Milk and Meat in the Kitchen
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Revision as of 14:01, 11 February 2019 by YitzchakSultan (→Cooking and Benefitting from Milk and Meat)
For halachic questions of Kashrut a person should consult a rabbi because the laws of Kashrut involve many complexities.
- 1 The Biblical Prohibition
- 2 Cold Meat and Dairy Touching
- 3 Utensils
- 4 Food that was on the Table
- 5 Stovetops
- 6 Meat and Dairy Equipement
- 7 Transference of Taste
- 8 Kashrut of Ovens for Meat and Milk
- 9 Microwave for Meat and Milk
- 10 Toaster-Oven for Meat and Milk
- 11 Washing Meat and Milk Dishes in One Sink
- 12 Soaking as a Form of Cooking
- 13 Inedible Foods
- 14 Negative Taste (Noten Taam Lifgam)
- 15 Sources
The Biblical Prohibition
- Take note that we are only speaking in terms of the Torah prohibition here. There are many cases where it is rabbinically forbidden to have meat and milk even though there is no Biblical prohibition. For example, it is Rabbinically forbidden to eat milk and meat together even if they were not cooked together.
- The Torah states three times "לֹא-תְבַשֵּׁל גְּדִי בַּחֲלֵב אִמּוֹ" "You shall not cook a kid (baby goat) in its mother's milk". Our Sages learn that the repetition of this prohibition three times teaches us the prohibitions of cooking, eating, and having any type of benefit (monetary or feeding one's animals) from milk cooked with meat.
- Our Sages teach us that the language of "לא תבשל"-"You shall not cook" implies that the Biblical prohibitions only apply if the meat and milk are cooked together.
- There is discussion whether frying or roasting meat and milk together is included in the biblical prohibition. There is also discussion about if it is forbidden biblically to cook meat and milk together in a pot that is hot (Yad Soledet Bo), but is no longer on the fire.
- There is a discussion if microwaving meat and milk together is considered cooking meat and milk together.
- If you mix milk and meat in a kli sheni (a pot that was never on the fire), even if it is hot, you do not biblically violate cooking meat and milk.
- If a mixture of meat and milk is not biblically prohibited then one may derive benefit from it so long as one doesn't eat it.
- The Torah only refers to a "גדי"; however, our Sages have taught us that a "kid" refers to all kosher domesticated animals (e.g. sheep, cows). We were also taught that all types of kosher animal milk are prohibited to cook meat with, not only the milk of the mother. Rather, the reason why the Torah was so specific is because it was speaking in the present (i.e. that the verse spoke in terms which are similar to the way the world functioned at the time).
Cooking and Benefitting from Milk and Meat
- There is a biblical prohibition to eat or benefit from meat and milk cooked together. Additionally, cooking meat and milk together is itself a biblical prohibition. 
- For example, it is forbidden to use butter cooked in a meat pot as a candle for light or for Chanuka candles since that would be benefitting from milk and meat.
- This biblical prohibition only applies to milk and meat from kosher species. Therefore, it is permitted to cook and benefit from the mixture of milk and ham or meat from another non-kosher species. Similarly, it is permitted to cook or benefit from the mixture of pig milk or milk from another non-kosher species with meat.
- However, it is forbidden to cook or benefit from meat from an unslaughtered animal and milk or meat from an animal that was Tereifah and milk. 
- It is forbidden to eat chicken cooked with milk, however, it is permitted to cook and benefit from chicken and milk.
- Even if one isn't going to eat the food, it is forbidden to cook meat in a dairy pot that was used within the last 24 hours for dairy.
- It is forbidden to stoke the coals under a pot that is cooking meat and milk.
- According to many poskim it is permitted to smell meat and milk together such as a cheese burger since they are cooked as food and not for the smell.
Feeding An Animal Meat and Milk
- Some hold that it is forbidden to feed meat and milk cooked together to animals, even ones that belong to other people, since there is a benefit in having one's desires fulfilled and by using the meat and milk one was able to fulfill his intentions.
Cold Meat and Dairy Touching
Dry Foods Touching
- Initially it is forbidden to have meat touch cheese if it is uncommon to wash the meat and cheese before eating them because one might forget to wash them before eating them.
- If a cold piece of cheese or dairy food touched a cold piece of meat or a meat food, the area of their contact each needs to be washed before being eaten.
- If both foods were dry and don’t make crumbs then they don’t even require washing and it all depends on what you see.
- When washing the area of contact the food should be washed in water and rubbed gently to clean it from anything being stuck onto it. If a dairy food was cut with a knife with meat fat the dairy food needs to be washed and rubbed well.
- After the fact if the dairy and meat foods touched and were cooked separately without being washed beforehand, some say that it is kosher while others are concerned unless there is certainty that there was 60x the crumbs that transferred.
