Gemara

(Rav Dimi): A non-Kohen is lashed for eating a k'Zayis (of Terumah) bi'Chdei Achilas Pras (within the time to eat a half loaf, i.e. three or four eggs).

37a (Beraisa - R. Akiva): If a Nazir soaked his bread in wine and ate a k'Zayis of bread and wine, he is liable.

Isurei Nazir are not permanent, they do not forbid all benefit, and there is a Heter for the Isurim. Still, the Torah forbids Ta'am k'Ikar;

Kil'ayim (crossbreeds) in a vineyard are permanently forbidden, all benefit is forbidden, and the Isur cannot be permitted. Two of these stringencies apply to Orlah (fruits of a tree before its fourth year). All the more so, Ta'am k'Ikar is forbidden!

Pesachim 42a (Mishnah): One transgresses for the following during Pesach: Kutach of Bavel (a dip made with bread), beer of Madai (which has barley)...

43a (Rav Nachman): Our Tana is like R. Eliezer:

(Beraisa - R. Eliezer): One who eats proper Chametz of grain is Chayav Kares. One who eats a mixture of it transgresses a Lav;

Chachamim say, one who eats a mixture does not transgress anything.

R. Eliezer forbids mixtures due to "Kol Machmetzes Lo Sochelu."

44a - Question (Abaye, against Rav Dimi): If the Torah forbids a k'Zayis bi'Chdei Achilas Pras, why do Chachamim exempt one who eats Kutach on Pesach?

Answer (Rav Dimi): Kutach is different. It is not normal to eat a k'Zayis (of Chametz in it) bi'Chdei Achilas Pras.

Zevachim 78a (Reish Lakish): If one ate Pigul and Nosar that were mixed together, he is exempt. Surely, one of them was the majority, and the other is Batel. This teaches that if an Isur gives taste to a food of a larger volume than itself, it does not forbid it (to obligate lashes) mid'Oraisa.

Avodah Zarah 67a (R. Avahu): If the Isur gives taste and is intact, if it has k'Zayis bi'Chdei Achilas Pras, it is forbidden, and one is lashed for eating (a k'Zayis of Isur in) the mixture. If the Isur gives taste and is not intact, it is forbidden, but one is not lashed for it.

Rishonim

The Rif and Rosh (Avodah Zarah 32a and 5:10) bring R. Avahu's teaching about an Isur that gives taste.

Rambam (Hilchos Nezirus 5:6,7): If wine mixed with honey, and it all tastes like wine, and there is not a Revi'is of wine within a Pras of mixture, it is forbidden mid'Rabanan. If he ate it, he is lashed mid'Rabanan.

Ra'avad: Ba'al ha'Halachos (BaHaG) forbids mid'Oraisa, but one is not lashed for it. I brought proofs for BaHaG elsewhere.

Kesef Mishneh: The Rambam rules that Ta'am k'Ikar, therefore one is lashed for it. The Ra'avad holds like the Poskim who forbid only due to Chetzi Shi'ur mid'Oraisa, and one is not lashed for this.

Rambam (Hilchos Ma'achalos Asuros 15:2): If Chelev became dissolved in grits, if it can be tasted and there is substance to the Chelev, the Torah forbids it. If it can be tasted but there is no substance, it is forbidden mid'Rabanan.

Rambam (3): 'Substance' is when there is a k'Zayis of Chelev within the volume of three eggs of the mixture. If one ate three eggs of the mixture, since it contains a k'Zayis of Chelev he is lashed, since he tasted the Isur and its substance. If he ate less than three eggs, he is lashed mid'Rabanan. Similarly, if there was less than a k'Zayis of Chelev within three eggs of the mixture, even if it has the taste of Chelev and he ate the entire pot, he is lashed only mid'Rabanan.

Rambam (Hilchos Chametz u'Matzah 1:6): One is Chayav Kares only for eating Chametz itself. For a mixture, such as Kutach in which Chametz is mixed, one is lashed and there is no Kares, for it says "Kol Machmetzes Lo Sochelu." If he ate a k'Zayis of Chametz within the volume of three eggs he is lashed mid'Oraisa. For less than a k'Zayis of Chametz within three eggs he is lashed only mid'Rabanan.

Magid Mishneh: This is like the Rif, who says that Chachamim say that there is an Isur without lashes for a mixture, i.e. when there is not a k'Zayis of Chametz within three eggs. All obligate when there is a k'Zayis within three eggs. However, why does the Rambam exempt from Kares? The Gemara connotes that he is fully liable. Chachamim do not expound a separate verse for a mixture. We must say that the same Isur (with Kares) applies!

Kesef Mishneh (DH v'Lachen): It seems that the Rambam rules like R. Eliezer. However, if so Heter Mitztaref l'Isur, and he should obligate Kares even for a k'Zayis of Kutach! Rather, the Rambam rules like Chachamim. Ta'am k'Ikar applies to all Isurim. "Kol Machmetzes" is needed to obligate for Kutach, since it is not normal to eat it straight. Alternatively, since "Ki Kol Ochel Chametz..." excludes a mixture from Kares, one might have thought that it is permitted. "Kol Machmetzes..." forbids it. If so, regarding other Isurim of Kares, there is Kares for a mixture with k'Zayis bi'Chdei Achilas Pras. One could say that R. Eliezer and Chachamim argue about two matters. When there is k'Zayis bi'Chdei Achilas Pras, Chachamim obligate Kares, and R. Eliezer says that it is only a Lav. When there is not k'Zayis bi'Chdei Achilas Pras, or when it is eaten abnormally, R. Eliezer obligates a Lav, and Chachamim exempt. The Rambam rules totally like R. Eliezer. "Kol Machmetzes" and "Ki Kol Ochel Chametz" exempt a mixture from Kares, and we learn to all Chayavei Kerisos. This is why in Hilchos Ma'achalos Asuros the Rambam did not mention Kares for a mixture.

Ran (Chulin 35a DH Garsinan): If Ta'am k'Ikar mid'Oraisa, surely one is lashed! The Isur itself is mid'Oraisa. Ta'am k'Ikar merely reveals that the mixture is like the Isur itself, so one is lashed, like for the Isur itself! The Torah warned not to eat Neveilah, nor its taste. There is an Aseh regarding Pesulei ha'Mukdashim "Tizbach v'Ochalta" - you will eat, but you may not shear or work it. One is lashed for shearing or working it. The above verse reveals that the Lav to shear or work with Kodshim applies also to Pesulei ha'Mukdashim.

Poskim

Aruch ha'Shulchan (OC 442:20): Barley beer and whiskey are total Chametz. There is Kares for them, even according to Rif and Rambam who say that there is no Kares for k'Zayis bi'Chdei Achilas Pras. This is because the essence of the grain enters the water. This is like drinking melted Chametz. Even if the whiskey is partly (vodka) from potatoes, most Poskim say that there is Kares if there is a k'Zayis of grain bi'Chdei Achilas Pras. Also, the barley primarily ferments it.

Aruch ha'Shulchan (YD 109:4): The Rosh holds that an Isur that was Batel becomes Heter. All the more so, Heter becomes Isur through Ta'am k'Ikar. Poskim other than the Rambam obligate lashes for each k'Zayis.