Gemara

The Torah needed to teach about absorptions of Chatas and of Minchah. Had it taught only about Chatas, one might have thought only Chatas forbids, for fatty meat penetrates deeply.

Chulin 96b (Mishnah): If a thigh was cooked with the Gid ha'Nasheh inside, if it is Nosen Ta'am (imparts taste) to the thigh, the thigh is forbidden.

(Shmuel): This is only if the thigh was cooked (with water), If it was roasted, it is permitted (but one may not eat the meat around the Gid).

Question: Rav Huna taught that if a kid that was roasted with its Chelev, even the tip of the ear is forbidden! (The Chelev gives Ta'am to the entire kid.)

Answer: Chelev is different. It bubbles up and spreads.

Question: Does Chelev really forbid the rest of the animal?!

A case occurred in which a kid was roasted with its Chelev, and R. Yochanan permitted it, except for the areas around the Chelev!

Answer #1: It was a thin kid.

Answer #2 (Rav Huna bar Yehudah): The case was, a kidney was roasted with its Chelev. R. Yochanan permitted the kidney. (The membrane around it blocks the Chelev.)

97b: Some thighs were salted with the Gid ha'Nasheh inside. Ravina forbade them, and Rav Acha permitted them.

Question (Rav Acha, to Ravina): Do you forbid them because Shmuel taught that salted food is like Rose'ach (boiling hot), and pickling is like cooking?

Shmuel forbade the thigh only if it was cooked with the Gid. If it was roasted, it suffices to peel off the layer around the Gid!

Do not say that Shmuel said that salted food is like Rose'ach, i.e. like cooking. Since he said that pickling is like cooking, we infer that salting is not. Rather it is like roasting!

108a (Mishnah): If a drop of milk fell on a piece of meat in a pot, if the milk gives Ta'am to the piece, it is forbidden;

If the pot was stirred, if the milk gives Ta'am to the contents in the pot, it is all forbidden.

Pesachim 75b (Mishnah): If juice dripped onto the oven (it is cooked by it) and splattered back onto the Korban Pesach (which must be only roasted), one must remove Kedei Netilah (more than just a layer) from that place (for the drop gets absorbed);

Zevachim 97a (Mishnah): If a wafer touched a wafer, or a piece (of meat) touched a piece, it does not forbid all of them. It forbids only the place it touched.

Rishonim

Rambam (Hilchos Ma'achalos Asuros 15:2): If a kid was roasted with its Chelev, it is totally forbidden. If it was lean, and the Chelev was only one part in 60, he peels and eats, until he gets to the Chelev. The same applies to a thigh roasted with the Gid ha'Nasheh. He peels and eats, until he gets to the Gid, and discards it. The same applies to an animal roasted whole, without removing the forbidden veins and membranes. One peels and eats, until he gets to the Isur, and removes it. Gidin do not give taste, that we should need to estimate them.

Rosh (Chulin 7:24): The Gid should forbid the entire animal, since its fat (which is forbidden) penetrates! The Rivam says that since the fat is forbidden only mid'Rabanan, Chachamim were not stringent (to forbid) more (due to it) than (due to) the Gid. Rashi and the Rif did not say so. They must say that that fat of the Gid does not penetrate, like we say about a lean kid.

Tosfos (97a DH She'ani): Chelev penetrates, therefore even the head is forbidden. In Pesachim, we say that Kedei Netilah suffices. Gravy does not penetrate as much as Chelev, for it is mere brine.

Gra (YD 105:34): The Rashba holds that gravy penetrates. Even so, it forbids where it fell. In Pesachim, there was not enough to give taste. It seems that Tosfos disagrees. Also the Rambam, Sefer ha'Terumah (i.e. R. Baruch, in Mordechai Sof 884) and the other Poskim (hold like the Rashba).

Tosfos (97a DH She'ani): R. Tam said that fat of the Gid does not penetrate, therefore it does not forbid the entire animal. The Rivam says that since fat of the Gid is forbidden only mid'Rabanan, they were not stringent (to forbid the entire animal). If so, since we hold that Gidim have no taste, even if the Gid was cooked with the animal, it does not forbid. Ravina forbade. He must hold that Gidim have taste, like the Tana of our Mishnah.

Ran (Chulin 34b DH Gedi): Chelev did not forbid the kidney. This is because the membrane interrupts. We learn from here that all membranes and strands forbidden due to Chelev do not forbid. If a bird was cooked without cutting the veins, it suffices to remove them. The veins block the blood from diffusing. Above (93b), we say that blood (in the veins) in the head can forbid! Perhaps they do not block diffusion because they are soft.

