1) PEELING OFF THE OUTER LAYER
QUESTION: The Mishnah (75b) states that if the Korban Pesach was smeared with oil of Terumah, only the outer layer must be peeled off even if the Korban was hot. The Gemara says that according to Rav, who maintains "Ila'a Gavar" (the temperature of the substance on the top "overpowers" the temperature of the substance on the bottom, so that if a cold substance falls into a hot substance, the cold overpowers and cools the hot), it is clear why only the outer layer must be peeled off and we are not afraid that the Korban absorbed the oil of Terumah. Since the oil itself is not hot, it cools off the Korban Pesach at the moment of contact and does not get cooked into the meat.
RASHI (DH Tzonen Hu) asks that according to Rav, it should suffice to rinse off the oil. Why must one peel off a layer of the Korban? Rashi answers that since the animal has been roasted, it is soft and absorbent, and even if it is cold it absorbs some of the oil. Therefore, the outer layer must be peeled off.
However, the Gemara itself continues and explains why the animal absorbs some of the oil. The Gemara says that it takes a few moments for the oil to temper the heat of the meat. Consequently, before the meat is cooled it heats and absorbs some of the oil. This apparently is the reason for why peeling is necessary. Why does Rashi not mention this reason? (SEFAS EMES)
ANSWER: The DEVAR SHMUEL (Mahadura Basra) points out that Rashi's words are based on the Mishnah. The Mishnah says that if the meat is raw (it has not yet been roasted), one may simply wash off the oil of Terumah. Once the meat is roasted, however, one must peel off the outer layer. The Mishnah does not differentiate between whether the Korban is cold or hot, but between whether the Korban is raw or roasted, as TOSFOS points out (75b, DH l'Fi). For this reason, Rashi writes that the outer layer must be peeled off because it has become soft and absorbent by being roasted, even though it is cold. For the same reason, Rashi later (DH v'Iy) explains that the Bar Gozla, young bird, that fell into the Kutach dip would have absorbed some of the dip had it been roasted and not raw, even though it was cold at the time that it fell in.
(What exactly the Mishnah means by "roasted" and "raw" is debated among the Rishonim. The RASHBA (Toras ha'Bayis, and in Chulin 112a) and the RITVA (ibid.) explain that when the Mishnah and Gemara refer to "roasted" as opposed to "raw," it actually means that the food is "hot" as opposed to "cold," unlike the Devar Shmuel infers (see Insights to Chulin 112:3). The Rashba and others write that this is Rashi's opinion as well. If this indeed is Rashi's approach, then the question of the Sefas Emes returns.)
2) "KAVUSH K'MEVUSHAL"
OPINIONS: The Gemara teaches that "Kavush k'Mevushal" -- "soaking is like cooking." Soaking an item in a liquid causes a transfer of taste, as cooking does, so that a permissible item soaked with a forbidden item becomes forbidden. What are the parameters of this principle? What liquids cause a transfer of taste, and after how much time?
(a) RASHI explains that "Kavush" refers only to an item soaked in vinegar or a similar substance that is very sharp, such as a liquid that contains sharp spices (as Rashi writes in Chulin 97b, DH Kavush). Rashi does not specify a minimum time duration for which the item must be soaked in order to absorb taste.
(b) The MORDECHAI in Beitzah (#674) argues and says that even if the liquid is not sharp, it transfers the taste of the items soaked in it.
The Mordechai, as well as the ROSH in Avodah Zarah (5:11), adds that taste is transferred only after 24 hours. He proves this from the Gemara earlier in Pesachim (44b). The Gemara there says that a mixture of meat and milk differs from other forbidden foods. A mixture of meat and milk is forbidden only when the meat is cooked with milk, but not when it is soaked with milk for one day. All other prohibited foods make another food forbidden even when they are soaked together for a day. It is evident from the Gemara there that "Kavush" transfers taste after 24 hours.
The Gemara earlier also implies that even soaking in liquids that are not sharp, such as milk, is considered "Kavush." How does Rashi understand that Gemara?
1. Rashi earlier (44b) explains that the Gemara means that when meat is soaked in milk, even if the milk becomes absorbed in the meat, it can be removed by squeezing and wringing out the meat. The reason why the taste of other forbidden foods cannot be removed from the food it enters is not because of "Kavush," but because of "Nosen Ta'am" (they give their taste to the permitted item). "Nosen Ta'am" of all other forbidden foods is prohibited, but for meat and milk it is permitted until they are actually cooked together. (See also TOSFOS to Chulin 108a, DH Chidush.)
2. The CHASAM SOFER (OC 129, DH Ivra) explains that since milk becomes sour after 24 hours, it has the same quality as vinegar and is able to cause a transfer of the taste of the meat that soaks in it.
3) HALACHAH: "REI'ACH"
OPINIONS: Rav and Levi argue whether the "Rei'ach" (aroma) of a forbidden food is significant enough to forbid another item. Rav says that if kosher meat was cooked in the same oven with Neveilah meat, the Rei'ach of the Neveilah meat becomes absorbed into the kosher meat and forbids it. Levi argues and says that "Reicha Lav Milsa Hi"; the vapor is insignificant and does not forbid the kosher meat. What is the Halachah?
