“We learned the hard way,” says Jeff Stuffings, founder of the farmhouse-inspired Jester King Brewery in Austin. When filling a foeder for primary fermentation, be sure to leave room for the kräusen—or else expect a mess to ensue.
On the other hand, if you’re adding fruit to a foeder for secondary fermentation, fill it completely to reduce oxygen exposure. At Jester King, they use a stainless-steel paddle to occasionally push the fruit down into the beer—a winemaking technique called a “punch down”—but only while there is still enough refermentation activity to create carbon dioxide, which pushes out any ambient air. Jester King's foeders came from a winery, so they were made for handling fruit.