Liquid and Solid Touching
- If a cold piece of cooked meat fell into liquid milk according to Sephardim the meat should be washed and it is permitted, while according to Ashkenazim the meat is forbidden up to the depth of a peel and the milk is permitted.
- One shouldn't drink from the same cup when one was eating meat and when one is eating dairy.
- One can use the same cup to drink a parve drink while one is eating meat or milk as long as one wipes one's lips before drinking so that a fat residue doesn't stick to the lip of the cup.
Food that was on the Table
- Sliced bread that was on the table during a meat meal should be treated as meat and can't be eaten with milk since meat residue may have touched it. Some question how it is permitted to save that bread for another meat meal since one might forget and eat it with milk. However, the minhag is to save the cut up bread on the table to save that bread for another meat meal.
- Some poskim suggest being strict to use separate grates for stovetops that are designated for meat or dairy, however, many other poskim aren’t concerned and permit using the same grates.
Meat and Dairy Equipement
- Parve food cooked in a meat pot is considered meat-equipment and according to Ashkenazim it shouldn't be eaten together with dairy but after the fact if it was cooked with dairy it would be permitted to be eaten. According to Sephardim it is permitted even initially to eat the parve food made with meat equipment with dairy.
- Parve food cooked in a dairy pot is considered dairy equipment and according to Ashkenazim it shouldn't be eaten together with meat but after the fact if it was cooked with meat it would be permitted to be eaten. According to Sephardim it is permitted even initially to eat the parve food made with dairy equipment with meat.
- If the meat pot wasn't used within 24 hours for meat, then if something parve cooks in it, the parve food can be eaten together with dairy even initially. However, one shouldn't use a meat pot even if it hasn't been used within 24 hours to cook parve food that one intends to eat with dairy. The same is true of dairy and meat vice versa. According to Sephardim all cases of cooking parve food in a meat pot in order to eat it together with dairy are permitted.
Transference of Taste
If there's no liquids
- If a hot meat pot touches a hot dairy pot and there’s no liquid in between the pots the two pots are still kosher because taste doesn’t transfer between pots without liquid.
- If a hot piece of meat touches a hot piece of dairy and there’s no liquid between the pieces and neither piece is fatty only a finger-width of each piece surrounding where they touched is made non-kosher. Ashkenazim are strict to forbidden the entire piece of meat and dairy.
- If a hot piece of fatty meat touches a hot piece of dairy and there’s no liquid between the pieces the entire piece is forbidden since the fat spreads taste.
- A piece of meat which absorbed a dairy taste is considered completely forbidden. Therefore, if it is hot and touches a piece of kosher food it will cause that other food to become non-kosher up to the depth of a finger-width.
If there's liquids
- If hot or even cold meat fell into hot milk or vice versa everything is forbidden. 
- If hot milk falls onto cold meat the meat is only forbidden up to the depth of a peel. Sephardim hold that the milk is permitted, while Ashkenazim hold that the milk is forbidden unless there is sixty times the area of contact to the depth of a peel.
- One shouldn’t pour water from a faucet or parve cup into a hot meat pan since the stream will affect the faucet making it meat like the pan, otherwise the faucet or parve cup will become meat. The same is true with a milk pan.
- If a hot piece of meat is cut with a cold dairy knife if the knife was used for dairy within 24 hours the amount of a "peel" needs to be removed from the meat. If it wasn't used within 24 hours the meat is kosher. The knife in either case needs to be cleaned off but not koshered. However, many poskim hold that the meat is completely not kosher and the knife needs to be koshered.
- This applies even if the meat isn't yad soledet bo as long as it is hot.
Kashrut of Ovens for Meat and Milk
- To avoid all issues one should choose the primary use of the oven for meat or milk and then cover the other type. For example, if he chooses that it be used as a meat oven, the meat foods can be cooked in there uncovered. The dairy dishes with liquid should be covered, however, the dry dairy dishes can be cooked uncovered. Also, if within 24 hours of cooking a meat dish with liquid one wants to cook a dairy dish with liquid one should wait until after 24 hours even if it is covered.
- According to Sephardim, many Poskim rule that nowadays a person should have separate ovens for cooking meat and for cooking milk. In cases where this is difficult, one can be lenient to use one oven so long as one covers all food placed in the oven. Alternatively, if one cooks a solid food in the oven over 24 hours apart from of the opposite type and one also preheated it for 20 minutes. After the fact, one may be lenient.
- According to some, if the foods are dry foods that don't produce vapors, then one may place the foods in the oven one after the other (but not at the same time). Others rule that one should wait 24 hours between cooking the foods and that one should first let the oven run for 15 minutes before placing the second food into the oven.
- In a case where someone has only one oven, he does not need to have separate oven grates for meat and milk.
- If one cooks two foods in the oven at the same time there is a smell (reycha) that is transferred from one food to the other. After the fact the food is permitted.
