i have no idea whether this is relevant, but i once read while wizzing around the net that the meat industry uses (or used to) a mix of peroxide and vinegar to kill off the surface layer of bacteria on aging cuts, to keep it pink and to mask the bitter taste this bacterial overgrowth causes.
since then i've noticed that if i have been eating starches and don't swish out the food particles between my teeth or brush before sleeping i sometimes wake up with a bitter taste on my tongue. i've assumed since reading that, that the bitter taste is is from an overgrowth of oral bacteria and since there's a lot of die off going on throughout the digestive system and mouth while fasting it may have something to do with it too.