Milk and Meat in the Kitchen
This is the approved revision of this page, as well as being the most recent.
For halachic questions of Kashrut a person should consult a rabbi because the laws of Kashrut involve many complexities.
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 informs 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.
- Can you sell a utensil which has absorptions of meat and milk? Some poskim say that it is permitted since one isn’t directly benefiting from the absorptions as much as the actual pot. Others hold that one should only sell it after 24 hours.
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.
Cups and Glasses
- One shouldn't drink from the same cup when one was eating meat and when one is eating dairy unless one cleans the cup in between in which case it is totally permitted.
- 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.
- Some say that there's a minhag to have separate dairy and meat drinking cups. Others argue that one can use the same glasses for milk and meat meals as long as they are washed in between. They can be used for cold or hot parve drinks.
- It is permitted to use a cold drinking glass in a non-kosher restaurant for a cold drink. But one should only have a hot drink in a disposable cup.
- One shouldn't use a knife that was used to cut a dairy food to cut bread that one is planning to eat with meat. The same is true of the opposite case. The reason is because we're worried that a knife in general has a bit of fat stuck onto it.
- One shouldn't use a knife that was used to cut a meat food to cut a dairy food or the opposite.
- Some have the practice to have a parve bread knife that is designated to cut bread so that the leftover bread can be eaten without a concern of mixing meat and milk.
- If a person ate meat on a tablecloth he should change the tablecloth before eating milk. The same is true vice versa.
- Some poskim are lenient that one doesn't need to change the tablecloth between meat and milk since nowadays we eat on plates. Others are strict.
- One shouldn't leave an open bowl or cup with salt next to a dairy liquidy food since accidentally some of the dairy might splatter into the salt and later one might use that salt for meat.
- It is recommended to have two salt shakers since the shakers might touch food or residue on people's hands might get on the shaker.
- If a person cooked parve food in a meat pot and then dairy food was cooked in that pot within 24 hours of the meat the dairy food is considered non-kosher and the pot needs to be koshered.
Hot Cover on Hot Pot
- If one uses a meat pot cover to cover a milk pot while it is on the fire or the opposite, the food, pot, and the cover are non-kosher.
- If the pot is currently cooking parve and the pot or the cover weren't used within 24 hours, the food would be kosher and the utensil which was used within 24 hours remains kosher, but the one which wasn't used within 24 hours is non-kosher. If both weren't used within 24 hours the food, the pot and the cover are kosher.
- If the food in the pot is meat then that makes the pot and the cover absorb taste of meat immediately even if it wasn't used within 24 hours for other meat. The same is true of milk.
- The same is true whether or not the cover was hot as long as the pot was hot.
- Some say that we can't assume a pot cover that we're unsure if it was used within 24 hours that is considered eino ben yomo.
Hot Cover on Cold Pot
- If one accidentally placed a hot cover on a cold pot, some say that the food, the pot and the cover are all still kosher. Other poskim hold that the pot needs to be koshered. Poskim are strict unless there is financial loss involved.
- The above only applies when the hot lid was wet or moist, but if it was totally dry then everything is kosher. Without any more information we have to assume that a hot lid that was on a cooking pot and removed is still moist.
Cold Cover on Hot Pot
- If someone accidentally places a cold cover on a hot pot, and it was left there for a while it is treated like we treat a hot cover on a hot pot, described above.
Chumra of Non-Ben Yomo Lids
- In the Ashkenazi rishonim there was a minhag to be strict on all lids to treat them as though they are ben yomo even if they are certainly eino ben yomo. Some try to provide justification for the minhag but ultimately it is a difficult minhag to explain. Although the Rama 93:1 tried to abolish the minhag, nonetheless, in his notes to Shulchan Aruch he writes that some have this minhag. He adds that if there are any other factors to be lenient in this case one can be lenient so as not to add to this minhag which is difficult to begin with.
- The Maharshal (Chullin 8:46:7) explains that the chumra (stringency) only applies to cover pots with a section that are hard to clean and so there could be actual food frosted on the inside of the lid that can't be cleaned and therefore it could forbid the food even when it is not ben yomo since food doesn't spoil by sitting out for 24 hours. Therefore, if one can be sure that the lid doesn't have any area that is hard to clean or had enough in the pot to nullify the food that could be stuck to that section of the cover the food is kosher. This approach is adopted by the overwhelming majority of poskim. It follows that for the common modern pot covers this isn't an issue.
- Sephardim don't have this minhag.
- According to the chumra, some say it that if one cooked parve in a ben yomo meat pot and covered it with a eino ben yomo dairy cover or the opposite, the food should be eaten with meat since it is meat equipment but regarding the pot cover some say that it is forbidden, while others are lenient.
- It doesn't apply to anything besides a designated cover and not if you put a pot on top of another pot.
