Corks - Past,
Present, and Future
Cork is a spongy material which comes from the
bark of the cork tree and has traditionally been used to seal
wine bottles. The best cork is said to come from Portugal.
Currently, there is a movement to turn to man-made materials to
replace cork due to the shortage of cork trees and problems with
tainted corks. This month we will begin a series of articles
looking at the history of cork, why it is used and why its
future may be limited. The following is based on material from
Simi Winery, Healdsburg, California.
A Brief History of Corks in Bottles
Corks have been used as bottle stoppers for as
long as we have had wine. The Greeks in the 5th century BCE
sometimes used corks to close wine jugs. Following in their
footsteps, the Romans also used the cork as a stopper and also
coated corks with pitch to seal the closure.
Corks, however, were not the closure of choice
in those ancient days: the most common closures for wine jugs and
amphora were a coating of pitch or gypsum over the opening of a
vessel or a film of olive oil floating on the surface of the
wine. The use of corks was apparently completely given up in the
medieval times. Paintings from that era depict twists of cloth or
leather used to stop the jug or bottle, sometimes with sealing
wax to make a secure closure.
We find corks beginning to be mentioned again
at the end of the 16th century. By the time Shakespeare wrote
As You Like It (Between 1598 and 1600) they were well
enough known for Rosalind to say impatiently to her cousin Celia:
I pray thee take thy cork out of thy mouth, that I may
drink thy tidings. The marriage of cork and bottle, at
least in England, took place by degrees over the first half of
the 17th Century. The alternative closure of the time, stoppers
of ground glass made individually to fit the bottle neck, held
their own for a remarkably long time. Worlidges
Treatise of Cider, published in 1676, declares that
great care is needed in choosing good corks, much liquor
being absolutely spoiled through the only defect of the cork.
Therefore are glass stoppels to be preferred... Each
stoppel had to be ground to fit a particular bottle, using emery
powder and oil. The stoppel was then tied to the bottle by a
piece of packthread around a button on top. Glass stoppers were
finally abandoned around 1825, primarily because they were often
impossible to extract without breaking the bottle.
Corks success as a closure depends upon
its fitting snugly into an opening with a relatively uniform
diameter. Thus, it was not until the 17th century, when glass
bottles were first made with more or less uniform openings, that
the cork truly came into its own.
Many wine historians have linked the
development of the glass bottle and its cork stopper as two
necessary prerequisites for the modern international wine trade.
Wine no longer need be shipped in bulky, awkward clay vessels or
wooden barrels. The economies of space enabled ships to carry
more wine and the wine was much less subject to spoilage in the
Because cork stoppers prevented oxygen from
spoiling the wine, both in shipment and in subsequent storage, it
became evident that wine benefited from its maturing time in the
bottle. The desirable properties of aged wine made it more
valuable, and collecting and cellaring wines from many different
regions became both feasible and profitable.
One thing remained to be invented before the
cork closure became truly practical: a corkscrew so that the cork
could be driven all the way in, not left half-out like a stopper.
The first printed reference to a bottlescrew was in
1681 by one N. Grew: a steel worm used for the drawing out
of corks out of bottles. They had been used for at least 50
years to draw bullets and wadding from fire-arms. The term
corkscrew was not coined until 1720.
Why Do We Use Cork Today?
In the almost three hundred years intervening
between the renewed usage of cork and the end of the 20th
century, cork has been the overwhelming closure of preference for
fine wines around the world. The very sound of a corks
firm, round plop from the mouth of a bottle sets
taste buds salivating in anticipation of a delicious treat. There
are several very sound reasons for corks continuing
popularity: corks component materials and structure give it
a unique set of physical and mechanical properties that make it
ideal as a bottle closure. They are:
- Lightness: cork is very light in weight
and low in density.
- Impermeability: cork is very resistant to
moisture penetration. (Its used in life jackets!)
- Compressibility: cork can be compressed to
half its dimension with no loss of its flexibility. And
it can be compressed in diameter without expanding its
- Flexibility: when removed from
compression, cork will recover about 85% of its initial
volume immediately and more than 98% after 24 hours.
- Adherence: the slicing of the surface
cells in forming a cork stopper produces an extraordinary
cupping effect. Millions of cells are opened and function
as suction cups. This provides an exceptional power of
adhesion to wet, smooth surfaces.
- Temperature and age stability: cork
retains its properties at both high and low extremes of
temperature and usually lasts 20 years without
- Cork is the only material known that
compensates for small imperfections in glass.
- Cork is biodegradable.
