 After the boil , the beer was cooled and fermented in open wooden vats at 14 – 16 ° C ( 57 – 61 ° F ) . The two strains of yeast were added at the same time , and fermentation would proceed rapidly . After one day of fermentation , the thick foam that formed on the top of the fermentation vat was skimmed off and discarded . After 60 hours , up to 50 % of the available sugars would have been fermented by the yeast , and thick clumps of yeast were skimmed off of the surface and harvested for reuse . The beer was then pumped with sterile air into clarification tanks . Isinglass was added to clarify the beverage , and then it was bottled . 
