There are nearly 300 of these venerable vessels, with a total capacity of 10 million liters. Their maintenance requires the employment of two full-time coopers. Strict adherence to such traditional methods is one reason why Rodenbach beers are so expensive to produce. Yeast is pitched into open copper vessels, but the wort remains there only during the lag phase. Once fermentation begins, the active wort is transferred to closed rectangular tanks, primarily to facilitate easy removal of CO2 from the cramped fermentation rooms.
Primary fermentation lasts for seven days.
After primary is completed, the brewery has large amounts of yeast slurry to dispose of. The latest analysis from the University of Leuven identifies 22 different strains of yeasts and bacteria. They have been brewing with the same yeast from the beginning, without reculturing. Thus Rodenbach's yeast and resultant beers are constantly evolving. Up to primary fermentation, two worts receive identical treatment, but after primary they are handled differently.