Burnt taste

[HillsideMaple]

So a friend of mine has a hobby rig and boiled a batch this weekend. He poured off what he boiled to finish in a stock pot as I have done in the past. He left it for a little while and came back to it smelling burnt. It didn’t boil over but really stunk. I told him cool it down and check it with the hydrometer. He said it was under done still. I’ve never had this happen. I’ve overdone syrup and had that result. Anyone have an idea of what caused this. Underdone burnt syrup?

[MapleMark753]

The "just walked away from the pans for a minute" deal got me. OK, so I'm not an expert but near syrup that boils up (foam) can leave the pan bottom empty, and that pan bottom can then burn. Even a small burn spot or scorching can make an entire batch stink and taste horrible. If propane is on high the flames can shoot up the pan sides and the foam or bubbles can over caramelized and burn on the sides of the pan. That too can add to a burnt smell and taste.
Niter baked on the pan bottom can cause a small burn spot underneath it, and that can make a batch smell and taste bad too.
There's lots of possibilities, but all of the above have happened to me, mostly during the first couple of seasons when I started, but can happen to anybody at any time too.
What do the pans/pots look like?

[HillsideMaple]

The pan and pot look fine. I’m thinking like you said, excessive foam and too hot on the burner. When I had a smaller operation I always tried to get it as close as i could to syrup, then finish it in small batches over propane. Worked pretty good for me. Thanks for the input.