cross posted from another forum:
I just tested a cider which I am fermenting where I was having trouble placing the bad flavor. I was first thinking that it was too tart for the gravity measurement, then a week later on another SG measurement/taste I was thinking it was vinegery, but that wasn't quite it. Since we had that off-flavor tech meeting recently, my memory was jogged yesterday night, and I believe that the flavor is ethyl butyrate ( http://en.wikipedia.org/wiki/Ethyl_butyrate ) in the form of Butanoic Acid ( http://en.wikipedia.org/wiki/Butyric_acid ), which smells and tastes moderately like vomit.
I might see if sweetening masks it, but this might be the first batch of anything that I trash -- I p[resume there is no reasonable way to remove / precipitate this chemical out?
Not sure where the infection was introduced. I took a yeast cake in the carboy from a cider (from juice) which turned out quite well, and immediately after racking to a keg, I added more apple juice/nutrient/O2 to the carboy, sanitizing all juice container lids with StarSan before opening, and boiling the nutrient before addition.
edit: Anyway, it got dumped long ago. First time I've had to dump something.