- Some say that after two months certainly the pot lids have the status of eino ben yomo.
- If there is an area that is hard to clean and we assume that there's some food that collected there, some say that it is necessary to have 60 times that area to nullify it, while others say you need slightly less than 3660 times that area to nullify it.
- It is permitted to leave meat and milk foods in the refrigerator without designating an area or shelf for meat or milk. However, one should be careful to make sure not to leave something that could spill or leak such as a milk carton over a meat food.
Food that was on the Table
- 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.
Drop of Milk on Meat in Pot
- If a drop of milk falls into the liquid part of a soup or stew it is nullified in that soup if the food in the pot is sixty times the drop's volume.
- If a drop of milk falls onto a piece of meat that is partially submerged in the liquid of a soup or stew it makes that piece forbidden unless there is sixty times its volume in that piece. However, with respect to the rest of the food in the pot it wouldn't forbid it unless there wasn't sixty times the drop's volume in all of the food of the pot.
- If a drop of milk falls onto a piece of meat that is completely out of the liquid part of the soup or stew that piece would be forbidden unless there is sixty times its volume in that piece. According to many poskim the rest of the food in the pot would be kosher.
Drop of Milk on Outside of Pot
- If there's a drop of milk that splashed on a meat pot on the fire that is cooking, if the drop falls on a spot that corresponds to the area where the food is cooking that drop of milk is nullified in sixty times the contents of the pot. 
- However, if the drop falls on a spot that is above the area where the food is cooking the top of the pot becoming non-kosher and then the food in turn is non-kosher unless there is 3600 in the contents of the pot to nullify the drop.
- The only time that 3600 is necessary is if the pot was used within 24 hours for meat and then the drop fell on the outside of the pot above the line where it is was cooking on the inside of the pot meat. However, if it wasn't used in 24 hours for meat then only sixty is necessary.
- In all of the above cases even if the food is kosher the pot needs to be koshered. In a case of need, Sephardim are lenient if the milk fell on the outside below the line where the food was cooking. The pot should be koshered on the inside and outside.
Meat and Dairy Equipment (Nat Bar Nat)
Parve Food Cooked with Meat or Dairy Equipment
- Parve food cooked in a meat pot according to Ashkenazim should not 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 according to Ashkenazim should not 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. Some even permit cooking the parve food in the meat pot to eat it with dairy.
- 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.
- Parve food cooked in a meat pot can be eaten with dairy utensils but the parve food shouldn't be poured directly from the meat pot onto a dairy utensil. The same is true of the opposite case. It is permitted to even initially indirectly pour (Iruy Shenifsak Hakiluach) the food into the dairy utensil from a meat pot.
- It is permitted to reheat food made with dairy equipment in a meat pot.
- According to Ashkenazim and some Sephardim one shouldn't cook the parve in a meat pot in order to eat it with dairy or vice versa. After the fact even if one intentionally made the food in a meat pot in order to eat with dairy most poskim hold that it is nonetheless permitted to eat with dairy.
- Tasting transferring from food to food isn't considered nat bar nat. Nat bar nat needs to involve a utensil. If there is a transfer from food to utensil to food that is nat bar nat. From utensil to food to food or food to food to utensil are questionably nat bar nat.
Roasting, Baking, Soaking
- Some Ashkenazim are more strict if the parve food is roasted or baked as opposed to cooked in a dairy equipment to treat it like dairy food, while others consider it like the general category of dairy equipment. This has ramifications even after the fact if that parve food is mixed with meat. The same is true of parve food roasted or baked with meat equipment. Most poskim hold that roasting is no different than cooking.
- If the parve food soaked in a meat or dairy pot there is an unresolved discussion if that would render the food meat or dairy equipment.
Dairy Spoon Used to Mix Meat Pot Cooking Meat
- If a dairy spoon was used to mix a meat pot with meat in it, if there isn't sixty in the pot relative to the amount of the spoon that was inserted into the food, the spoon, the food, and the pot are rendered non-kosher. If there is sixty in the pot the food and pot are kosher but the spoon is rendered non-kosher. Alternatively, if the spoon wasn't used for dairy within 24 hours the food and pot are kosher but the spoon is rendered non-kosher.
Dairy Spoon Used to Mix Meat Pot Cooking Parve
- If someone used a dairy pot, used for dairy within 24 hours, to cook parve and stirred it with a meat spoon, used for meat within 24 hours, the pot, the food, and the spoon are forbidden according to Ashkenazim.
- If the dairy pot and the meat spoon weren't used within 24 hours for their respective kind then they have no effect upon each other. Nonetheless it is best to eat the food parve not with meat or milk.
- If the dairy pot or the meat spoon weren't used within 24 hours for their respective kind then they have no effect upon each other. However, the minhag is to treat the food like the one that wasn't used within 24 and the utensil that wasn't used within 24 hours as non-kosher.
