Practical Winery
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January/February 1997

Ensuring Quality in Corks from Portugal
By Mike Miller
Ensuring cork quality begins in the forest where weeds that may host damaging insects are cleared.

Growers, manufacturers, processors, and suppliers of natural oak bark in Portugal are increasing efforts to produce corks that ensure a tight, lasting, untainted seal.

Providing high quallity corks, however, is no easy task. A natural product typically harvested every nine years from the Quercus silber oak trees of central Portugal, cork bark is subject to a host of complications. For suppliers in Portugal, the world's largest cork growing and processing region, the quality objective has led to a critique of existing programs and introduction of technological innovations.

Responding to the growing demand for quality cork, especially in the U.S. and Australia, suppliers have taken a greater role to ensure that all possible steps are taken to eradicate common quality problems such as taint, worm holes, and green wood.

Cork growers
In the forest, it is no longer enough simply to harvest on a nine-year Cycle. Portuguese cork growers are improving the forest environment to facilitate growth of quality cork bark. Removing grass around the base of the trees destroys natural habitats for destructive insects that burrow into the tree bark and form holes that can lead to leaking wine corks. Trees are pruned and thinned to promote healthy, upright growth that provides clean and straight bark.

Growers are also improving harvest practices. Some growers delay harvest for an additional one or two years, which allows for the development of thicker bark that yields a higher percentage of wood suitable for wine cork production. Increased wood percentage means a reduction in the level of critical defects.

Corks are punched from bark.

Sources of taint (2,4,6-Trichloroanisole or TCA or other taints) have been connected to the forests, prompting growers to rethink the age-old tradition of drying cork on the forest floor and to experiment with improved drying methods, such as storing cork bark planks on plastic pallets to avoid ground contact.

Preparadores Once harvested, nearly all cork is sold to preparadores, who sort the slabs of cork bark according to quality, reserving the top 15% to 20% for the wine cork industry and selling the remainder for a broad spectrum of industrial uses. In addition to sorting the cork, preparadores boil all raw cork bark to kill harmful molds and reduce tannin levels. The molds, however, can and do grow back, so processors must keep the wood dry throughout processing. This raw material is then sold to individual cork factories or to cooperatives that resell the cork to smaller "home" producers who punch cork for resale.

Preparadores have been identi fied as a possible source for contamination and taint. Pressure is being exerted on these middlemen to ensure that their operations are run cleanly - that boiling water is changed regularly and cork is dried and stored properly. Partly as a result of improvements by preparadores, taint rates have dropped from the 6% to 8% levels of a decade ago to present levels of approximately 1%.

Factories - the final process Though the efforts of growers and Preparadores are critical, most responsibility for quality lies with the cork manufacturer. Whether a large facility employing dozens of workers and producing millions of corks or a small independent, family-owned operation, the Portuguese cork factory holds responsibility for reboiling, punching, and washing the cork, and most important, for conducting countless cork quality sorts, checks, and examinations.

Because factories re-boil and store the cork, issues of taint arise once more. Here again an emphasis on clean water and storage conditions is being stressed to overcome taint problems that have plagued the industry in the past.

One focal point for quality issues related to taint is washing and bleaching of the corks. Traditionally, all corks were washed in baths of chlorine and oxalic acid. Studies by CTCOR (the Portuguese cork industry's research arm), however, have shown that the traditional method of controlling the chlorine concentration in these baths may have facilitated development of TCA. Most factories have installed computerized equipment to consistently maintain the ideal chlorine level and/or have introduced hydrogen peroxide washes. The hydrogen peroxide wash provides a deeper, more thorough cleaning than the lighter, more cosmetic wash provided by traditional chlorine washes.

Corks are washed in computer-monitored baths.

Despite these precautions, elimination of TCA remains an overriding concern for the cork industry. The widespread presence of this compound in air and water make it impossible to completely eradicate. Since chlorine is known to be a potential contributor to TCA, peroxide washes, while not a cure-all, are widely seen as a better alternative to traditional chlorine washes.

