Foam?

[frjeff]

What exactly is the white foam I am skimming off the top of my boil (in evaporator pan)?

[happy thoughts]

I think it's sugar and debris including niter. If you taste it it can be sweet depending at where you are in the boiling process. If you collect it, the foam will eventually break down and turn into a small amount of muddy looking liquid. At least that's been my experience.