Brett The Viking was fermented and aged in oak barrels with lactobacillus, brettanomyces, and kveik yeast. Lactobacillus creates lactic acid, which makes the beer sour. Brettanomyces (brett) is a wild yeast that creates a distinct sweet-tart flavor with a hint of tart cherries. Kveik (pronounce kuh-vek) is an ancient Norwegian yeast strain that ferments very clean with notes of citrus fruits. Brett The Viking is truly a one-of-a kind sour beer.