While cork is still primarily punched by hand, machinery is being introduced and playing an increasing role in routine dimension and quality checks. Ultimately, however, the factory's largest payroll is attached to the sorting tables where many women are employed. Their visual checks (often complemented or replaced by machines) sort out defects such as cracks, worm holes, green wood, bilrk fragments, and size problems. In most CilSCS, these women Me the lilst check on cork 'luillity before the product is sold ilnd shipped out of Portugal.

Industry structure influences quality
Appreciating the efforts to ensure cork quality requires some understanding of the cork industry. While the number of growers and preparadores is relatively small, the number of factories is immense. Concentrated in the corkproducing region surrounding the city of Oporto are more than 600 factories - ranging greatly in size. Most of these are small "mom-and-pop" businesses operating out of garages, punching cork; which is then resold to larger manufacturers. The majority of exported cork, however, is produced by about 20 larger factories.

Since most of these factories are located in a collection of towns close to each other, the industry tends to be close-knit. It is not uncommon for brothers and cousins to compete against each other for the same business, and competition is fierce. The very number of factories and the expense of holding inventory fuel an aggressive fight to sell the punched corks as quickly as possible, but at a price high enough to pay the preparadores' costs and still make a profit.

Finished cork from the factories is primarily marketed in three ways. First, most large factories sell their product directly through importers. This gives the factory control over quality and supply, but may also mean that their sales are driven by production. They generally have exclusive agreements with one importer in each country, or they distribute their own cork, taking on the responsibility to sell what the factory makes rather than offering what the market wants.

Corks before preliminary inspection.

Second, a network of agents is also active in the region. They buy directly from different factories on behalf of their clients, who are the importers in the U.S., Australia, Europe, and other markets. This system allows the agent's clients to tailor all purchases exclusively to their needs. However, in many cases, agents simultaneously represent both the interests of their client and the factory. In these instances, the agent is not only working to procure cork that meets his client's needs, but he may also be retained by a factory to sell its production. This can lead to a division of loyalties and conflicts of interest.

Finally, an importer may purchase cork from a number of factories and then sell that cork to a wide range of wineries. Since there are no alliances with a particular factory and no profit motivation to buy from any given manufacturer, in this system the supplier can avoid conflicts of interest and focus on quality and value. However, the independent nature of this system can make the task of procuring cork a greater challenge for the importer.

The industry is further complicated by thousands of miles that separate the U.S, wine market from Portugal. Given that distance, buyers can unknowingly purchase Portuguese cork that does not meet the quality demands of U.S. wine producers. Dealing with these complications, as well as a different language and a foreign business culture, is a challenge.

Shaking up the structure to ensure quality
One American cork supplier has overcome the complexities of this industry structure to ensure reliable, impartial quality examinations of all cork before it leaves Portugal.

Cork Supply USA (CSU), a Bencia, California-based provider of wine cork stoppers, has supplied cork to the U.S. wine industry since 1981. Raised in Portugal and apprenticed in the cork industry, owner Jochen Michalski experienced first-hand many of the complications and difficulties cork suppliers face in their efforts to meet American winemakers' demanding quality standards.

For years, Michalski relied on his personal association with cork factory owners to ensure a consistent, reliable source of quality cork. Despite his personal relationships and stringent quality control methods, he received an occasional poor cork shipment. Returning low quality shipments to Portugal was an expensive venture that not only jeopardized important supplier relationships, but also left CSU in a short supply situation. Accepting the corks, however, meant absorbing the cost of selling poor quality cork purchased at a premium price.

Sensory testing lab at Global Cork in Portugal (Cork Supply Group).

Faced with this conflict, CSU in 1991 created a quality assurance laboratory in Portugal: Global Cork. While several cork factories have quality control labs to examine their own production, Global Cork was established to operate entirely independent of production quotas and cost issues.

Given complete autonomy, Global Cork's exclusive charter is to ensure that every cork lot purchased by Cork Supply passes the company's quality standards. To ensure that level of independence, Global Cork holds absolute authority to refuse a lot if it fails any of the company's quality tests.

Global Cork provides a quality funnel through which cork shipments must pass before being exported. This places yet another layer of tests and examinations on Portuguese corks and offers the cork industry a model for insuring quality shipments.

To manage the fledgling experiment, Michalski hired Isabel Allegro, whose reputation as Seagram-Portugal's independent and uncompromising cork quality director had attracted his attention.

