At the heart of the wild fruit cave is a coolship termed a koelschip in Flemish used to cool fresh wort, gradually, in open air. It is a large, shallow, square, stainless steel tub that has no cover. The coolship sits in its own room with windows on either side; these are left open to slowly cool the wort and allow wild yeasts from the outdoors to find their way in. Such cooling of the wort in this manner usually happens overnight, and during cooler months of the year. The ceiling above the coolship is a combination of concrete and exposed wooden beams and slats that increases the room’s humidity and collects moisture, making for a ready home to microorganisms essential to this type of brewing.

via New Glarus Brewing debuts the first sour beers from its wild fruit cave — Isthmus | The Daily Page — Madison, WI.

New Glarus Brewing debuts the first sour beers from its wild fruit cave