Expert: Ed Westemeier - 7/14/2006

Question
Okay, so larger is bottom fermenting and ale is top fermenting. Is there a diffrence in the yeast itself or is it just the tempeture of fermenting that makes the diffrence?

Answer
Yes, you're on exactly the right track.
Originally, all beers were ales, and they all used similar yeast strains.
In the 19th century, it was discovered that one or two of those yeast strains had acquired a mutation that made them much more comfortable doing their work at a colder temperature. There were a few other changes as well, but the one that stood out was that they also worked particularly well with the newly popular light colored beers -- even  better than with the darker traditional ales. They also gave a cleaner, crisper, less fruity flavor and aroma to the beer.

These new lager strains were carefully cultured, and today they make most of the beer in the world.

Incidentally, all yeast does the job throughout the fermenting tank. Those other terms just came about because at one point in the process, ale yeasts tend to clump and float at the top, while lager yeasts tend to settle to the bottom.