- There are some instances where it is possible for reycha to forbid food even after the fact according to Ashkenazim:
- If the oven is small and completely sealed reycha forbids the food even after the fact.
- If one of the foods is a Dvar Charif then reycha can forbid the food even after the fact.
- If the prohibition under consideration is forbidden in any amount, such as Chametz, reycha can forbid the food even after the fact.
How to Prevent Reycha
- There is no reycha when foods are cooked one after another.
- If the oven is large enough to hold 12 isaron of bread and has a large vent there is no reycha. The poskim assume that our ovens today do not have that requisite size.
- If one or both of the foods were in pots with walls those walls separate between them and prevent reycha from transferring from one to the other.
Reycha for Parve
- Bread: If one cooked open meat simultaneously with open bread in the oven at a temperature above Yad Soledet Bo one shouldn't eat the bread with dairy.
Microwave for Meat and Milk
- If one uses a microwave for meat and for dairy (at different times), some authorities hold that one should preferably double wrap all foods, however, some authorities hold that covering it well with one covering is sufficient. Some also advise using different trays one for dairy and one for meat. 
- Alternatively, one can cover all dairy foods with one large plastic microwave container and all meat foods with another container designated for meat. Also, the plastic containers may have tiny holes to let out steam.
- If one wan't careful to keep his microwave kosher in these regards, one can make it kosher again by cleaning the microwave out and boiling water in the microwave for a few minutes until it steams. Some also recommend adding soap to the water that is to be boiled in the microwave.
Toaster-Oven for Meat and Milk
- One should preferably designate his or her toaster-oven specifically for meat or for dairy, since it is small and hard to clean out. The concern is that small particles remain behind in the toaster-oven and would then make it impossible to separate between meat and dairy foods.
Washing Meat and Milk Dishes in One Sink
- Ideally, one should have two sinks one for meat and one for milk.
- It is permissible to wash meat and dairy dishes in a sink one after another as long as there are two boards or racks to raise the dishes off of the bottom of the sink, one for meat and one for dairy.
- Hot water poured from a kli rishon can cause flavors to be imparted up to the depth of a peel.  Therefore, if hot water from the sink hit a piece of meat and then a piece of dairy it would be impart flavor up to the depth of a peel in the dairy food. There is a dispute if the hot water from the sink hit a piece of meat and then a dairy pot if it would impart flavor up the depth of a peel in the pot.
- Some are lenient to allow washing dirty meat and dairy dishes together in one sink.
- Most assume that this would cause the meat and dairy dishes to become non-kosher. However, they would be lenient to clean the meat and dairy dishes together if the dishes didn’t have any pieces of dairy, meat, or even grease on them.
- Some are strict and hold that even if the dishes had no pieces of meat or dairy on them nonetheless washing them together under a hot faucet would cause them to become non-kosher.
- For Ashkenazim, the halacha is that one should not wash dishes together in the sink at the same time and if one did, even if the dishes were clean beforehand they should be koshered. 
- Hot water which was poured and the stream was broken is considered hot enough to impart flavor up to the depth of a peel but can’t transfer taste from one food to another. 
- If one is cleaning meat dishes in the same sink in which one cleaned milk dishes, if there's still some dairy remnant in the sink, it's forbidden to pour hot water there because at the time one pours the water the meat and milk are halachically cooked together even though one has no intention of using those remnants. 
- According to Ashkenazim, it is forbidden to use a dishwasher for meat and milk even one after the other and even if one uses different racks.  Sephardim only allow using one dishwasher for meat and milk if they are used in different shifts and the dishes are first rinsed to remove the large pieces.
- If there is porcelain in the walls of a dishwasher it can't be koshered. If it is metal then it can be koshered but not the plastic parts. There is a dispute if the hard plastic racks can be koshered.
Soaking as a Form of Cooking
- Milk and meat that were soaked together for 24 hours or were salted together may not be eaten, however, they are permitted to benefit from.
- If a prohibited item was soaking together with a permitted item in a liquid for 24 hours the permitted item is. If it was there for less than 24 hours it is sufficient to just wash off the permitted item. 
- If it soaked for 24 hours non-consecutively, such for 23 hours and then for another hour, after the fact most poskim assume that it is permitted.
- If a prohibited item was soaking in a pot for 24 hours, the pot absorbed the prohibited taste and would need to be koshered. 
- If a permitted item was soaking in a non-kosher pot for 24 hours, after the fact the food is kosher according to most opinions, yet a person should only be lenient in cases of great loss. 
- Prohibited food which is inedible is permitted on a biblical level and forbidden on a rabbinic level because when your eating the food indicates that it is good.
- Medicines made with prohibited ingredients which were made inedible, some poskim permit this since one’s intent isn’t to eat the prohibited item, while others are strict.
Negative Taste (Noten Taam Lifgam)
- If a forbidden item fell into a mixture and adds a negative taste, if the actual forbidden item was removed the mixture is permitted.
