it’s easier to peel off the bark, as the level of sap present between the cork bark and the underlying surface of the tree trunk is higher during this period. However, it’s a very delicate, highly skilled and entirely manual operation (attempts to develop an automated process have not been successful).

Mesquita adds: “Harvesters use a particular type of axe to carefully peel off the bark without damaging the tree, otherwise successive harvests will be compromised or even prevented. Only about half of the tree’s bark is peeled off during each harvest, which may cause temporary stress to the tree but also makes the bark grow back more vigorously.”

Following the harvest, planks of bark are carefully stacked in the open in order to air dry the sap within the bark, which takes six to nine months.

The cork planks are then boiled in water which increases the thickness of the cork, while also removing any foreign objects (eg. dust, insects) embedded in the rugged surface. Natural spring water is used, not tap water, because tap water contains chlorine which reacts with naturally occurring micro-organisms (present for example in the soil) to promote a mould containing TCA (technically 2, 4, 6-Trichloroanisole). This mould emits a pungent, musty odour referred to as ‘corked’. Even a very low concentration of this odour (measured in parts per trillion) can be so dominant that it could suppress the aromas of a malt whisky. Moreover, the human nose is particularly sensitive to this type of odour, rendering the contents of the bottle undrinkable.

A subsequent safeguard is steam cleaning the cork to remove any residual micro-organisms with TCA potential, as above 60 degrees centigrade they become volatile (ie. vaporise) and the vapours are ‘hoovered’ up by extraction units. Additionally, batches of corks are analysed during the production process, to check for signs of TCA and micro-organisms with TCA potential.

Finally, cork planks are cut into strips from which individual cork stoppers are punched out, either manually or mechanically. A head (with the logo or branding of the malt whisky) is then attached to the cork.

Corks arrive at bottling halls in Scotland within sealed, air-tight bags to maintain hygiene. Corks can be applied to bottles as part of an automated process, within an ‘enclosed’ bottling line. Alternatively, a manual process requires precautions such as handling the corks in gloved hands, with corks spending the minimum amount of time outside an air-tight bag.

Cell structure
Cork possesses a natural flexibility as it comprises millions of minute cells that each contain air, which is actually trapped within each cell. This means that after a cork is compressed into a bottle neck (the cork being slightly larger than the opening), the air within each cell attempts to expand and regain its original dimensions, forming an air-tight seal.

Beyond these technicalities, there’s also the question of what customers expect.

Antony Wills, managing director, Kilchoman, says: “With a prestigious product like single malt consumers have come to expect cork stoppers. Popping a cork and being able to replace it is an essential part of the experience, and we certainly wouldn’t want to disappoint anyone. With all the safeguards in place when cork stoppers are produced, and also on the bottling line, I’m not concerned about the possibility of TCA and the malt whisky becoming corked.”

14 January 2017 - Ian Wisniewski The Drinks Report, editorial assistant