- If a dairy spoon was used to mix a parve soup or food with liquids in a meat pot, according to Ashkenazim the food and the pot are rendered non-kosher. If there is sixty times the spoon in the soup the food and pot are kosher and the dairy spoon is rendered non-kosher. If the dairy spoon wasn't used within 24 hours the food and pot are kosher and the spoon is rendered non-kosher. According to Sephardim the food, pot, and spoon are kosher.
- If a dairy spoon was used to mix rice, noodles, or vegetables in a meat pot on the stove where there aren't liquids between the foods, some poskim hold that for Ashkenazim the food, the spoon, and the pot are rendered non-kosher. Others hold that only the food is non-kosher, while still others hold that everything remains kosher. Sephardim hold like that everything remains kosher.
Dairy or Meat Spoon Used in Parve Pot
- If a person used a dairy spoon to mix parve food in a parve pot then we should treat that pot like it is dairy. The same is true if a person used a meat spoon to mix parve food in a parve pot.
- If a person used a dairy spoon to mix parve food in a parve pot rendering the pot dairy and then a meat spoon was used to mix parve food in that same pot, that pot shouldn't be used at all. Some are lenient to say to treat the pot as meat. After the fact if a person used that pot the food isn't forbidden.
- If the parve food in the pot was sixty times the spoons then they have no effect on the pot. Additionally, if the spoons were not used within 24 hours for meat or milk they have no effect on the pot.
Transference of Taste
See the complete topic here: Transferring 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 you mix rice with milk or butter and the milk or butter is completely absorbed and then the rice is mixed with a meat spoon the rice is non-kosher but the spoon and pot are kosher, some however hold that the rice, spoon and pot aren’t kosher.
If there are 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 hot meat from a kli rishon fell into cold milk or hot milk from a kli rishon fell onto cold meat the meat needs to have a klipah removed and the milk is permitted.
- If the klipah wasn’t removed and it was cooked after the fact the it is permitted but if the klipah is recognizable it needs to be removed.
- If a cooked piece of hot meat from a kli rishon fell into a cold dairy salty liquid if it was raw it could be washed off. If it was cooked it would be forbidden up to the level of a klipah. If it was spiced or has cracks it would be entirely forbidden. According to Ashkenazim even cold meat that fell into a cold dairy salty liquid is forbidden up to a klipah. If it has cracks or is spiced it is entirely forbidden.
- 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. Some Ashkenazic poskim permit using an oven for meat and milk consecutively as long as there are no spills or residue on the oven walls or floor.
- 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.
- The primary reason that cooking in an oven could transfer taste from the food to the walls or the opposite is through the mechanism of zeyia, steam, or more accurately defined water vapor. Generally, the poskim hold that the zeyia of a food that is cooking contains the taste of the food and transfers its taste.
- Some poskim hold that there is no issue of zeyia in an open area but most poskim disagree. Therefore, it is advisable not to pour salt from a saltshaker into an open pot cooking on the fire since the zeyia from that food will get absorbed in the saltshaker. If that happens if the food is meat then the salt becomes meat and if the food was dairy then the salt becomes dairy.
- Some poskim considered the possibility that zeyia doesn't get into a food that is itself steaming. It certainly isn't accepted but some poskim use it as a factor. Others disregard this idea.
- Some poskim hold that solid foods do not have any zeyia and only liquids have zeyia. Many poskim disagree.
- A minority opinion holds that zeyia is burnt up in an oven but isn't accepted.
- A minority opinion holds that zeyia dissipates in an oven that has a small vent.
- Zeyia which isn't yad soledet bo can't transfer taste. Therefore, if meat is hanging above a pot of dairy that is cooking if it is so high above the pot that the steam isn't Yad Soledet Bo there is no concern.
- A covered pot can't transfer zeyia.
- If one cooks two foods in the oven at the same time there is a smell (trans. reycha; Heb. ריחא) 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. Ashkenazim are lenient if one doesn't have other bread available.
- If a person cooked parve bread in the oven at the same time as he cooked a dairy bread it is best to treat the parve bread as 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 wasn'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.
Steam Jacket Kettle
- Some permit using a steam jacket kettle even if that steam was previously used for non-Kosher.
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 hits 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.
- One should certainly not clean dirty meat and dairy dishes together in one sink. After the fact some poskim hold that the dishes are fine, while most assume that this would cause the meat and dairy dishes 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. 
- After the fact, if there were no pieces of meat, dairy, or even grease on the dishes and they were washed together in the same sink the dishes are all kosher. 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.
- After the fact, if the meat dish had a piece of meat on it or even just grease and the dairy dish was clean, and they were washed together in the sink, both dishes are considered non-kosher. In a case of a large loss some say that the meat dish would be considered kosher, while the dairy would be considered non-kosher. The same is true of the opposite case.
