WAG: I think it’s the foamed-up residue of whatever goes into the toothpaste (aluminum or calcium carobonate depending on what brand you use). Might also be the saccharin that they also put in there. I’ve noticed with my toothpaste (not that I recall what brand it is) that I don’t have that problem, but do get some fizziness in the ol’ yakker when I’m done that takes a couple mouthfuls to get rid of.