Czech brewers use secret recipe to prepare barrels for world's oldest Pilsner

As autumn arrives in Western Bohemia, a Czech region, a group of craftspeople are carrying on a tradition largely unchanged for over 180 years.

In a process that involves flaming, scraping, rolling and a lot of hammering, the coopers of Pilsner Urquell brewery in Pilsen clean out and re-tarr empty barrels, readying them to take on a fresh batch of the city's famous beer–Pilsner.

"They must be retarred after every use, after every time beer is tapped. This creates a protective coating—a glaze—on the inside of the barrel," says Head Cooper Karel Hofman. "The tar forms a protective layer so that the beer does not come into contact with the wood," he adds. 

The process starts with removing the old pitch—made from a mix of tree resins and oils. The barrel is heated with a flame until the thick tar coating inside melts and can be poured out. 

Tarring has to be done in autumn and spring, explains Hofman, as extreme winter and summer temperatures can damage the oak barrels.

With the old tar out, a fresh layer needs to be poured into the barrel. Coopers then vigorously hammer and roll the giant barrels to make sure the inside is evenly coated.

"We learned this from our predecessors, who in turn learned it from their masters," says Hofman. "We're continuing this tradition."

In the brewery cellar, Chief Brewer Emeritus Vaclav Berka walks around by candlelight, just as his predecessors did when the brewery opened 184 years ago, brewing the world's first Pilsner. 

Berka explains how the tarring process upstairs is crucial to preserving the original taste of the beer brewing down in the cellar. 

"The resin inside the oak barrel is meant to isolate the beer from the wood," says Berka, "since wood is porous and breathes, it could impart a certain flavour to the beer."

The tar, Berka says, is made up of a mix of oils and tree resins from around the world.

"It's almost as secret a recipe as the beer itself," says Berka. 

The Pilsner Urquell brewery was opened by Bavarian brewer Josef Groll in 1842 and is credited with creating the world's first pale lager. 

Camera: Eva Korinkov. Editing, production and writing: Thomas Holdstock/Reuters