Following the questions that I asked Pax in the last show, I ran a couple experimental batches. I've done plenty with spent grains before, but hadn't tried doing it with beer until then, mostly because I wasn't sure how the sitting warm and exposed for 18 hours would treat the beer. I did a side by side half batches with straight plain bread and the same bread with Great Lakes Edmund Fitzgerald Porter in place of the water. Didn't warm it, de-gas it or de-alcoholize it, just poured it straight in. The rise after 18 hours was about half that of the one done with water. I doubt that the temperature would matter too much as I'm using relatively cool water too, and over that long of a rise, I don't think the initial difference would account for that amount of variation. CO2 from the beer likely isn't a factor, since it makes bread rise, and most of it likely was lost when I poured it in and mixed it (lots of foaming). That potentially leaves me with the alcohol content stunting yeast growth, which is what Pax said may be an issue in response to my question.
I read someplace that most commercial baker's yeast ferment in the 5.0-5.5 range, and they don't do well at PH<5.0. Any chance the beer drops the PH that low?