there would not have been a universal answer to this until around the 1600s. medieval europe was decentralized, and technology was scattered and varied from place to place. read more
The strains of yeast used for brewing and baking are the same species, and have become separate in modern times, but before the 1800s, barm (from the top of a vat of actively brewing beer) was the most common source of yeast for baking. read more