"When Global Cork started, I Wanted to be able to offer CSU more than the same visual tests that are common in the industry," explains Allegro. "Now, every lot that we sample is not only visually inspected, but also undergoes a sensory examination to detect cork taint or 'off' aromas." Today, the lab examines shipments representing more than 250 million corks annually.

Pass or Fail
Uninvolved in purchasing decisions, Globill Cork steps into action once CSU concludes negotiations to purchase cork from a supplier. When an order is placed, the factory is informed that all purchases are contingent upon the approval of Global Cork. To protect its impartiality, Global Cork receives a copy of the order with the pricing information removed . Upon preparing the order, the factory contacts Allegro's staff, who, within 24 hours, will collect a sample based on the U.S. military standardized sampling system.

At the factory, a Global Cork employee randomly pulls a representative sample from the prepared order, systematically selecting corks from the top, middle, and bottom of each sampled bale. That sample (twice the Military Standard) undergoes a series of sensory and visual tests. For example, the sensory test for a 51O,OOO-cork lot would require 39 individual corks from the 13 separate bales. One hundred corks from this sample are individually placed in "1OOml flasks with wine and allowed to soak for 24 hours. The wine is then examined for any taint or mold odors. If the sample lot does not pass this test, the entire lot being offered for sale is refused. No additional tests will be conducted.

"All testing stops if the cork sample does not pass our taint test," declares Allegro. "No amount of resorting can eliminate taint, and we will not take any chances. What the factory does with that cork is their business, but it won't end up with a Cork Supply customer."

While the sensory test is being conducted, a portion (100 corks from a lot of less than 200,000, 150 corks from a lot of 201,000 to 500,000, 200 corks from a lot of 501,000 to 700,000, and 350 corks from a lot greater than 700,000) of the sample undergoes a battery of visual quality tests. Every defect is scored, and the lot is compared against a control sample lot for the quality of cork being purchased. To speed up the tedious task, electronic measuring devices tied directly to computer evaluation forms are used to ensure an exact measure of dimensions, weight, density, ovality, and moisture levels. Ultimately, however, it is the trained human eye that determines if the sample meets Global Cork's quality expectations.

Most cork fails
"Based on our records, most of the cork we evaluate fails. This is usually due to physical defects including nonconforming moisture levels and quality not meeting our standards," Allegro reports. "The factory then must resort the product and submit it for a second or even third evaluation where all the same quality tests will again be conducted."

When a lot receives Global Cork's approval, the original sample is divided in half; one portion is held at Global Cork and the other is packaged and shipped along with the approved cork to CSU's quality control lab in Benicia. When the lot arrives in California, the sample is opened and used as a check against the actual shipment to ensure that the cork that passed Global's inspection is the same cork that has arrived in the U.S.

Global Cork's charter to ensure quality is even more difficult in short supply years, such as 1996. Limited supplies tighten the market, giving more power to factories, which enjoy a wealth of anxious alternative outlets. "In short years, you really have to rely on the relationships that you've built," says Michalski. "This is a long-term business and factories know and appreciate the importers that have an established track record. If you have those relationships, quality can still be had, but maintaining our standards means paying higher prices to manufacturers."

Fortunately, the short 1995 Portuguese crop that drove up prices and left suppliers desperately searching for cork that met quality standards has ended. The 1996 Portuguese harvest produced approximately 50% more excellent quality cork wood than the small 1995 crop. Price spikes continue to reflect inventory shortages, but Michalski expects those prices to eventually stabilize as the pipeline is filled with a sufficient supply of high quality cork wood.

Global Cork assists manufacturers by providing technical know-how to facilitate improvements which will benefit the manufacturer and ultimately the buyer. Through its involvement and recommendations, Global Cork has helped several factories improve their processing procedures, tighten controls, and make improvements that lead to a more hygienic processing facility and better quality cork.

Corks undergoing physical testing to ensure quality before export.

Quality in transit
"I get criticized for going overboard, but cork can suffer as much in shipment as it can in the washing or manufacturing process," Allegro explains as she inspects a shipping container. Inside the blackness of the 40-foot container, she and her staff search for holes, leaks, or off odors that could cause problems for the cork during the five-week trip to the U.S. Just like the cork, every shipping container, must pass Global Cork's inspection.

