I compare it to boiling dirty socks, not because I ever got it, but that was a description in something I read.
My suggestion is when you start to suspect it, run a test boil before you add that sap to the rest to boil. If the test reveals that smell, toss it. Then finish off what you have that is still in need of finishing. If you just add every day, once you get the bad smell you have ruined the good sap still in the pan.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.