Poskim

Shulchan Aruch (YD 105:5): Roasted meat forbids only Kedei Netilah in cases like the thigh and its Gid, i.e. something lean without strength to penetrate into the entire piece. However, if a fatty kid was roasted with its Chelev, if the entire kid is not 60 times all the Chelev, it forbids the entire animal. If it is lean, even if there is not 60 times all the Chelev, it forbids only Kedei Netilah.

Beis Yosef (DH veha'Rashba): Similarly, all membranes forbidden due to Chelev, if they were salted or roasted with meat, one peels and eats until he reaches them. The Rambam and Ramban say so.

Shulchan Aruch (ibid.): Anything that penetrates that fell into a known place in a roasting piece, even if the piece has 60 times and can be Mevatel the Isur, one must remove Kedei Netilah from where it fell.

Taz (11): The place where it fell is considered like intact Isur. We learn from Pesach.

Shach (15): The Terumas ha'Deshen holds that whenever there is 60, we need not remove Kedei Netilah or a Kelipah (peel off a layer). The Maharshal says that for lean meat, we require Kelipah if it was salted, and Kedei Netilah if it was roasted, even if there is 60. If the meat is fatty, it is folly to forbid. Maharam Padova says similarly. The Rema holds like the Mechaber (who requires Kedei Netilah Stam, i.e. even if it is fatty).

Pri Chodosh (20): The Rashba says 'since there is in the kid enough to be Mevatel it, it forbids only Kedei Netilah, if the place (where it fell) is known.' It follows that if one does not know where it fell, we are not stringent to require Kedei Netilah from every piece due to Safek, since it is Batel in 60. However, a lean Isur that does not penetrate is not Batel in 60. If we do not know where it fell, one must remove Kedei Netilah from all the pieces, due to Safek. A proof is from a piece that was partially in brine. (If a piece of salting meat was partially in the brine (the blood that drips down), we forbid the part in the brine, and permit the rest.) If we do not know which part was in the brine, all of it is forbidden.

Rebuttal (R. Akiva Eiger): Even if it is lean, if we know that it touched only one place and we do not know which, it is Batel in the majority of places from which we might need to take Kedei Netilah. We need not be stringent lest one cook them, for then there would be 60 in all the pieces against the Chelev.

Note: The Pri Chodosh would have a good proof if there are at least three places on the piece about which there is a Safek (which place was in the brine, and even so we do not rely on the majority of Heter), and we are stringent. However, the usual case is that one of the two ends was in the brine. We are stringent because there is no majority!

Pischei Teshuvah (11, citing Shemen Roke'ach and Mishbetzos Zahav 11): If it was cooked without removing Kedei Netilah, it is permitted even if there is not 60 times as much as Kedei Netilah. This is if it is fatty. If it is lean, letter of the law one must remove Kedei Netilah, so we forbid everything if there is not 60 times Kedei Netilah.

Rema: Anything cooked without gravy, or baked, is like roasted in every way.

Gra (35): In Zevachim 97a, we say that if a wafer touched a wafer, or a piece (of meat) touched a piece, it forbids only the place it touched. This is without liquid. Something that penetrates totally forbids, like through roasting. We learn from the drop of milk (108a).

Rema (ibid.): Some say that we are not expert regarding what is called lean or fatty, so we forbid in every case, unless there is 60 times. Even if there is 60, one must take Kedei Netilah. This is the custom.

GRa (36): I.e. even those who disagree with the Rashba (and hold that what penetrates becomes Batel in 60) are concerned lest it does not penetrate.

Rema (ibid.): This refers only to Chelev or other Isurim that can be fatty. Regarding an Isur that cannot be fatty, it is surely lean, so it forbids only Kedei Netilah.

TAZ (12): 'Can be fatty' alludes to what Sha'arei Dura wrote, that one needs 60 times as much as Heter as brine of Neveilah, even if they were (not cooked together, merely) salted together. Darchei Moshe says that brine of Neveilah and similar things penetrate like Chelev. (However, an Isur that surely does not penetrate, forbids only Kedei Kelipah.) We must say so, to resolve this with the ruling that Kedei Kelipah sufficed for thighs salted with the Gid ha'Nasheh.

Shach (16): It is clear from many places that fattiness does not apply to blood. However, the custom (Rema, end of Sa'if 9) is to estimate 60 times for salting, even if the meat is totally lean. If so, the same applies to roasting, for it is like salting. Torah Chatas and the Maharshal and other Acharonim say so. Here the Rema says that a lean Isur forbids only Kedei Netilah. This is for an Isur mid'Rabanan.

Gra (37): There is no custom to be stringent about an Isur that cannot be fatty, for no expertise is required!