Cork Supply USA isn't the only company that appreciates the merits of this type of thorough, independent evaluation. Nearly 30% of all the cork tested by Global is now being contracted by other cork buyers around the world.

The real challenge to Global Cork's independence, however, is when it evaluates cork from Cork Supply USA's sister company, Cork Supply Portugal Despite being a member of the Cork Supply Group (which owns Cork Supply USA, Global Cork, and Cork Supply Australia), the Portuguese production arm of the company gets no breaks. About 25% of Cork Supply Portugal's output is purchased by Cork Supply USA, but every shipment undergoes the same exhaustive Global Cork tests as any other shipment purchased by Cork Supply USA.

"Quality is blind," states Allegro. "I know they don't always appreciate it, but ultimately I help Cork Supply Portugal by holding strictly to our quality standards. Ultimately it is the wineries that establish the standards we have to meet. Either the corks pass or they don't. Cork Supply Portugal has learned not to expect any favoritism from us."

The relationship between the two companies is softened, however, by a special service Global Cork provides exclusively to Cork Supply Portugal "We examine and test all the raw cork they buy. If we can catch TCA problems there, they have a real advantage over the competition."

In an industry where cork quality cannot be compromised, Global Cork offers an added level of protection to cork suppliers and wineries alike. Since Global Cork was founded, no lots received by CSU have had to be returned to Portugal. In the pre-approval process, wineries have declined lots, but this is generally because grades did not meet their specific needs.

"Now cork factories know what we are demanding," says Michalski. They know they can't slip anything past us. and they know that no cork is going to leave Portugal until it has Global's stamp of approval. Global Cork has made a tremendous difference, and I expect more cork companies will follow suit by contracting with Global or starting their own independent labs."


Freelance writer Michael Miller's report was edited and closely reviewed by PWV staff and industry experts.

Global Cork inspection procedures

All cork purchased by Cork Supply USA must first undergo and pass the following tests conducted by Global Cork in Portugal.

Sensory test: Samples are pulled from the lot using a detailed sampling plan. Individual corks are soaked in 100ml flasks of neutral white wine for 24 hours. After 24 hours, the wine is transferred into glasses and sniffed to determine acceptability of final sensory properties. The test is designed to reject lots with excessive levels of TCA or taint, and if a lot fails this sensory evaluation, the lot is permanently rejected.

Visual quality: The remaining corks are evaluated for wood quality and categorized as Extra, Superior, 1, 2, 3, or 4. In addition to the cork by cork evaluation, a random sample from the lot is compared to established reference samples to determine acceptability. Corks not meeting the standards for specific qualities are rejected and must be reselected by the processors. After reselection, samples are again pulled by Global Cork. There is no limit to the number of times a lot may be rejected.

Residual oxidants: To detect the presence of oxidants on the cork's surface, Global Cork agitates corks in a solution of potassium iodide and acetic acid. If residual oxidants are present, the solution will turn a distinct violet color. The test is a pass/fail for residual oxidants and lots with any traces are permanently rejected.

Moisture test: That corks meet moisture specifications is paramount for controlling microbial activity during transit, and lots with unacceptably high (above 8%) moisture levels are rejected for reprocessing until they meet acceptable levels (5.5% to 8.0% moisture). To measure moisture levels, Global Cork utilizes a DC-2011 moisture meter.

Physical characteristics: Length, diameter, and weight are evaluated for conformity to specifications. Diameter is measured at two points which are perpendicular to each other. The final diameter value is an average of those two readings. By performing diameter analysis in this fashion, ovality (roundness of the cork) information is available. There is a +/- 1mm tolerance for length. From the sampling of each lot, if more than 5% of corks are not within the acceptance range for diameter and/or if more than 10% are not within acceptance range for length, the lot is rejected.

Additionally, length, diameter, and weight values are used to determine cork density. The cork weight is divided by the volume after volume is determined using the calculation: πr2 x length.

When corks arrive in the US, each of these tests (with the exception of physical characteristics) is again performed by quality control lab technicians at Cork Supply USA. Additionally, after cork has been treated with a coating of paraffin and silicone, Cork Supply USA conducts several treatment evaluations including dust, capillary, carousel, extraction force, microbiological, and sensory tests.