- If a forbidden item fell into a mixture and adds a negative taste, if the actual forbidden item dissolved completely and there is greater detriment from the negative taste than the increase of volume, the mixture is permitted.
- There is a dispute whether or not this leniency of spoiled tastes applies to chametz on pesach. 
- Noten taam lifgam applies to all prohibitions including combinations of milk and meat.
- Shemot 23:19; 34:26; Devarim 14:21
- Shulchan Aruch YD 87:1. The Rambam (Maachalot Asurot 9:2) explains that when the Torah only mentions the prohibition of cooking milk and meat together, it means to say that in addition to not eating or having benefit from it, cooking is also prohibited. This is similar to how the verse only prohibits one to have relations with his daughter's daughter, but makes no mention of not having relations with one's own daughter; the latter, unmentioned portion, is taken as a given.
- Chullin 108a, Shulchan Aruch YD 87:1.
- Pri Chadash 87:2 (p. 20) says that frying is included in cooking. However, the Chavot Daat (biurim 87:1) points out that many rishonim hold frying isn't cooking. Included in those Rishonim is Rashi (Sanhedrin 4a s.v. derech bishul) who explicitly states that frying in fat isn't considered the biblical prohibition of cooking meat and milk.
- Sefer Kashrut HaShulchan (Baser BeChalev 6:1) writes that there is a dispute between the Pri Chadash and the Machaneh Yehuda whether frying milk and meat is included in the biblical prohibition or is only rabbinically prohibited. He concludes by quoting the Ben Ish Chai Bahalotcha who rules like the Pri Chadash that it is biblically forbidden. This is also the position of the Gra YD 87:13. Pitchei Teshuva YD 87:3 rules to be stringent like the Pri Chadash but quotes the Pri Megadim to say that if there is significant loss, one may be lenient to derive benefit from the mixture so long as one doesn't eat it.
- The Pri Chadash rules that roasting meat and milk together is likewise prohibited by the Torah. The Ran quoted in Rabbi Akiva Eiger (Shulchan Aruch 87:1) rules that meat and milk roasted together are only forbidden midirabbanan. The Aruch HaShulchan 87:11 rules that one may consider fried and roasted meat with milk to be on the level of a rabbinic prohibition.
- Yalkut Yosef Isur Viheter vol. 3 87:9 quotes opinions on both sides and is strict.
- Minchat Shlomo 1:12:2 writes that just like microwaving food isn't considered bishul since it doesn't involve fire, it isn't considered cooking for the purposes of meat and milk. See however, Igrot Moshe OC 3:52 who holds with respect to Shabbat that microwaving food is considered cooking biblically.
- Yalkut Yosef Isur Viheter vol. 3 87:11
- Rama YD 87:1. Even though the Taz 87:1 cites the Maharshal who argues with the Rama and is strict not to benefit from derabbanan forms of meat and milk, the Shach 87:2 accepts the Rama.
- Rambam (Machalot Asurot 9:3), Shulchan Aruch YD 87:2. Maimonides in Guide to the Perplexed 3:48 even suggests that the practice of cooking a kid in its mother's milk may have been an idolatrous one.
- Chullin 115b, Rambam (Machalot Asurot 9:1), Tur and Shulchan Aruch YD 87:1
- Kaf HaChaim 87:16
- Mishna Chullin 113a states that there's no prohibition of cooking or benefitting from the combination of milk from a non-kosher species and meat or meat from a non-kosher species and milk. This is codified by the Rambam (Machalot Asurot 9:3), Maharshal (Yam Shel Shlomo 8:100), and Shulchan Aruch YD 87:3. In any event the combination is biblically forbidden to eat. Even though the Bach 87 writes based on the Tur that there is an additional rabbinic prohibition to eat the combination because of meat and milk, the Taz 87:2 and Shach 87:3 argue.
- The Rambam (Pirush Mishnayot Keritut 3:4) writes that there's no prohibition to benefit from or cook meat from a nevelah or teriefah and milk since the meat was already forbidden from beforehand and the additional prohibition of meat and milk can't apply afterwards. The Rashba (Torat Habayit ch. 4, p. 85a) and Ran (Chullin 113b s.v. Vesvar) write that we consider the prohibition of meat and milk to apply on top of other prohibitions since it includes a prohibition of benefit (isur mosif). The Tosfot (Chullin 101a s.v. Isur) has a slightly different terminology, see Dagul Mirvavah 87:1 and Binyan Yakov (p. 81) regarding a dispute about Tosfot's opinion. In any event, Dagul Mirvavah writes that a person who relies on the Rambam in cases of lose has what to rely upon. Chatom Sofer YD 92 writes that the halacha does not follow the Rambam. Badei HaShulchan 87:26 writes that many poskim are strict to forbid cooking and benefit from a combination of unslaughtered animal and milk or meat from an animal that was Tereifah and milk.
