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I noticed several small pink colonies on the foam of a second stage starter that I made from recovered yeast (originally Gigayeast: GY 054) that was mixed with 25% glycerin and frozen. They were not fuzzy as might be expected for a mold or mycelial species. I've done some research on the Web and PubMed in particular and have found two genera of wild yeast that might be responsible: Rhodotorula and Sporobolomyces. I didn't find any representations that these were feared contaminants in beer and decided to pitch the starter but also decided to attempt making a second batch with similar ingredients using fresh GY 054 to make another 1L starter. I'll keg them both in a few weeks and report back, but in the meantime I'm wondering if anyone else has experienced the same issue?

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I suspect that the source of the infection was glycerin I used, since I perhaps erroneously assumed it should be sterile from a previously unopened container. I found citations that Sporobolomyces in particular had been isolated from "technical glycerin". Will now run my glycerin supply through the pressure cooker "autoclave". (My wife, trained as a microbiologist, is suspicious that the glycerin will all boil over during autoclaving, but I found with a simple search that glycerin boils far above the temperatures you get with a pressure cooker.)

Further note: Also saw an article that colored mutants of Saccharomyces cerevisiae (the species we are usually using for beer making) can occur when they lose certain enzyme capacities, so this may be more common than I realize.

And final note: Was unable to taste any skunky or "off" odors or tastes in the final product. No differences between the recovered yeast batch and the new commercial batch. And no one got sick or died.

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