Winemakers the world over
ask, "Should I base my cork
purchasing decision on
appearance and functionality
or the budget? Do the two go
hand in hand?" The fact is pricing and
cork appearance do go together. To a
large extent, cork pricing has been based
on the visual appearance of corks. Thus,
the use of visual analysis to determine
both cost and quality is the generally
Because the surface appearance of a
cork continues to be the primary factor in
price, visual grading of corks is a critical
issue for both wineries and suppliers.
Should visual analysis be the sole criteria
for cork pricing? This is one of the issues
that will be addressed below.
Cork grading began with a system that
included six grades: from Primeira
(First) to Sexta (Sixth). But as wineries
demanded higher qualities and more
careful selection, the grading system
grew to include "Superior," then
"Extra," and finally, "Flor y Nata"
(Flowers and Cream!). As a way of standardizing
these grades, various cork
companies in Portugal, in concert with
their marketing departments, developed
a system of visual grading for each
grade. Due to the fact that there were no
industry standards, there was substan~
tial variability within each grade.
At the same time, cork producers discovered
that bleaching corks disinfected
them and helped eliminate or mask discoloration,
thereby increasing its apparent
visual quality at very little cost. Most
winemakers approved of bleached corks
and were willing to pay a premium for
them. They quickly became the standard,
and experimentation continues on
In this system, confusion often
reigned, and winemakers were frustrated.
There seemed to be virtually no
quantitative analysis for the visual grading
of corks, and the cork industry itself
seemed disinclined to change the situation.
Because the visual asp.ect of the
cork is one of the key elements upon
which pricing is based, a more complete
method was needed to help winemakers
determine the value of cork lots submitted
Several years ago, the Cork Quality
Council in the U.S. developed a series of
visual grades that were described in
terms of qualitative analysis. This analysis
provided a method to visually identify
the physical characteristics of, and to
place various types of corks into separate
categories within a given sample. This
was an important step which helped to
form the foundation for some of the
analysis presented here.
During development of the procedure
for visual analysis the following goals
were perceived to be the most important:
- Visual grading should play a key
role in establishing the anticipated
levels of defects in a cork lot.
- Visual grading can be used to monitor
consistency of quality over time.
- Visual grading can be used to compare
lots of corks from different suppliers
to determine their relative value and
- Finally, visual grading may be used
to generate specifications for minimum
levels of quality categories and maximum
defect levels within a cork lot.
Cork grading is designed to detect the
level of defects present in the cork.
Defects may be separated into two categories,
critical and non-critical. Both
types are related to functionality in that
they may affect sealability. Critical
defects, though, cause a loss of revenue
due to leakage and seepage.
Critical defects are those that run 75%
to 100% of the length of the cork, including
insect holes, channels, cracks, and
Non-critical defects are mainly cosmetic
in nature and alter the perception
of quality. However, they can interfere
with the effectiveness of bottling equipment
and add dust and cork particles to
the wine. This type of defect is used as
the basis for the quality grading of
a specific lot of cork, and includes
porosity, chipping, dry years, deep holes,
belly, bark, hardwood, end notching,
and short cracks.
Non-critical defects may be classified
as "minor" or "profound." Minor defects
may be defined as those that run up to
25% the length of the cork. In some
instances, a small crack originating on
the end of the cork may cause slight
seepage along the length of the crack.
Generally, the seepage will go no further.
Profound defects may affect the performance
of the cork to a greater extent.
For example, a very large chip in the end
of the cork may allow wine to enter the
area between the cork and the bottle,
with subsequent heavy seepage.
Profound defects may run from 25% to
75% of the cork length.
Defects arise from two different
sources: those that occur naturally in the
cork wood itself, and those that have
been caused by faulty manufacturing
Suppliers in the U.S. that regularly
monitor defects are working to reduce
their incidence. Education of the cork
producer as to how the market perceives
specific defects is key. Therefore, winemakers
may want to consider investigating
the relationship that their supplier
has with the overseas producer.
Natural defects include those caused
by growing conditions - such as cracks,
dry years, porosity, hardwood, and
greenwood - and those caused by parasites,
such as insect larvae or molds
growing in the forest.
Cracks appear as dark brown or
black fissures in the wood, and can run
either vertically or horizontally through
the finished cork. They are usually
caused by uneven expansion of the cork
bark as it grows. Depending upon the
length, location, and depth of the crack,
they can prevent a cork from forming a
Dry Years are similar to cracks, but
appear as a narrow dark line down the
length of the cork. This defect is caused
by low moisture during the growing season
which prevents the new cork bark
from growing evenly into the existing
layer. The result is a narrow band of
hard, dry wood that is inconsistent with
the rest of the cork. Creasing, and thus
seepage, may occur along this dark band
which has a higher resistance to compression
and lack of resilience. In addition,
very large dry years may crack off
as they go through the corking jaws.
Porosity is easily defined as the
number and size of holes in any given
cork. In many ways, this is the parameter
upon which visual grading is based.