- Shulchan Aruch YD 87:3. The Taz 87:1 and Shach 87:2 disagree with the Maharshal and Bach who are strict regarding the benefit from chicken and milk. Kaf HaChaim 87:21 follows Shulchan Aruch.
- Pri Megadim (Siftei Daaat 87:18) holds that recooking meat in a dairy pot that was used within 24 hours for dairy is considered like cooking meat and milk together which is forbidden.
- Rama 87:6
- Sh"t Rashba 3:234 writes that it is permitted to smell something that is forbidden if it isn't made for its smell. Shulchan Aruch Y.D. 108:7 codifies this Rashba. Shach 108:27 quotes the Tosfot Avoda Zara 12b who argues with Tosfot.
- Pitchei Teshuva 94:5 quotes the Isur Veheter, Maharshal, Taz, and Rama who are forbid giving meat and milk to a dog even if it doesn't belong to you. Then he cites the Mekor Chaim and Gra who permit this and explain that chametz is unique that such a benefit is forbidden. The Shaar Hatziyun 448:75 sides with the strict approach though he cites the Gra as being lenient.
- Shulchan Aruch 91:3 based on the Baal HaItur
- Mishna Chullin 107b says that it is permitted to wrap cheese and meat together as long as they don’t touch. The gemara explains that even if they touch they just need to be washed to be permitted. The Shulchan Aruch YD 91:1 codifies this.
- Shach 91:1, Kaf Hachaim 91:1, Badei Hashulchan 91:3
- Badei Hashulchan 91:3-4
- Badei Hashulchan 91:5
- Taz 91:6
- Badei Hashulchan 91:6
- Shulchan Aruch YD 91:7 and Rama
- Rama 88:2
- Aruch Hashulchan 88:11
- Yerushalmi Pesachim 6:4, Shulchan Aruch YD 91:3. Igrot Moshe YD 1:38 holds that the actual prohibition of eating bread from a meat meal with dairy only applies to the bread which was eaten with meat or sliced pieces that were intended to be eaten. Still it is preferable not to eat with dairy any of the bread that was on the table during the meat meal. Badei Hashulchan 89:98 and The Laws of Kashrut (p. 212) adopt the opinion of the Igrot Moshe.
- Badei Hashulchan 91:17 quotes this minhag and questions it.
- Igrot Moshe YD 1:40 was asked about using meat and dairy grates initially and he responded that it is permitted. He writes that we don’t need to be concerned since there’s no chance of taste going from your pot into the grate based on Rama 92:8 and Shach 105:22. Igrot Moshe YD 3:10 reiterates that his opinion isn’t in opposition to the Rama’s insistence on initially avoiding this concern. Badei Hashulchan 92:183 disagrees and advises using different stovetop grates for meat and milk so that there’s no transference of taste from pot to pot. [Rabbi Baruch Simon (yutorah.org Yoreh Deah Shiur 34 - Melicha K’rote’ach)] explains this dispute and added that the minhag is like Rav Moshe.
- Shulchan Aruch and Rama YD 95:1
- Shulchan Aruch and Rama YD 95:1
- Rama 95:2 writes that if the pot was eino ben yomo there's no issue of nat bar nat. Badei Hashulchan 95:33 and Chachmat Adam 48:2 clarify that this means after the fact that the parve food was cooked in a meat pot it is considered parve and can be eaten with cheese even initially. However, one shouldn't cook the parve food in an eino ben yomo meat pot if one plans to eat that food with dairy. Yet, the Gra argues that it is permitted even initially.
- Shulchan Aruch YD 95:1
- Mordechai (chullin no. 690) says that two pots can’t transfer taste without liquid. Rama 92:8 and 93 rules like the Mordechai. Shach 105:22 agrees.
- Generally, Shulchan Aruch 105:4 rules that roasting, dry heat, only causes a transfer of taste up to a fingerwidth. The Rama 105:9 writes that since today we’re not experts in determining what is considered fatty and what isn’t we always consider it like it is fatty and there is a transfer of taste completely throughout the food.
- Nonetheless, the Rashba (Torat HaBayit HaKatzar 10a) limits this concept of tastes being transferred during roasting to when the food is intrinsically forbidden. The Tosfot (Chullin 96b s.v. afilu) and Rosh (Chullin 8:24) also subscribe to this concept. Tur and Shulchan Aruch 105:7 codify this idea but adds that meat and milk is considered an example of something that is intrinsically forbidden. The Maharshal (Chullin 8:45) and Taz 105:13 argue with the Shulchan Aruch on the grounds that meat which absorbed a milk taste is still considered something with a forbidden taste and not intrinsically forbidden. Nonetheless, they agree that if a hot piece of meat touches a hot piece of dairy that there would be a transfer of taste into the meat and dairy pieces and it isn’t considered like absorbed tastes.
