Topic
Some yeasts are not very attenuative or alcohol resistant. Windsor ale yeast is just one example, but in the wild there are millions of others, and they all do their own thing. If you don’t pitch healthy yeasts, but then some wild yeast takes over that is not very attenuative or does not like alcohol, stalling could occur. Unlikely for all these problems to line up but it could happen. An infection to an otherwise reasonably healthy fermentation should not cause the healthy yeast to die; rather the two would work in parallel, and odds are that one of the two or more critters would keep on fermenting way down to normal dryness or beyond.
What about an infection that isn’t yeast? Like just something that occurred from improper cleaning/sanitation? Could this cause it to stall out?
Bacteria behave a little differently, but in general the same things apply that I mentioned above. They will not cause the fermentation to stall though.
Do you have a specific case that you can explain? What was the original gravity, what yeast did you use, did you mash, what temperatures and for how long???
Possibly pitched a killer wine strain perhaps?
I’m not aware of any kind of contamination that would cause fermentation to stall.
This page from the Yeast book says a stuck fermentation can be the result of bacterial contamination. I guess I’m just wondering what kind of bacteria?
Well, there’s probably a million kinds of bacteria, and they’ve all got to eat something, so I suppose it stands to reason that a few types of bacteria could murder yeast to use as their energy source. Just about anything’s possible where survival is at stake. Animals get sick, plants get sick… Can yeast get sick? catch a deadly virus? Yeah, maybe, I suppose.
So basically it’s possible, but unlikely.
I don’t know if I’d go that far…more like very unlikely to the point of almost impossible. I’ve never heard of anyone encountering it. Doesn’t mean it doesn’t happen, but you’d think one of us would have heard of it.
If acid-producing bacteria (or wild yeast, for that matter) out-competed your sacc and produced enough of a ph drop then conceivably the ph could drop low enough to stall sacc fermentation. But we’re talking around 3.0 or below. That would be highly improbable under normal conditions. It would require a large pitch of acid-producing bacteria and a fair amount of heat. A typical infection by wild bacteria would not get that done.
More likely cause of a stalled fermentation is failing to pitch enough healthy yeast. There are other possible causes but I would start by ruling out a pitching problem.