apushatyid: Of course taste is the main ingredient how to grade food. What else do you think they had two thousand years ago? Electronic microscopes? Come on, we should have a modicum of sense here. “Nossen taam” appears in a multitude of halochos so ,yes, taste is the standard that we use. The prohibition of certain foods is based exclusively on how and whether they taste in a certain way. The question of “rou’i leachilas kelev” is the lowest common denominator that we find in halocho concerning the edibility of food. Soap is not a food nor is it edible to humans and dogs. You may find individual dogs (or people for that matter)who relish it. It does not make it “rou’i leachilas kelev”.
yehudaona: Emulsifier may have been my mistake. It is “saponification” that I meant. (kudos to google for the explanation) And, indeed, if the actual subject is changed chemically, maybe it would become edible and would change its halachic definition. For now, soap is still not a food and tastes terrible.