Inexpensive cork may appear to have a
network of larger holes and cork bark.
However, even the very finest corks will
have some minor porosity. Porosity as a
critical defect is only seen in corks where
the wood structure of an entire cork face
is affected with a crumbly texture or the
porosity is focused along a growth-line.
Hardwood is seen as small sections
of bark that do not
have the same
resiliency as the
rest of the cork.
from factors that
influence the formation
lenticels of the
bark itself. Because
it is not resilient,
hardwood can prevent
the cork from
an unusual defect
that prevents the
cork wood from
seasoning completely. After harvest,
most cork wood is aged in the forest for
six months or more before processing,
then boiled and aged again for a short
period. This process produces seasoned
cork wood - a material that is unique
for its . many physical properties.
Greenwood prevents the proper seasoning
of the wood. After ageing and boiling,
greenwood does not achieve dimensional
stability It will appear as a ruffled
or bulbous area on the cork, while the
remainder of the corkwood surrounding
the area appears smooth and consistent.
Insects have traditionally been the
bane of cork producers. Small (1-2 mm)
holes are created by insects that burrow
through the cork bark leaving either
open or dust-filled tunnels behind them.
Insects such as Coroebus Undatus
("Bicho" or worm), Cremastogasterscutellaris
Olivo ("Formiga" or ant), and
Dermestes vulpinus ("Traca" or butterfly)
create open tubes that - when present
from end to end of the cork - cause
leakage, oxidation, or both.
Yellow stain is apparent in the raw
cork wood and easily identified by its
pale yellow color. It is caused by growth
of the mold Armillaria mellea on the wood
in the forest (personal communication
with Dr. Mario Borges, Juvenal Ferreira
da Silva, Lda., Santa Maria de Lamas,
Portugal). It is readily detected at the
cork processing facility in Europe and
eliminated at that point. However, it is
important to note any discoloration due
to this defect as it lends a moldy flavor
and aroma to the wood.
Manufactured defects are those caused
by human hand during processing of the
cork wood. While some have their origins
in the material itself (belly and bark),
most are caused by an incorrect application
of the production process.
Belly is that part of the cork that is
closest to the tree. Occasionally, when the
cork stopper is punched out from the cork
slabs, the puncher does not avoid this
belly layer, and the exterior of the cork
shows the ruffled texture of the belly of
the tree. Because this belly is both rough
in texture and less resilient than the rest of
the cork wood, this can create sealing
problems. However, the more severe
belly defects are sorted out in Portugal.
The defects seen in the U.S. tend to be
limited to small, non-critical spots.
Bark is the outside of the tree, the
part that has developed a dark brown or
black appearance due to interaction with
the air. The defect is a cutting error similar
to belly, but the error has been made
on the opposite side of the cork slab. The
results are equally poor.
Channels are purely manufacturing
errors. The corks have been punched too
close together, and one cork displays the
cylindrical notch that was created by the
previous cork. When very shallow and
short, these channels may not affect performance.
When deep, they can create
Chipping can be caused either by
inconsistencies in the wood itself or by
poor manufacturing technique. The
result in either case is the same: large
chunks or chips of the cork are missing. It
is interesting to note that the Portuguese
have two separate terms for this noncritical
defect. "Falha Corpo" is a chip in
the body of the cork, while "Falha Tapa"
occurs on the end of the cork.
It is important to note that the results
of any analysis for these defects in cork
are affected by both the branding and
coating. Branding the cork can introduce
minute ink spots that may affect the
visual appearance. Coating the cork can
make the surface look more porous. The
combination of the two processes tends
to darken the cork. For this reason, suppliers
often provide coated cork samples,
which alleviates mistaking the appearance
of a finished cork for discrepancies
in the quality or identity of the lot itself.
Finally, while it is not a defect of a single
cork per se, it is important to inspect
corks for excess dust or particulate matter
on their surface. This dust can affect
both the seal of the cork and its visual
appearance (poor printing) and should
be noted on any qualitative analysis.
Cork is a particularly forgiving material.
Small numbers of the above flaws
will have virtually no impact on performance
of the cork in the bottle. A large
defect that runs across one end of a long
two-inch cork will not have an appreciable
impact on. the seal, while one which
runs the length of the cork may leak
immediately. Distinguishing between
critical and non-critical flaws is a function
each winery approaches differently
due to their experiences with bottles and
bottling equipment, wine types, and
In addition, insertion of the cork into a
standard bottle decreases the effect a
defect may have upon the cork. The cork
is compressed from 24mm down to 18 or
19mm, so non-critical defects are less
threatening to the integrity of the wine
A good seal depends on several elements:
the cork, the coating, the bottle,
and bottling conditions. The coated cork
works in combination with the interior
neck of the bottle. While bottle necks do
not suffer from the defects listed above,
they may not conform to the same rigid
criteria for dimensional consistency
either. Significant variations in bottle
neck diameters can introduce defects in
the seal between cork and bottle.