- Shulchan Aruch 105:5 writes that if a piece of meat is fatty it can transfer taste into a kosher piece of meat completely without without any liquid since fat spreads taste. Also, according to Shulchan Aruch 105:7 meat and milk combining is considered like something intrinsically forbidden.
- Tur and Shulchan Aruch 105:7 codify this idea but adds that meat and milk is considered an example of something that is intrinsically forbidden. The Maharshal (Chullin 8:45) and Taz 105:13 argue with the Shulchan Aruch on the grounds that meat which absorbed a milk taste is still considered something with a forbidden taste and not intrinsically forbidden. However, the Shach 105:17 defends Shulchan Aruch based on the Ran (Chullin 43a s.v. tenan) and concludes that one should be strict to assume that meat which absorbed milk taste is considered intrinsically forbidden and can forbid something else by its touch. Nonetheless, the Shach says that in this case Ashkenazim do not have to be strict to consider all foods to be fatty whether or not they are since anyway it isn’t clear that meat with milk taste is considered intrinsically forbidden.
- Shmuel in the Gemara Pesachim 76a says that the temperature of the bottom item overcomes the top item and if the bottom one is hot it'll heat up the top one. The Ran (Chullin 41b s.v. garsinan) quotes the Raah who says that the cold meat which fell into milk is only forbidden up to a fingersbreath but the Ran argues that it is completely forbidden. The Rashba (Torat HaBayit HaKatzar 5b) and Rambam (Machalot Asurot 9:17, as cited by the Bet Yosef 91:4) agree with the Ran. Tur and Shulchan Aruch YD 91:4 codify the opinion of Shmuel that if a cold item falls into a hot one the top one becomes hot and everything is completely forbidden.
- The gemara Pesachim 76a says that if hot milk falls onto cold meat the meat is forbidden up to a peel. Tosfot s.v. tanya quotes Rabbenu Tam who says that the milk isn't forbidden at all since it is impossible to remove a peel of a liquid. However, the Riva is quoted by Tosfot as disagreeing and stating that the milk is forbidden unless there is sixty times the depth of a peel where the contact was made. The Tur 91:4 infers from the Rambam that he sides with the Rabbenu Tam. Shulchan Aruch YD 91:4 holds like Rabbenu Tam, however, the Taz 91:7 and Badei Hashulchan 91:34 are strict for the Riva.
- Rama 105:3 cites the Mordechai who learns that pouring is considered a connection to transfer taste as we find by tumah (nisok chibur). The Rama concludes that it isn’t an issue after the fact. Gra explains that it isn’t possible to learn from tumah since that is dependant on a connection whereas kashrut depends on taste being transfered. Badei Hashulchan 92:180 concludes that it is initially forbidden to pour water from a parve cut or faucet into a hot meat pan since doing so could make the parve cup or faucet meat. Horah Brurah 105:41 agrees.
- Rama Y.D. 94:7
- Badei Hashulchan 94:101 cites the achronim who argue with the Rama including the Maharshal and sides with him unless there is a case of great loss. That is also the view of the Aruch Hashulchan.
- Badei Hashulchan 94:100 based on Tosfot and Rosh on Chullin 8a. Gra explains that this Rama is based on Chullin that with duchka dsakina a kli sheni is boleh a klipah.
- The Badei Hashulchan 92:183 presents this set of laws to prevent any prohibition. However, in cases of mistakes one should ask his rabbi. The following is a summary of halachic discussion on this topic:
- Mishna Machshirin 2:2 says that steam is considered liquid to cause a fruit that came in contact with steam is considered susceptible to impurity since it came in contact with liquid. The Rosh (responsa 20:26) infers from this mishna that steam has the same status as the actual food where it came from. Therefore, he writes that if there’s a milk pot cooking below a meat pot the meat pot is forbidden because the steam is considered like hot milk. The Trumat Hadeshen (responsa 103) agrees but stipulates that it is only an issue if the steam reached the temperature of yad soledet bo. Therefore, it is permitted to cook a pot of milk food beneath hanging meat, such as salami, if the meat is far away and the steam won’t be yad soledet bo once it reaches the meat. Shulchan Aruch 92:8 codifies the opinion of the Rosh, while the Rama quotes the Trumat Hadeshen.
- Because of a concern of steam coming from an uncovered pot of food in an oven, some poskim forbid using one oven for meat and milk even not at the same time. Rav Moshe Feinstein in Igrot Moshe 1:40 writes that there is an issue. See what he writes regarding after the fact. He rejects 3 concerns that could be used to argue that it is permitted.
- (1) Perhaps since the oven is large the steam doesn’t get absorbed into the walls just like the Trumat Hadeshen said that steam that’s less than yad soledet bo isn’t absorbed. Rav Moshe argues that even if the steam weren’t yad soledet bo nonetheless they would be absorbed into the ceiling of the oven since it is very hot and heats up the steam upon touch.