- 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.  Some say that technically, it is permitted but best to avoid.
- According to Ashkenazim, some say that 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.
Accidental Placement of Wrong Type in Sink
- If one accidentally placed a utensil of the wrong type in the sink and it was cold everything should be washed with cold water and is permitted. If the utensil was hot but was placed there with only parve food in the utensil then it is permitted, especially if the area between the utensil and sink was dry but it is permitted even if it was wet.
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.  The definition of liquid for this purpose is something that jiggles when a person moves the container.
- 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. There is a discussion if metal and glass pots absorb taste through soaking.
- 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.
- If an onion or another sharp food is cut with a meat or milk knife or is cooked in a meat or milk pot see the Sharp Foods page.
- 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.
- Tosfot Avoda Zara 32a s.v. vhacha cites a dispute whether one could get benefit from a pot that has absorptions of something that itself is forbidden in benefit. Rabbenu Tam is lenient. Maharshal Yam Shel Shlomo 8:46 (cited by Shach 94:11) is strict not to benefit from meat and milk cooked in a pot by selling the pot, however, the Chatom Sofer 98 is lenient. Badei Hashulchan 94:35 writes that someone who is lenient has what to rely upon.
- 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. Laws of Kashrut p. 182 is strict.
- 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
- Hagahot Ashri 8:20 from Rav Klonimus and Rav Yehuda, Rama Y.D. 88:2
- Knesset Hagedola (Hagahot Hatur 88:7), Darkei Teshuva 88:29
- Aruch Hashulchan 88:11
- Badei Hashulchan 89:110
- Rabbi Forst (Laws of Kashrut p. 359)
- Rashba teshuvot meyuchasot 172 writes that one shouldn't use a dairy knife to cut meat or bread that one is going to eat with meat. He explains that a knife generally has smeared fat or grease stuck to it. Shulchan Aruch Y.D. 89:4 codifies this Rashba.
- Shulchan Aruch Y.D. 89:4
- Laws of Kashrut p. 211 citing Darkei Teshuva 89:53, Badei Hashulchan 89:111 citing Maharshal, Pri Chadash, and Aruch Hashulchan. The Maharshal Yam Shel Shlomo 8:8 writes that he saw some had the righteous practice to have three knives, one for meat, one for milk, and one for bread. Pri Chadash 89:24 agrees. Badei Hashulchan Biurim 89:4 s.v. lihiyot isn't sure if this applies to all foods and not just bread. Also, he isn't sure if it applies if one is cutting bread that one plans to eat in that meal or only for bread that one is planning on leaving over after the meal.
- The Rashba (Teshuvot Meyuchasot 1:76) writes that one can't use the same tablecloth for meat and milk since there is certainly going to be spills of meat and milk on the tablecloth. This is codified by the Shulchan Aruch Y.D. 89:4. The Gra 89:19 sources this practice in the Yerushalmi Pesachim 6:4.
- Radvaz 2:721 writes that since people eat on plates and not directly on the tablecloths perhaps there's no concern about the tablecloths and they can be used for meat and milk. He concludes it is proper to switch the tablecloths. This is codified by the Pitchei Teshuva 89:8. Horah Brurah 89:52 writes that it is permitted to use the same tablecloths and someone who is strict will be blessed.
- Badei Hashulchan 89:102 is strict because of the opinion of the Minchat Yakov 76:17 who says that even if you eat with plates there's a concern that food will spill on to the tablecloth and so it needs to be switched. He says that this is the minhag.
- Badei Hashulchan 95:92 writes that there's no fixed distance. It depends on where this is a concern that food might splatter from one utensil to the other.
- Badei Hashulchan 95:90 writes that some say that it is only an issue with Kutach, some say that it is also true of milk, and he isn't sure if potentially it applies to any dairy solid food.
- Gemara Chullin 112, Shulchan Aruch YD 95:5
- Laws of Kashrut p. 360
- The Rashba (Torat Habayit 38a) writes that if someone cooks vegetables in a meat pot we can treat that cooking like a mini-koshering of the pot and it can be used for dairy afterwards. His proof is Rava in Gemara Avoda Zara 76a who says that cooking korbanot in a pot koshers it from its previous absorptions. The Rashba explains that cooking is only a mini-koshering if the absorption was permitted such as milk or meat or korbanot. However, it isn’t a sufficient koshering for a forbidden absorption. Raah Bedek Habayit 38a agrees with the Rashba and Orchot Chaim Isurei Machalot s.v. hachamishi, Rabbenu Yerucham 15:28 138b, and Tur 93:1 codify it.
