FIGURE 1: Perpendicular section of the
Quercus suber tree showing wood, cambium
zone, and corkwood. Natural corks
are punched from the corkwood in a direction
parallel to the axis of the tree.
Natural Cork
Closures made from cork material
come from the bark of Quercus suber,
a native oak tree that grows in the
Mediterranean region. Most natural
cork closures are manufactured in
Iberia, primarily in Portugal.
The bark is harvested every nine
years from mature trees between
the months of June and August.
Corkwood, once aged, conditioned,
and cleaned, is sliced in sections proportional
to the length of the closure
and punched parallel to the axis of the
trunk of the tree (Figure 1). Then, the
corks are rectified (sanded), peroxidewashed
(to whiten them), sorted by
visual grades,4 and bagged in quantities
of usually 10,000 corks (a bale). At
this point, the corks are exported to
wine-producing regions throughout
the world where they are then printed,
moisturized, and treated (usually with
paraffin and silicone).
The spent corkwood, a byproduct of
the natural cork punching, is not wasted
but used to manufacture “technical”
corks. The material is ground and the
cork granules are sorted by particle size.
Through various cleaning techniques,
the particles are washed to reduce or
eliminate any traces of potential taint
and/or other unusual aromas. The cork
granules are finally glued together to
make micro-agglos, 1+1s (dual disk),
and champagne cork closures.
Synthetic Closures
Synthetic closures first appeared in
the early 1990s and were probably the
first wine 
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sealing alternative that significantly
competed with natural cork.
They are mostly made out of a combination
of polyethylene and other
trade-secret adjuvants. There are two
types of synthetics with two distinct
manufacturing technologies: injectionmolded
and co-extruded polymer.
Some of the first attempts to create
a synthetic closure yielded products
that were difficult to extract from the
bottle, lacked the resilience of natural
cork and therefore created a few
problems during and after bottling.
Other issues included excessive oxygen
transmission rates, resulting in
premature wine oxidation. However,
by the late 1990s, most of these problems
were being resolved and today
synthetics are a popular choice for
wine products consumed within a
short timespan after bottling.
Screw Caps
The idea for sealing wine with a film
barrier under an aluminum cap (that
emulated a classic bottle capsule) was
first developed during the late 1950s
and early 1960s. Screw caps with various
types of liners had been used on
dessert wines and low-cost jug wines
during the following years, and were
sporadically applied to higher quality
wines with mixed results.
Early attempts met much consumer
resistance because they had a long
association with cheap finished products.
Also, the first screw cap liners
created some problems during wine
ageing, because the barrier requirements
were not totally understood
and perfected. Even though the screw
cap still is an evolving closure, by the
turn of the 21st century, it became the
wine closure of the new millennium
and was gaining wide acceptance in
Australia, New Zealand, and Great
Britain.
Today, there are basically two types
of screw cap liners made with Saran™,
(polyvinylidene chloride [PVDC]), an
extremely heavy fiber with a remarkable
barrier against water, oxygen, and
foreign aromas. The Saran/Tin liner is
a lamination of Saran™ combined with
an impervious thin layer of tin metal.
The Saranex™ liner has multilayered
films of Saran™, co-extruded and integrally
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sandwiched between outer layers
of other polymers. The Saran/Tin
liner provides a greater barrier against
oxygen than the Saranex liner.
Considerations in choosing
a closure type
Personal, financial, and technical
reasons lead winemakers to choose
one type of closure over another. Some
winemakers are traditionalists and
appreciate the qualities of natural cork,
even if they have to accept a low rate
of tainted bottles. Others will require
total product consistency and would
probably only look at man-made closures
such as synthetics or screw caps.
Environmental concerns also play a
big role in the decision-making process.
Closure cost and sub-type classification
differences, particularly for
natural corks, are as wide as differences
in the prices of wine offered by
the market.
Certain technical considerations
must be accounted for before, during,
and after bottling. Wine style and
preparation, expertise during closure
application, equipment availability
and/or investment, closure robustness,
and the logistics of product storage,
transportation, and commercialization
must all be kept in mind in
selecting a closure type.
Comparisons
A systematic approach for looking at
closure selection includes five factors.
· Closure Taint
This might not be the most important
reason for closure selection, but it
certainly has been the original impetus
for why, today, there are multiple
choices of wine closures. TCA, probably
the most significant cork taint, is
a biochemical transformation product
that starts in the bark of the cork tree. 5
The process has been summarized in
Figure 2. This pathway of formation
was ironically encouraged by the use
of chlorine as a bark disinfectant and
cork bleaching agent.
However, this practice was discontinued
by the mid-1990s, and
increased quality control gates and
improved processing procedures were
developed. Today, taint levels have
come down to an average rate of 1%
to 2%, depending on what releasable
TCA (RTCA) level is considered unacceptable.
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