I was hoping to premiere DCY01 to interested parties at the January DC Homebrewers’ meeting. Since it’s this Wednesday, and I haven’t started culturing it yet, that’s not going to happen.
Every once in a while I realize that this is much more difficult than it looks, and I have the results to prove it. It starts to feel like I’m in way over my head as a total amateur. My past two efforts at growing the cultures have resulted in sour starters. I’m not quite sure at what point the contamination is setting in so it’s difficult to know what to change at this point. The plates and slants are clean, as far as I can tell, but they seem turn sour in the second step up. It turns cloudy with a less cloudy layer on top of the liquid. So far that looks like a telltale sign of infection.
One possibility that comes to mind is the fact that my building’s heat is quite intense right now and the radiator in the kitchen leaks a bit. The air is humid and probably teeming with bacteria. It doesn’t take much to contaminate a tiny sample of wort with an initially tiny population of yeast. I’m not sure how much the flow from the single alcohol burner’s flame helps avoid things from falling in. It’s worth trying to do it in another space.
There are other possibilities I’m less likely to be able to work around. I remember reading somewhere (a forum post) that yeast colonies on plates are often contaminated with bacteria. I haven’t been able to find anything more on this in my limited research. It seems a bit surprising due to the idea that an isolated colony on a plate is supposed to be descended from a single cell, and on my plates there are no visible bacteria colonies. But this would support the claim I’ve seen in some places that plating is not effective at isolating yeast from bacteria without some sort of antibiotic media (out of my reach as an amateur, as far as I can tell). It doesn’t help that I don’t have access to a microscope to test this out or help identify the source of my problem.
In spite of all of this, I am working on what will hopefully be DCY02. One of the two step-up cultures I just talked about is this one, so I had to restart the process. Hopefully by next week, when I will be off work, it’ll be ready to go so I can brew a one-gallon batch with it.
Perhaps I should attempt more straightforward yeast ranching using known-good cultures. I have a slant of WLP001 I cultured out of the remnants of a tube. I should try stepping that up to a starter since it is more likely to be clean than my wild cultures.
Update: I have now seen this with cultures that were not infected with bacteria–at least not noticeably–so it is probably not a telltale sign of infection after all. In the most recent case it is just yeast that refuses to settle, even after refrigeration. It’s worth noting, though, that according to this document Pediococcus form a clear layer on top when cultured in liquid.