One of the sources in the Gemarah (Pesachim 44-45) for the din of טעם כעיקר is from the parsha of kashering kelim from the absorption of issue flavor after the battle in Midyan. It is a machlokes Rishonim if the bottle line pesak is that טעם כעיקר is a Bibical prohibition or only a Rabbinic prohibition. It is is Rabbinic why does the Torah say to kasher the vessels belonging to the Midyanim? Simply understood it was a temporary גזה"כ and that is the simple read of the Gemarah. However, the Reah cites the Ramban (cited in Aruch Hashulchan Yoreh Deah 98:10) says that even though taste imparted from food is only Rabbinic taste absorbed in a vessel must be kashered just as one must be tovel a new vessel purchased from a gentile, it is a מעלה for kelim brought into the jurisdiction of a Jew to be totally pure. (The Meshech Chachma points out the Torah introduces the law with the words זאת חקת התורה indicitating it is a גזה"כ and he asks if טעם כעיקר is always prohibited and the Torah is only prohibiting אעם לשבח what is the חוק? His starting point it to assume this parsha is a chiddush not the normal law.)
The Chasam Sofer Yoreh Deah (#94) recounts a story of a live bird that flew into a pot of boiling cream. The C.S. recounts an esteemed Rabbi (he doesn't say who, I have read that tradition says it was the Haflah) ruled it is בשר וחלב and אסור בהנאה. After everything was spilled out, it was pointed out to him that it was only בשר עוף which does not create an issur hanah? He immediately responded that since the chicken entered the butter while alive and died from the heat, the boiling butter absorbed the flavor of a live animal's limb before it died, hence it is אבר מן החי which is forbidden even to a non-Jew and therefore it would not be permitted to sell to a non-Jew either. The C.S. notes that it was obvious that he had made a mistake and came up with the justification afterward to save face but Hashem protects the righteous. The C.S. notes the novelty here is assuming that even a live thing can give off טעם and he cites his student cites a proof from the Rosh in Avodah Zarah who proves there is no טעם from a cold item from the fact that Haman told Achashverosh it a fly falls in wine a Jew will remove it and drink the rest, what about טעם? We see no טעם in something cold, asks the C.S.'s student, maybe there is no טעם because the fly is alive? We see live items also give off טעם.
The Sdey Chemed cites the Teshuvot Yad Eliezer asks on the pesak that who says the law of טעם כעיקר applies to a gentile, that is a chiddush din said to a Jew? He answers that we only need the chiddush since I would say it is less than the shiur of the issur but for goyim where there are no shiurim, I would know the law of טעם כעיקר anyway. The S.C. goes on to be מפלפל in this point.
The question assumes that טעם כעיקר is a new halacha of issur but if we say it just tells us that the original issur extends to טעם as well, we can here it is merely expanding the boundaries of the issuim that preexist and if that is an issur that applies to a gentile, the taam will be assur as well (the Achronim discuss this chakirah at length, see לקח טוב, גר"ח מאכ"א.)
The S.C. cites the Nodeh B'Yehuda suggests we don't apply the rule of טעם כעיקר only by issurim that have malkus. I don't understand since the Gemarah applies טעם כעיקר by mitzvot as well, so why should an issur without malkus be worse? (See Zevachim 78, though there may be other ways to learn the Gemarah, he doesn't even reference it?)