(a) RASHI (DH Amar Lach Rav) says that the Halachah follows the opinion of Levi, even though Rav was able to explain that all of the Tana'im agree with his opinion. The reason the Halachah follows the opinion of Levi is because elsewhere (Avodah Zarah 66b) Abaye and Rava also argue about Rei'ach, and Rava agrees with the opinion of Levi. Since the Halachah always follows Rava when he argues with Abaye, the Halachah is that Rei'ach is not considered significant.
However, it is evident from the words of Rashi (DH Avad Uvda, as pointed out by TOSFOS DH Mai) that according to Levi, Rei'ach is insignificant only b'Di'eved. Levi agrees that l'Chatchilah we consider Rei'ach to transfer taste. This is also the ruling of the RIF in Chulin (32b of the pages of the Rif), who rules like Levi but only permits the food b'Di'eved, and the ruling of the RAMBAM (Hilchos Ma'achalos Asuros 15:33).
(b) TOSFOS (DH Asrah) cites RABEINU TAM who rules like Rav and says that Rei'ach is considered significant, even b'Di'eved. Even though Rava in Avodah Zarah (66b) seems to agree with Levi that Rei'ach is not significant, Tosfos asserts that Rava's words there apply only to the specific case that is discussed there. In all other cases, Rava agrees that Rei'ach is significant. This is also the opinion of the ROSH (Avodah Zarah 5:8) according to the TUR (see, however, BEIS YOSEF who points out that the Rosh cites both opinions and does not seem to side like one in particular). (See Insights to Avodah Zarah 66:2.)
HALACHAH: The BEIS YOSEF (YD 97, DH ul'Inyan Halachah) rules like the Rif and Rambam. He writes that l'Chatchilah one may not cook a permitted item with a forbidden one in a small oven (in a large oven, or in an oven with an exhaust fan, it may be permissible even l'Chatchilah), but b'Di'eved the food is permitted. This is the ruling of the SHULCHAN ARUCH (YD 108:1).
The REMA adds that this also applies to a meat food and a milk food cooked together in the same oven; b'Di'eved the foods are permitted even if they were cooked in a small oven at the same time. (There are some authorities, however, who prohibit the food in this case unless one stands to suffer a great loss.) However, if either food was very spicy, then even b'Di'eved they are forbidden.
The SHULCHAN ARUCH (YD 92:8) adds, based on the ROSH (Teshuvos 20:26), that when two items are cooked in the same oven, there is also a problem of steam ("Zei'a"). Steam from a milk food that rises up to a meat food forbids it. The Shulchan Aruch rules that steam of milk actually makes a meat item prohibited even b'Di'eved, unlike Rei'ach, which prohibits the other food only l'Chatchilah, but not b'Di'eved.
In practice, there are three general opinions as to whether one may cook meat after milk in the same oven.
1. The ARUCH HA'SHULCHAN (YD 92:55) writes that steam is not a problem in modern ovens. Rei'ach, however, is a problem. Nevertheless, Rei'ach is only a problem when the meat and milk are in the oven at the same time. One may cook meat right after milk in modern ovens. (The oven, however, must be clean.)
2. The MINCHAS YITZCHAK (YD 5:20) rules that since the difference between Rei'ach and Zei'a is not well defined, it is preferable to have two ovens, one for meat and one for milk. He says that steam left in the oven from the meat food might enter the milk food, or that the steam of meat and the steam of milk might become absorbed into the walls of the oven, rendering the oven itself not kosher (due to the mixture of meat taste and milk taste in its walls). One who does not have two ovens should either kasher the oven between cooking meat in it and cooking milk in it (such as by heating it to the highest setting for an hour or so), or double-wrap the meat or the milk item. This is also the ruling of the BE'ER MOSHE (3:105:2).
3. RAV MOSHE FEINSTEIN (Igros Moshe YD 1:40 and end of 1:59) rules that steam is a problem only when the food is a liquid. We do not have to worry that a solid food will produce a significant amount of steam (unless one clearly observes that it does). Similarly, the REMA (YD 92:8) writes that steam is not a problem when the food is in a pot that is covered. Therefore, Rav Moshe suggests that one may cook meat in the oven immediately after he cooks milk once the oven cools down, as long as neither food item is a liquid, or as long as one of them is covered. Accordingly, if one's oven is a meat oven, he may cook meat in it in the normal manner, and after he removes the meat and the oven cools down, he may cook a solid milk food in it, or even a liquid milk food which is covered. Alternatively, a liquid milk item may be cooked in the meat oven uncovered if one waits 24 hours from the last time the oven was used for meat (so that the steam that is absorbed becomes Nosen Ta'am li'Fegam). (See also Rav Ovadyah Yosef's ruling in YABI'A OMER 5, YD 7:5 and 7.)
Of course, with regard to how to conduct oneself in practice, one should consult a competent rabbinic authority.