Last summer, two “strippers” from Portugal walked into the parking lot at Napa Valley College, wielding axes seemingly forged in medieval times. They chopped deftly at the trunks of cork oak trees, expertly peeling bark from the trees and removing it in large sections.

While Napa County is home to some of the oaks, known as quercus suber, used for the cork stoppers that top wine bottles, Portugal is the point of origin for most of the millions of corks stuffed into bottles of Napa Valley wine each year.

According to a 2005 survey by wine industry analysts MKF Research, corks, capsules and screwtops represent an $83 million industry within the county, generating $1.9 million in wages.

At its facility near the Napa County Airport, cork supplier Portocork America tests and cuts pieces of cork, branding them with winery names before bagging them, sealing them and shipping them to wineries.

The Napa-based arm of Amorim Group, the largest manufacturer of natural corks worldwide, is the largest supplier of cork stoppers in North America.

More than 250 million corks are finished each year at the Napa plant, part of a century-old company that employs some 4,500 people around the world, including 30 or so here.

Unique as a fingerprint

Dustin Mowe, president of Portocork America, marvels at the shady, thick-trunked cork trees, many of which are hundreds of years old. Vintners and viticulturists talk about the patience needed to work with grapevines, but more patience — much more — is required for cork.

“The way the trees grow is just amazing,” said Mowe. “Once you plant a tree, you can’t harvest it for 40 to 50 years.”

To top it off, “That first harvest is unusable. You strip it but you can’t make a cork for another eight to 12 years.”

As for the cork stoppers themselves, Mowe said each one is unique, “like a fingerprint.”

Mowe joined Portocork six years ago. He recently guided the business through a $1 million, one-year overhaul of the south Napa facility.

Besides all-natural cork, Portocork also creates what’s called composite or particle cork stoppers. Their “twin top” product is a combination of cork punched freshly from strips of bark, and shredded and reformed cork bits, which are placed on each end of the stopper.

Mowe estimated that 30 percent of product Portocork sells is natural cork and 70 percent is composite, particle or “twin top.”

Battling TCA

The nose knows at Portocork’s laboratory. That, and a pink fairy wand left on a lab shelf.

While the wand is just a whimsical testing talisman, Portocork clients come to the plant to test the corks for themselves. Their job: look for TCA contamination.

TCA (2,4,6-Trichloroanisole) is a mold that can cause wine to become “corked” or undrinkable.

Portocork lab workers immerse corks in small vials of table wine. Then the wine is drained away from the stoppers, and representatives of Portocork’s winery clients come for a sniff test.

About six to eight customers a week come to Portocork for sensory testing of the corks that are destined to top their bottles, he said.

The company also puts corks through a rigorous lab test for TCA, sending cork samples to a lab in St. Helena.

Between a million and 2 million stoppers are finished a day in Napa, said Mowe. Bales contain 5,000 corks, and 50 of them are pulled from every bale for TCA testing, he said.

While sensory testing is confined to one of the smallest rooms at the plant, Mowe said it’s a critical part of the process. Computers and lab testing can reveal contamination, he said, but “sensory is still integral.”

Once the stoppers pass the smell test, Portocork then uses fire to brand the cork with the name of a winery, or prints on the side of the cork with edible inks.

From there, stoppers are sent to a humidifier room to add moisture and make the stoppers more pliable. A giant clothes dryer-like machine tumbles the corks to remove dust. Then a machine blows a silicon/paraffin coating onto the stoppers to smooth the surface.

After a final visual inspection, the corks are then bagged, sterilized and delivered to clients.

Rejected corks are sent back to Portugal, said Mowe, where they are recycled into flooring or insulation, or are put to other uses.

“Our customers are very demanding,” said Mowe. “In one year we sent back more than 6 million corks.”

Screwing things up?

While Napa Valley is home to hundreds of wineries, much of the industrial activity in the industry takes places elsewhere, according to Terry Hall of the Napa Valley Vintners, a trade association of some 300 wineries. Warehouses are located in areas including Benicia and Vallejo, for example, and other wine-related facilities are in the Central Valley, around cities like Lodi. The reason is simple: Land is generally cheaper and better suited to industrial purposes in those locales.

Portocork has mulled relocation, but many of its 600 client wineries are here in the North Bay.

“We’ve considered moving out of the area,” Mowe said. “But we’ve got to be where our customers are. We’re not going anywhere.”

Working for a cork company, Mowe hears the inevitable questions about other types of wine bottle closures. He acknowledged that screw caps — which are becoming increasingly popular enclosures despite the fact that they don’t give that satisfying pop when they open — are the biggest threat to his side of the industry.

Screw caps, along with synthetic corks, are perceived as being as or more effective at preserving wine while posing no risk of TCA.

“Screw caps are here to stay,” he said, admitting “it’s a good closure.”

Price now drives many producers. According to Mowe, “the cheapest natural cork closure is around six cents, while you can find synthetic for four cents.”

Mowe said the new synthetics have their limitations, as well. “Virtually no premium wineries are using synthetic because of performance issues such as rapid oxidation and extraction,” he said. “However, the five largest wine companies in the U.S. represent 75 percent of the synthetic cork sold in this market.”

Many of those wines, including some from Gallo and Constellation Brands, retail for $6.99 a bottle or less, he said.

Before the rise of synthetic closures, most producers used natural cork. Today, Mowe estimates that of the 1.8 billion stoppers used annually, 920 million are natural cork closures, 750 million synthetic closures and 130 million “other” closures, such as screwcaps.

“Our market has shrunk,” he acknowledged, “but we’re still king.”

Old-fashioned craftsmanship is a powerful force in the wine industry, and corks are a part of it.

“This is a business of tradition,” said Mowe. “Customers want cork.

“Whenever I’m at a restaurant, and there’s a bottle of wine opened, I’m the first to grab for the cork,” said Mowe. “It’s more than just a stopper. The amount of work that goes into that stopper is just tremendous.”