For instance, seepage may occur when
the cork length extends beyond the flare
point of the bottle and does not form a
continuous seal. Squared off entry bores
may also affect the cork, causing slight
chipping to occur during compression
and insertion into the bottle. For this reason,
it is recommended by the authors
that the winemaker set up bottling trials
with a supplier when considering purchase
of new glass.
Visual grading analysis
Traditionally, corks have been graded
by the number and size of visible flaws
on the surface of the cork. Historically,
this began with wineries and cork companies
inspecting the corks for visible
structural flaws, such as worm holes and
long cracks. These flaws would have a
direct negative impact on the
performance of the cork in a bottle.
It was then applied to corks which had
no structural flaws, but did have visible
surface anomalies. Of course, corks with
perfect exterior appearance are rare and
command much higher prices. Such
corks also project an image of quality
and integrity for the bottle of wine itself
which is an important marketing aspect
of corks that cannot be overlooked.
It is the defects, or lack of them, which
allow one to determine the overall consistency
in a lot of corks. A specific lot of
corks may have a continuous spectrum
of qualities, but with a simple grading
method in place, the cork buyer may
determine the general quality profile of
the lot and identify possible problems.
Some California cork suppliers use a
grading scale detailed in terms of five
grades: "A" type corks have very little
porosity and no defects whatsoever, "B"
corks have slightly more porosity and
very minor defects, "C" corks have moderate
porosity and larger defects, "D"
corks are extremely ugly but functional,
and "E" corks have critical defects. Noncritical
defects are usually placed into C
and D categories, depending on whether
the defect is minor or profound. The
authors recommend the "Three strikes
and you're out" rule: If a "B" or "C" cork
is found to have three minor defects, the
grade is downgraded to a "C" or "D."
In the more expensive grades, there
will be very few C through E corks. In
lots of lower quality corks, the number
of Cs and Ds will be greater. Those
wineries that grade a variety of qualities
will notice that the defects in a "c" cork
in an "Extra" grade will be minimal
compared to a "C" cork in a "Third"
quality. Differentiating between the two
comes with the winery's familiarity with
the product, and in working with their
supplier to define the categories.
Within a lot, there will be a certain
number of each cork category. Once the
numbers are generated, a computergraphing
program such as Excel can be
used to depict the qualities of the lots
received over time (see fig. 1)
and between the actual shipment and
the reference sample.
Because cork is a natural, agricultural
product, the quality will vary slightly
from lot to lot. Quantifying the categories
within the lot gives the winery the
advantage of determining when this
variation becomes a cause for concern.
The data base can be used to develop
specifications for the product. By creating
specifications, the winery will be able
to reduce variation and purchase corks
that fit their needs.
In order to achieve this important goal,
sampling procedures must be statistically
sound, random, and representative
of the entire lot. Several sampling procedures
are available, such as the
ANSI/ASQC (mil-spec 105) and the
Sequential Probability Ratio Test. In
addition, coming from the University of
California at Davis, the Fraction
Defective Sampling Plan should soon be
With whichever procedure you use, no
fewer than 100 corks should ever be
graded. Due to the range of quality
within the cork lot (even within a bale of
10,000) it is difficult to determine the percentage
of defects and overall quality
profile when analyzing very small samples.
By grading a larger sample, the
winery reduces its risk of accepting cork
lots with critical defects which exceed
acceptable quality limits.
A minimum of 100 corks should also
be used when working with a supplier to
set up a reference sample. As seen in fig.
I, there may be fluctuation in quality levels
over time. With a reference sample
that has been agreed upon by both the
winery and the supplier, qualities are
easier to match, thus ensuring consistent
shipments throughout the year.
Visual grading facilitates the tracking of
quality levels between suppliers as well.
Because of confusion caused by the many
different trade names and qualities used
by suppliers, frequently cost is the only
facet that can clearly define what the
buyer is comparing. Using a program as
presented above should help to make
the decision-making process easier.
Winemakers can improve the functionality
and appearance of corks and reduce
the overall costs of corks through an
effective cork quality assurance program.
The purchasing decision based on visual
appearance (and functionality) can tie
into the budget. However, performance
of the cork is not strictly related to these
criteria: One must consider the sensory
attributes of the cork, the coating, bottles,
and bottling equipment and conditions.
In order to develop an effective cork
quality assurance program, the goals of
the program must be clearly defined.
The goals will vary depending on the
type of wine, quantity of corks purchased,
budget requirements, type of
bottling equipment, storage methods,
and winemaker preferences. By using
the criteria provided above, the winemaker
will be able to determine what is
required, communicate those requirements
to suppliers, evaluate samples
and shipments, improve consistency,
and reduce costs.
At JuvenaL Direct, Inc., Michelle Bowen is
director of Quality Assurance, Paul Wagner is
president, and Pern) Teaff is general manager.
Authors can be contacted at Juvenal Direct, 120
Dodd Ct, American Canyon, CA, tel: 707/254-2000
; fax: 707/642-2288.