Historically, artisanal breads were produced by capturing wild yeasts from the air. Prior to the development of modern yeast strains, the production of artisanal breads was long and laborious because many batches of dough ended up being discarded. Can you explain this fact?

The baker's yeast.

Explanation:

Prior to development of modern yeast strains, artisanal breads were produced by capturing wild yeasts from the air. However this process used to be very laborious and wasteful because most of the time the dough was spoiled and needed to be discarded. The dough got spoiled because, while capturing wild yeast several toxic spores were also trapped which made the dough toxic.

This problem was later solved when the particular strains of Baker's yeast was identified and separated.