- (2) Rav Moshe holds that a vent in the walls doesn’t allow all of the steam to exit.
- (3) Lastly, Rav Moshe doesn’t think that the steam was burned up before it was absorbed into the walls since we only can be sure that a drop of liquid is burnt up right near the fire (see Shulchan Aruch 92:6).
- Rav Ben Tzion Wosner (Or Yisrael 5763 year 8 no. 4:34 pp. 92-102), son of Rav Shmuel Wosner, writes that ovens aren’t an issue of steam because since the walls are so hot they burn up the steam before it is absorbed. This concept can be found in the Maharsham 3:208 though that isn’t his conclusion. Yavetz (1:93) is lenient and is adamant about this issue.
- Minchat Yitzchak 5:20 and Chelkat Yakov 2:136 are strict.
- Concerning dry dishes the Pri Megadim (seder v’hanahagot hashoel im hanishal b’isur v’heter seder 2 no. 37) says that zeiya is only an issur from liquids and not dry foods. Rav Moshe in Igrot Moshe YD 1:40 says that there is an issue of steam from dry foods if you can see it, otherwise you don’t need to assume that there is steam from dry dishes.
- Rabbi Mansour says this lechatchila at dailyhalacha.com.  This is also the opinion of R' Shmuel Pinchasi quoted at dailyhalacha.com. 
- Yabia Omer 5:7:8. Rav Moshe Feinstein in Iggerot Moshe Y"D 1:40. Chacham Ovadia Yosef quoted by Rabbi Mansour  writes that bedieved if one didn't wait 24 hours before cooking the opposite type of food, the food would nevertheless be permissible. However, R' Ovadia states that ideally one should wait 24 hours between cooking the two foods and that one should first let the oven run for 15 minutes before placing the second food into the oven.
- Yabia Omer YD 5:7:8. His reasons for being lenient include: zeyia is only rabbinic, it is a dispute, it might not come out into the new food, it might be burnt up, and it have dissipated through a vent. He is lenient after the fact even within 24 hours even on liquids cooked one after another.
- Rav Moshe Feinstein in Iggerot Moshe Y"D 1:40. Rav Ovadia Yosef in Yabia Omer 5:7:8 writes that those who follow this opinion have what to rely upon though he recommends being stricter.
- Chacham Ovadia Yosef quoted by Rabbi Mansour at dailyhalacha.com. 
- Rav Moshe Feinstein in Iggerot Moshe Y"D 1:40.
- Shulchan Aruch Y.D. 108:1
- Rama Y.D. 108:1
- Shulchan Aruch Y.D. 108:1
- Horah Brurah 108
- Bet Yosef 108:2, Shach 108:16, Horah Brurah 108:26
- Shulchan Aruch Y.D. 97:3, Shach 108:2
- The OU quotes Rav Yisrael Belsky who says that preferably one should double wrap food put in a microwave if it's used for meat and dairy (at different times).
- Yalkut Yosef (Isser Veheter, vol 3, pg 167) rules that if the microwave works only on radiation (without a heater) one should make sure to cover all food very well and then it would be permissible to use it for meat and dairy one after another. This is also the opinion of Rabbi Mansour at Dailyhalacha.com. Rabbi Aryeh Lebowitz writes that one covering should suffice to inhibit the splattering of food and steam from being released. He also mentions the point about using separate microwave trays.
- The Laws of Kashrut (p. 235)
- Rabbi Forst in The Laws of Kashrut (p. 234), Rabbi Mansour in the name of R' Shmuel Pinchasi at dailyhalacha.com. . Yalkut Yosef (Isser Veheter vol. 3, p. 167) writes that one can kosher a microwave by boiling water with a little soap in it for ten minutes and then cleaning the steam off the walls.
- Rabbi Mansour in the name of R' Shmuel Pinchasi at dailyhalacha.com. 
- Minchat Yitzchak 2:100, Badei Hashulchan 95:79
- Igrot Moshe YD 1:42 explains that even if one is concerned for the Rama and the Shach it is permissible to clean dairy and meat dishes in the same sink one after another. Primarily, there is little concern of a transference of taste. Primarily, the concern that potentially there would be taste in the bottom of the sink from a piece of dairy upon which hot water was poured and in that same spot also have taste from meat upon which hot water was poured within 24 hours of each other is highly unlikely. Nonetheless, avoid all issues one should have a board for meat and milk so that the dishes don't touch the bottom of the sink.