- However, Ritva Avoda Zara 76a s.v. vyesh and Ohel Moed Pesach 3:3 argue with the Rashba. The Ritva has another reading of Rava and the Ohel Moed is bothered how cooking can serve as koshering if it didn’t kosher the rim of the pot. (The Gra 94:11 also argues that it could be that we reject the opinion of Rava altogether. See however many rishonim who quote Rava as being accepted: Rashba Torat Habayit 38a, Ritva Avoda Zara 76a s.v. vyesh citing Raah, Maharam Chalavah Pesachim 30b s.v. vhilchata, Mordechai Pesachim n. 563, Rosh Chullin 7:31, Or Zaruah Basar Bchalav 1:467, and Bartenura Vayikra 6:21.)
- Bet Yosef 93 argues that we don’t follow the Rashba and demonstrates it from several places where we don’t assume that cooking is considered a mini-koshering. The Shach in Nekudat Hakesef 93:1 argues that the Rashba was talking about a complete koshering, that is, with cooking food up to the top of the pot and at a complete boil. He also thinks that this is completely accepted by the poskim to allow this as hagalah and hagalah is effective for a permitted absorption for everything. Either way, the idea of a mini-koshering because of cooking food isn’t accepted. This is also the conclusion of the Hagahot Shaarei Dura 85:10.
- Shulchan Aruch Y.D. 93:1
- Shulchan Aruch Y.D. 93:1
- Shulchan Aruch Y.D. 93:1
- Rama 93:1
- Darkei Teshuva 93:25 citing the Yavetz
- Rama 93:1
- Shach 93:6, Pri Chadash 93:9, Aruch Hashulchan 93:18. The Isur Vheter 31:18 holds that the moisture on the hot lid is considered like a kli sheni and not iruy since it is a drop and not a stream, while the Hagahot Shaarei Dura 56:1 disagrees and considers that drop to cause absorptions to forbid the pot and lid. Darkei Moshe 93:2 cites this dispute and in Rama 93:1 he rules like the Isur Veheter. Shach 93:6 holds like the Hagahot Shaarei Dura arguing that the lid is itself a kli rishon so the drop retains its status of iruy. Pri Chadash partially agrees with the Shach but only seems to forbid the pot and not the lid. Badei Hashulchan 93:1 s.v. vim hakisuy explains that the Pri Chadash holds that a drop doesn't have the ability to absorb taste and then transfer it. This is in line with the Chavot Daat 92:20 and 105:13 unlike the Yad Yehuda 92 (cited by Badei Hashulchan 92:8 s.v. ashtey kederot). Chavot Daat thinks that a small amount of moisture can absorb taste but not impart it because liquids also absorb tastes like foods and as such a food can't impart flavor to something else without a liquid or fat (Shulchan Aruch YD 105:7).
- Chachmat Adam 46:5, Badei Hashulchan 93:31, Laws of Kashrut p. 270
- Aruch Hashulchan 93:18, Chachmat Adam 46:5
- Shach 93:6, Chachmat Adam 46:5, Badei Hashulchan 93:31
- Rama 93:1
- Hagahot Shaarei Dura 35:5. However, see Iser Vheter 31:20 who assumes otherwise.
- See Darkei Moshe 93:2, Kereti 93:6, and Masat Binyamin 58 who give explanations for the minhag.
- The Pri Chadash 93:5 writes that the truth is that there's no basis for the minhag and it should be abolished. Levush 93:1 also rules entirely against this minhag.
- Bear Hagolah 93:4, Shach 4, Taz 93:2, Gra 93:7, Pri Chadash 93:5, Badei Hashulchan 93:20, Madanei Hashulchan 93:12
- For example, Laws of Kashrut p. 272 fnt. 37 doesn't cite this chumra at all other than in the footnote to see the Shach and Taz.
- Kaf Hachaim 93:20
- Badei Hashulchan 93:21
- Chamudei Doniel Tarovet 28, Darkei Teshuva 93:26
- Badei Hashulchan 93:22 based on Masat Binyamin 18
- Pitchei Teshuva 93:4 citing Bnei Chiya. See also Darkei Teshuva 93:23 regarding the dispute between the Knesset Hagedola and Chamudei Doniel.
- Pitchei Teshuva 93:3 citing the Shvut Yakov 1:21
- Taz 93:2 writes that it is sufficient to have 60 times the area, while the Shach 93:4 says you need slightly less than 3660 times the area since that food stuck there becomes forbidden in it of itself (chanan). Rabbi Akiva Eiger 93:1 sides with the Taz. Darkei Teshuva 93:21 cites the Minchat Yitchak 16b who makes a compromise. If the lid was placed on the pot before it started to steam up one should be strict, however, if the pot was already boiling when the lid was placed on then one could follow the Taz.
