As a brewer you want to have a consistent product. Once you'd got a brew with just the right amount of "horse blanket" you'd cultivate that yeast culture and us

As a brewer you want to have a consistent product. Once you'd got a brew with just the right amount of "horse blanket" you'd cultivate that yeast culture and use it again. To do that you need to make sure there are no other yeast strains in the raw ingredients. Louis Pasteur was working on beer brewing when he discovered the microbes, and invented the heating process to kill them.