My SWMBO, who is a PhD student of Biology at Northwestern in a yeast lab, told me how they keep their yeast from mutating too much. She says that the first trick is to re-plate the yeast often (their lab does it every day), because when the yeast deplete most of the material they normally feed on, they will start suppressing certain genes in order to keep going. Second, she says that when they propagate yeast from a plate, they choose a single well-formed colony. Those that look funky or are "clumped" are not propagated. She also said that I could probably keep some yeast frozen with glycerin in the freezer (there are tutorials around here for that) and grab some of the frozen yeast from time to time (without thawing) and put it on an agar plate to check for viability and to select a good-looking colony to propagate into a starter. I think this would keep mutation to a minimum. Anyway, just passing on the info.