New Scientist reports on the parallel evolution of Budweiser and Heineken:
Forced to produce their beer in the winter, brewers accidentally created conditions favouring the emergence of a hybrid yeast better suited to the cold. Researchers already knew that Saccharomyces pastorianus, now used to brew lager, is a hybrid produced through marriage between two yeast strains.
One was S. cerevisiae, the "brewer's yeast" on which the brewing industry is founded because it ferments sugars into alcohol so efficiently. The other was S. bayanus, a yeast strain seldom used alone in brewing because it ferments sugar into alcohol far less efficiently.
Now an analysis of the forensic ancestry of lager yeast has established that this same marriage happened independently at least twice, not once as previously thought, giving rise to two broad families of lager beer.
So, now you know. Oh, and this should be especially interesting if you've been following the Heineken storyline on Mad Men...