How Does Cork Seal The Bottle?
Corks natural vegetable tissue is
composed of closed air cells arranged with polyhedric geometry.
This allows pliability without leakage. The cells are banded
together in a perfectly regular manner at a rate of about 40
million per cubic centimeter. An average wine cork therefore
contains almost 800 million cells, which act as suction cups to
prevent seepage of wine from the inside of the bottle. Suberin, a
complex fatty acid substance, gives cork its basic composition.
It is the tissue that makes cork unique and gives it its
particular elastic characteristics.
The Source of Cork and Its Manufacture
Quercus Suber (thus, suberin) is the botanical
name for a slow growing, evergreen oak that grows well and
prolifically throughout specific parts of the Western
Mediterranean (mostly in Portugal), and only in these regions. It
requires a substantial sunlight and a highly unusual com-bination
of low rainfall and somewhat high humidity. (Experiments in
growing cork trees in North and South America, Russia and Japan
have so far proved disappointing.) The quality and thickness of
the bark will vary according to its specific growing conditions.
The tree has evolved the spongy substance of cork as protection
and insulation, particularly against fire.
Most trees will die if their bark is removed,
because the bark helps to carry the sap that is essential to the
life of the tree. The cork oak, however, has two layers of bark.
The inner layer is alive. It is the base on which a new inner
layer grows each year. As the old layers move outward and die,
they serve as insulation, protecting the tree from the hot arid
winds in the growing areas. The dead outer layer can be stripped
away without injuring the tree, but care must be taken not to
penetrate the inner living bark.
If you are planning to grow cork trees for a
living, be prepared to wait to least 25 years until the first
harvest of cork is mature. Cork from the first harvest, however,
is irregular in size and density, and not suitable for wine
stoppers. It will probably be used for floor tiles or sound
insulating materials. Nine more years must pass before the tree
can be harvested again. Even this second harvest of cork is not
good enough for bottle stoppers. It is not until the third
harvest, when the tree is 52 years old, that the regularity of
size and density of cells renders it acceptable for wine bottle
usage. A cork tree will yield 13 to 18 useful harvests in its
The cork is stripped by hand with the aid of
small sharp axes, and the resulting cork strips are then stacked
and weathered. The tree itself is carefully marked and numbered,
so that future harvesters will know that particular tree needs
nine years before it is harvested again.
The weathered stacks of cork are then inspected
by the manufacturers to determine if they will buy and at what
price. They truck the cork north from the cork forests to
Portugals cork factories, and stack the cork for an
additional 3 months or more to let it weather and dry. The
weathering process is designed to arrive at the optimal amount of
moisture in the cork. The proper moisture content is crucial for
the elasticity and compressibility of the cork.
After all this drying, the cork is immersed in
boiling water for at least 90 minutes to sterilize it and to
enable it to flatten from its original curved tree trunk shape.
The corks then ripen for 3 to 4 weeks to achieve the
desired moisture level. Next the cork is trimmed into strips and
holes are punched into it for the correct size and shape for the
bottle cork. The width of the bark strip forms the diameter of
the cork, not the length of the cork. Thus, growth rings of the
tree are to be found embedded longitudinally in the cork. This
stage of the manufacturing process requires a keen eye as the
hole punchers maneuver the strips for maximum quality. This is
one of many quality control steps. Next the cork heads are
polished so that the cork will have a specific, uniform length
and the body is polished for a specific, uniform diameter.
The corks are then washed and dried. Most are
bleached in either chlorine or hydrogen peroxide in order to
disinfect the cork, and some are just rinsed without bleach,
depending on the specific winerys request. Corks are graded
for quality and then each cork of comparable quality is branded
with the name of the winery that has ordered that particular
A final surface treatment, either silicone
and/or paraffin or a resin, is sprayed or tumbled onto the
surface of the cork. This treatment eases insertion of the cork
into the bottle and improves the seal against the glass. The
corks are bagged in plastic bags and shipped to their final
destination, the winery that ordered them.
There's Just One Little Problem.....
With all the beneficial properties of corks as
bottle closures, one significant defect rears its ugly head. This
is corkiness, a condition that exists when wine is
tainted by the presence of a chemical compound called
2,4,6-Trichloroanisole - TCA for short. This compound appears to
be caused in the cork by the interaction of moisture, chlorine
and mold. Corks are often exposed to these elements during their
production and TCA can form. (TCA also occurs naturally in the
wood and bark of many trees, including the oak family.)