- The Minchat Yitzchak 2:100 is stricter because of several concerns about the water being able to transfer tastes into the sink walls and potentially back into the dishes. He seems to be concerned for an expanded definition of hot water being poured which can transfer taste and also suggests that since the faucet water is connected to the urn which is on the fire the entire stream is considered on the fire and not like regular poured water (see there). Therefore, he holds that one should only wash meat and dairy dishes in the same sink one after the other upon the following conditions: 1) One washes the dairy dishes in a dairy sink insert and the same for meat. 2) The insert sits upon a board or rack so that the insert doesn't directly touch the walls of the sink. 3) The sink is cleaned well between each cleaning of the dishes. 4) The drain is open so that the hot water doesn't submerge the dishes.
- Both opinions are cited by the Badei Hashulchan 95:80.
- Hot water poured from a kli rishon, according to the Rashbam (Tosfot Shabbat 42b s.v. aval), is considered like a kil sheni which doesn’t cook. However, the Ri also cited by Tosfot holds that it cooks like a kli rishon. Finally, Tosfot holds that it cooks up to a peel. (See igrot Moshe YD 1:42 regarding whether poured hot water is formally considered cooking up to a peel or is it just an ability to impart taste). Shulchan Aruch YD 68:10 holds like the Tosfot that it only cooks up to a peel. Shach 105:5 and Kaf Hachaim 105:31 agree.
- Shach 95:20
- Rama 95:3 holds that hot liquids while being poured can only can taste from one food and impart it into another food but can’t impart taste into an utensil which is hard. The Shach 95:20 argues based on the Hagahot Shaarei Dura that poured hot liquids can cook up to a peel in a food or utensil.
- Rama 95:3
- Shach 95:20
- Pri Chadash (cited by Badei Hashulchan 95:77). See also Rabbi Akiva Eiger on the Shach 95:20 regarding if one was dirty that this should be an issue for both dishes.
- Badei Hashulchan 95:77
- Shach 105:5 says that a stream that was poured and was broken can’t cause a transfer of taste from one food into another but can cause a transference of taste. Kaf HaChaim 105:32 limits this to food and says that it wouldn’t cause taste to be imparted into a pot.
- Sefer Kashrut HaShulchan (Baser BeChalav 6:3) quoting Sh"t Yabia Omer 5:3
- Badei Hashulchan 95:81
- Yalkut Yosef (Otzar Dinim L'isha p. 618), Yalkut Yosef (Isur Vheter v. 3 p. 485), Yabia Omer YD 10:4
- Shulchan Aruch YD 91:8, Shach 87:2
- Shulchan Aruch YD 105:1
- Kaf HaChaim 105:6
- Kaf HaChaim 105:1
- The Shach 105:2 and Taz 105:1 argue that it is permitted since by the time that the forbidden taste can be absorbed into the permitted food the taste in the pot is already considered ruined. The Kaf HaChaim 105:3 writes that because it is a dispute and the Pri Chadash holds like the Isur Veheter a person should only be lenient in cases of great loss. The Laws of Kashrut (p. 269) seems to side with the opinion of the Shach but advises asking a rabbi.
- The Torah (Devarim 14:21) says that you should give the slaughtered meat (nevelah) to a non-Jew. From this the Gemara Avoda Zara 67b learns that any food which is inedible isn’t considered forbidden.
- Rosh Pesachim 2:1 says that it is asur on pesach to eat chametz that was completely burnt before pesach because your eating it indicates that you want to eat it. See the Ran Pesachim 5b who disagrees with the Rosh. The Taz OC 442:8 terms this achshaveh, you gave it significance, and he says that it is only derabbanan. Mishna Brurah 442:43 rules like the Taz.
- What about medicines on pesach is there achshaveh? Rav Moshe Feinstein (Igrot Moshe OC 2:92) held that there’s no achsheveh by medicines. Shagat Aryeh 75 held that eating medicines is like eating and there is achsheveh.
- Avoda Zara 75b, Shulchan Aruch 103:1
- The Ran (Avoda Zara 32b) forbids if there is a greater benefit in the increase of volume than the loss in taste since in the end of the day there is a benefit from the prohibited food. The Ran cites the Rashba that he would be lenient since actual forbidden item was nullified by a majority of permitted ingredients and the taste is detrimental. The Shulchan Aruch 104:2 quotes both opinions. The Rashba (Torat HaBayit 20b) implies that even if there is a majority of the detrimental taste it is still permitted. Aruch Hashulchan 103:5 makes this explicit. Kaf HaChaim 103:17 rules like the Ran unless there is great loss.
- Rashba (responsa 1:499) and Yereyim (Achilot Siman 52) are strict regarding the negative taste of chametz in a mixture on pesach since chametz is forbidden in any quantity. Tosfot Avoda Zara 66a s.v. meklal seems to permit it. This is also the opinion of the Rosh (Avoda Zara 5:6). Shulchan Aruch OC 447:10 permits noten taam lifgam of chametz on pesach, while the Rama there is strict.
- Kaf HaChaim 103:3, Chavot Daat (Biurim 103:1) unlike the Peleti 87:15