- Laws of Kashrut ch. 13 p. 350. Yalkut Yosef Isur Vheter v. 3 88:3 adds that one can leave the meat and milk next to eat other in the refrigerator as long as they don't spill upon each other but one should be careful about this initially. See however, the Badei Hashulchan 95:101 who writes that a person should designate areas or shelves in their refrigerator for meat and milk and not switch them. This is based on the Rama 95:6 and Maharshal who say that a person should not place dairy foods where they usually place meat foods in case something spilled. Zar Hashulchan 95:73 agrees with the Badei Hashulchan.
- 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 92:2
- Rama 92:2 according to Shach 92:13 and Taz 92:5. See however, the Gra 92:9 who emends the text of the Rama to require sixty times the volume of the entire piece the milk fell on in the rest of the pot for the rest of the pot to be permitted. Also, Rabbi Akiva Eiger 92:3 reads the Rama entirely differently. Badei Hashulchan 92:38 is unsatisfied with the Shach and Taz's explanation. Either way, the rest of the pot is kosher if there's sixty times the volume of the milk according to the Ri and the Shach 92:5 follows the Ri, as does the Gra 92:18. Badei Hashulchan 92:19 is strict for Rashi unless there's great loss.
- Tosfot Chullin 108a, Shulchan Aruch 92:2
- Both according to Rashi, Tosfot Chullin 108a, and Rashba (Torat Habayit Haaruch 8b) the rest of the food in the pot is permitted if the drop of milk fell on a piece of meat that is completely out of the liquid of the soup or stew. However, according to the Ran and Raah the rest of the pieces near that piece become forbidden up to the thickness of a fingerbreadth. The Shulchan Aruch 92:2 follows the opinions of Rashi and Tosfot. Taz 105:13, Gra 105:40, and Maharshal Chullin 8:45 also follow the Rashi and Tosfot. However, the Shach 105:17 follows the Ran and Raah. Badei Hashulchan 105:99 follows the Taz.
- Shulchan Aruch 92:5
- The Smag Lavin 140 holds that the drop of milk that falls on the outside of a meat pot which is cooking travels throughout the entire pot and makes it non-kosher even beyond sixty times its volume and even if there's more than sixty times its volume of food in the pot.
- The Tur's 92:5 first opinion is that the milk on the pot wall spreads itself out in stages and makes it completely forbidden even though it is larger than sixty times the volume of the milk. That is the understanding of the Bet Yosef. However, the Bach 92:10 and Taz 92:19 offer another approach in which there is no such opinion and the Tur was merely setting up the opinion of the Maharam.
- The Raavan 272 and 311 holds that automatically the drop is dispersed and nullified into the volume of sixty times itself in the walls of the pot.
- The Maharam (Parma 515, cited by Bet Yosef 96:5) and Ri hold that the drop can spread up to sixty times itself and make the walls of the pot forbidden. Then the content of the pot would become forbidden unless there is sixty times that area of the wall which became non-kosher, which itself is sixty times the original drop. Altogether the food would be non-kosher unless the meat food in the pot is 3600 times the volume of the drop.
- The Smak 213 has a compromise. If the drop falls on the outside of the pot below the line where there is food on the inside of the pot is cooking then the drop is completely absorbed into the food and is nullified in sixty. However, if the drop fell on the outside of the pot above the line where there is food on the inside of the pot then the drop makes the area of the wall it touched forbidden up to sixty times itself and in turn forbids the food unless there is sixty times sixty of the drop. Shulchan Aruch 96:5 follows the Smak.
- There’s three ways to read the conclusion of Zevachim 96b: either the pasuk shows us that absorptions travel completely throughout the walls of a pot (Smag) or that there’s no absorptions in the pot besides where you cooked (Maharam) or that it is a unresolved in the Gemara (Smak 213).
- The Shach 96:20 requires slightly less than 3660 in order to nullify the drop. Pri Megadim S"D 92:20 explains those who say 3600 based on the opinion of the Rambam that for derabbanan's you only require 59 to nullify it and milk absorbed in a pot cooking with meat is only rabbinic.
- Shulchan Aruch 92:5
- Rama 96:5
- Isur Vheter 31:7, Rama 92:6, and Shach 96:19 hold that in any event the pot needs to be koshered since the milk might have been absorbed in the walls and will come out next time it is used. However, the Taz 96:17 sides with the Maharshal who holds that the pot doesn't need to be koshered if the drop fell on the outside of the pot below the line where the food was cooking on the inside. Gra 96:31 implies that the pot only needs to be koshered if it is earthenware.
- Horah Brurah 92:68. In the biurim he explains that according to the Bet Yosef's explanation of the Smak it isn't necessary to kosher the pot. Either the milk traveled into the food or it spread itself out in the pot walls and became nullified by the absorptions in the walls. But either way the absorption isn't stuck in the walls.
- Pitchei Teshuva 92
- Shulchan Aruch and Rama YD 95:1. Gemara Chullin 111b cites a dispute between Rav and Shmuel whether a fish cooked in a meat pot can be eaten with dairy. The dispute depends on whether a taste imparted from a food into a utensil and back into a food is considered a significant taste. The conclusion of the gemara is that it is permitted.
