They actually discuss this at length in the Twitter 'thread'. Basically, they're pulling yeast spores from within the pottery, not from surface. Then they try t

They actually discuss this at length in the Twitter 'thread'. Basically, they're pulling yeast spores from within the pottery, not from surface. Then they try to keep everything as sterile as possible after that (kitchen autoclaves, sterile handling with gloves, UV lights, etc.)