Unfortunately, the human nose can detect this
corkiness at concentrations as low as 4 parts per
A lightly corked wine may simply smell like
cork, while a badly corked wine smells musty, like damp cardboard
or old newspapers. The usual rich aroma and flavor of the wine is
stripped away by the musty odor. A wine may be more or less
corked; that is, there may be just a trace of corkiness that will
be detected only if the person smelling the wine knows from prior
experience what aromas and flavors a good bottle should have. But
a bad case of corkiness is overwhelming and unforgettable! It
poses absolutely no health problems - it just doesn't smell or
taste good. Ultimately, the problem is that you have spent good
money for a bottle of bad wine, and you have absolutely no way of
knowing this until you have opened it and poured it.
Should You Worry?
Several years ago, the industry became aware of
what seemed to be a growing quality control problem with corks.
Portugal, which produces about 52% of the worlds cork
supply, had undergone a political revolution in 1974 that changed
the management of the cork tree forests and of cork factories. In
some cases, unskilled or inexperienced people were managing
groves of cork trees. Quality control standards were very poor by
todays standards. Almost 2.5 million acres of cork forests
had to be cleaned up, newly planted and reorganized.
Major investment got underway in 1978, and
today well-managed forests, stringent quality control in growing,
harvesting and processing are combating the problems of
2,4,6-TCA. Peroxide baths instead of chlorine bleaching (as well
as natural corks with no chemical bath at all) and
controlled humidity in shipment are also improving the quality of
Because world-wide concern with the perceived
degradation of cork quality had a tremendous impact on the cork
industry, a group of U.S. cork suppliers founded the Cork Quality
Council (CQC) in 1992. Its mission was to improve the quality of
corks at the source, to develop an educational program to assist
wineries and to develop industry standards for cork quality. The
CQC has been a major influence in persuading cork producers that
it was in their own best interests to monitor cork production in
a stringent fashion.
What About Plastic Stoppers and Screwtops?
In 1990, Harvey Steiman of The Wine Spectator
estimated that 4.7% of the wines submitted to that magazine were
contaminated by corkiness. Craig Goldwyn of The Beverage Testing
Institute (1992), estimated that up to 8% of wines are tainted by
the time they reach retail stores or restaurants. And Lyn Farmer
of The Wine News (1994) estimated that 3% of the wines submitted
for review were corked. Personally, I find these estimates quite
excessive. In my experience, a 1 - 1.5% corkiness rate seems
accurate. But even this is unacceptably high.
Bill Stephens, a respected wine writer for The
San Antonio Express-News, is vocally opposed to corks, saying
they should go back to the dark ages where they
belong. The revered tradition of cork as bottle stopper for
wine will die hard and slowly, but anyone who has an opinion or
preference appears to hold it strongly. At least it makes for
Not all the opposition to corks as wine bottle
stoppers is based on quality issues alone. Many people believe
that cork usage is an elitist holdover that renders opening a
bottle a difficult and esoteric process. And some consumers are
intimidated by the cork-sniffing, bottle-opening ritual in
restaurants. (You neednt be! When the waiter presents the
cork, just place it on the table and ask to taste the wine. The
cork reveals nothing about the wines quality.)
What about alternative closures? The two front
runners are a synthetic, plastic substance called Cellukork, and
the screw cap. Cellukork is made of ethylene vinyl acetate. It
looks and feels similar to real cork. A corkscrew is used to
remove it from the bottle. It has two drawbacks: one is that it
often fits so tightly in the bottle that it is very difficult to
remove (a problem that will no doubt be resolved through
research); the other problem is the question of whether the
synthetic material is truly non-reactive and inert over long
periods of time? Will it impart no flavors of its own to the
wine? Naturally, wineries using these plastic corks are
deliberately aging wines to see what happens, but it is too soon
by several years to know the outcome. None-the-less, more and
more low and mid range producers are switching to Cellukork.
And the screw cap? It provides an excellent
air-tight seal, although there is a question as to whether or not
it will protect the wine over a very long period of time. Aside
from the quality issue, the image of a screw cap is firmly lodged
in many minds as the epitome of cheap wine. Many fine
wine producers, sensitive to the fine wine market, hesitate to
switch to a screw cap because they do not want their wine to be
perceived as of inferior quality.
The best part for the consumer is that screw
caps are very user friendly - no more struggles if you forget to
bring the corkscrew to the picnic! Many wine marketers believe
that the down-home nature of screw caps can only benefit the
industry by making wine more accessible - figuratively as well as
literally - as we erase the elitist image associated with the
ritual of opening wine. However, no fine wine producers are
currently marketing their wines with screw caps.
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