- The Rivan (cited by Tosfot s.v. hilchata) holds that it is true only if the fish was placed on a hot plate but not if it was roasted or cooked on a meat utensil.
- The Sefer Hatrumah (61) holds that it is true only if the fish was cooked in the meat utensil but not roasted. The Rosh Chullin 8:30 agrees.
- Rashi (111b s.v. nat) writes that it is true whether the fish was cooked in the meat pot or was roasted on meat utensil.
- Shulchan Aruch and Rama Y.D. 95:1
- A few rishonim (Smak, Hagahot Maimoniyot, and Rabbenu Yerucham cited by Bet Yosef 95:1) write that nat bar nat is only permitted to be cooked with milk after the parve food was cooked in the meat pot and this is also the consensus of the majority of the achronim. The achronim debate why nat bar nat is only permitted after the fact. (See next note for discussion of those who hold it is totally permitted even initially.)
- Some (Minchat Cohen 1:12 s.v. vheneh dino as understood by Pri Megadim M"Z 95:4) say that it is like ein mevatlin isur lechatchila because nat bar nat is similar to nullification. Others (Pri Chadash 95:1) hold it is a unique prohibition in this context. This argue whether after the fact if one intentionally created nat bar nat whether it would be forbidden.
- Why is nat bar nat d'isura, secondary taste of something forbidden, forbidden? Rashba (responsa 1:516) and Ran Chullin 41a hold that it is forbidden because nat bar nat leaves over only a weak taste of the original food (Heb. טעם קלוש; trans. taam kalush) and for forbidden foods any remnant of the original taste is forbidden. For permitted tastes we are lenient since that weak taste in the first food became permitted, as though it is not there, and it is impossible to become forbidden even when it is later cooked with milk. They compare it to the leniencies we have for kashering spits for kodshim which began as something permitted as opposed to other prohibitions (Avoda Zara 76a). On the other hand, Smak (Hagahot Smak 213 Bitul n. 6, cited by Bet Yosef 94:5) forbids nat bar nat of something forbidden because we say that the forbidden taste makes the entire first food forbidden and is measured as though it is completely forbidden (Heb. תחיכה נעשה נבילה; trans. chaticha naaseh nevelah). For permitted things there is no concept that the food that was infused with a permitted taste is considered as though it is completely filled with that taste. This approach seems to think of nat bar nat as as way of permitting using nullification.
- Nat bar nat with isur mashahu: Ritva Chullin 112a s.v. garir writes that nat bar nat does not even leave a minute amount of the original food and as such is permitted. See also Magen Avraham 447:31. Gra YD 108:9 uses this to explain why nat bar nat is permitted after cooked and does not need to be avoided at that point. This seems to work best with the Rashba and Ran's approach above, but according to the Smak there is always a minute amount of the original taste that remains in the secondary taste although it is nullified.
- Rav Ovadia Yosef in Yabia Omer YD 9:4 and Rav Shlomo Amar in Shema Shlomo 1:2, 2:4-5. Or Hahalacha (by R' Makis 95:1) is lenient and cites Halichot Olam (Korach n. 11) and Tefila Lmoshe 3:12. However, Horah Brurah 95:1 holds that it is only permitted after the fact. He cites Zivchei Tzedek 95:2 Kaf Hachaim 95:1, Ben Ish Chai Korach 13, Rav Ben Tzion Abba Shaul, and Shemesh Umagen 2:42 who hold that nat bar nat is only after the fact.
- 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. Laws of Kashrut p. 162 fnt. 44 cites Rav Elyashiv has holding like the Chachmat Adam unlike the Gra. Rav Mordechai Willig in Chevrusa Oct. 1990 p. 5 agrees.
- Shulchan Aruch YD 95:1
- Rama Y.D. 95:2. The Shach 95:5 explains that one shouldn't pour directly from the meat pot onto the dairy utensil as the Rama writes however after the fact it is permitted since the infusions of nat bar nat taste are consecutive. Badei Hashulchan 95:27 agrees that we should be strict about pouring. He explains that the reason that we're lenient about using utensils of the other type for nat bar nat is because the entire concern of meat and milk here is rabbinic since there's no combination of actual meat or milk but only its tastes.
- Badei Hashulchan 95:72 quoting the Kereti Upeleti and Pri Megadim. Pri Megadim MZ 95:5 writes that one should not initially cook parve food in a meat pot in order to eat on a dairy plate. Badei Hashulchan 92:30 and Madanei Hashulchan 95:23 adopt this stringency.
- Rama 95:2 permits eating dairy equipment food on meat plates. Aruch Hashulchan 95:13 adds that it is permitted to even put it in a meat pot that is still very hot and within 24 hours of its original absorption of meat. The reason for this leniency is as the Shach 95:5 explains that once the nat bar nat taste is in the food without any introduction of a forbidden taste it is permitted. OU for this reason permits reheating a parve food made in a dairy pot in a meat pot. Rav Yaakov Yungreis in an lengthy article proves the same thing. However, Star-K argues that it is forbidden to reheat food made in dairy equipment with meat pot.
- Pitchei Teshuva Shraga p. 185 cites Pri Chadash 95:1, Kereti 95:1, Zivchei Tzedek 95:5, and Kaf Hachaim 95:5 as lenient, against Pri Megadim MZ 95:4 citing Minchat Kohen, Bet Meir 97 s.v. bs"a, Aruch Hashulchan 95:10 as strict. Ben Ish Chai Shana Sheniya Korach n. 14 is lenient.
- Pri Megadim MZ 95:1, Chavot Daat 95:1
- Pri Megadim MZ 95:1 holds that nat bar nat must be from a food to a utensil and then utensil to food. However, the Imrei Binah 95:1 writes that from the Shaarei Dura 60 nat bar nat can't be from food to food but with any utensil that is sufficient.
- The Rama Y.D. 95:2 rules that after the fact whether the parve food was cooked, roasted, or baked with the meat equipment it is considered parve. Shach explains that once the parve food is mixed together with the dairy it is permitted. However, the Maharshal (Isur Vheter 57:2) is concerned for the opinion of the Rosh that even after the fact if the parve food was roasted or baked with meat equipment they are considered meat. Shach 95:4 cites this opinion. Pri Chadash 95:4 and Minchat Yakov 57:3 disagree and hold that roasting is only forbidden initially not after the fact. The OU (Halacha Yomi, Feb 12 2020) was lenient in a case where dairy equipment bread crumbs were baked on chicken, even though it is a dispute between the Rama and Maharshal, since it is possible that the bread crumbs were made with utensils that weren't used within 24 hours. See Shaarei Deah 95:2 cited by Darkei Teshuva 95:17 who is lenient with nat bar nat on chicken. Badei Hashulchan 95:23 is strict.
- Badei Hashulchan 95:25 is lenient for the Rama after the fact based on the Pri Chadash, Minchat Yakov, and Chachmat Adam 48:1. Aruch Hashulchan 95:12 is lenient. The Laws of Kashrus by R' Forst p. 159 leaves the dispute unresolved.
- Rabbi Akiva Eiger 95:2 poses a question whether food soaked in meat or dairy equipment is considered like food cooked in meat or dairy equipment or it is less of a concern. Badei Hashulchan 95:24 cites this. Darkei Teshuva 95:17 quotes a discussion about it including the Kereti who is strict and Shaarei Deah who is lenient.
- Shulchan Aruch Y.D. 94:3
- Shulchan Aruch Y.D. 94:4
- Rama 95:3, Badei Hashulchan 94:57
- Levush 95:3, Shulchan Aruch 94:4
- Badei Hashulchan 94:62
- Rama 94:5
- Shulchan Aruch Y.D. 95:3
- Badei Hashulchan Biurim on 95:3 s.v. afilu cites the Chavot Daat 94:26 who writes that for rice or noodles where the water is poured out the taste of the dairy from the spoon goes directly into the food and the taste from the meat goes directly into the food rendering it nat bar nat of meat and milk together which is non-kosher. Therefore, everything else is rendered non-kosher. The Pri Megadim M"Z 94:7 agrees regarding the food but not for the pots since the absorbed taste of the meat and the milk in the food doesn't spread to the pot without a liquid. Badei Hashulchan comments that we follow the Pri Megadim on this point since we hold that meat and milk aren't considered forbidden in it of itself but an absorbed taste (Taz 105 unlike Shach 105:17). The Yad Yehuda argues further that the food isn't rendered non-kosher since only the pieces that are in direct contact with the pot absorb the meat taste and only the rice in direct contact with the spoon absorb the milk taste and since absorbed taste doesn't transfer without a liquid everything is permitted. Badei Hashulchan leaves this discussion unresolved.
- According to Shulchan Aruch 95:3 who follows the Ramban it is allows considered nat bar nat of kosher tastes since the milk taste from the spoon that goes into the food can't forbid the meat taste from the pot that came into the food without first becoming a third derivative taste which is permitted. Rama follows the Sefer Hatrumah who is strict and therefore leaves room for the above discussion.
- Shulchan Aruch Y.D. 94:5, Shach 94:15, Taz (Daf Acharon 94:5), Chachmat Adam 48:3, Aruch Hashulchan 94:19, and Badei Hashulchan 94:53. The Shach and Taz both explain that the reason a person should use the parve pot that was used with a dairy spoon only for dairy is because once a person is going to establish it as dairy or meat anyway he should make it dairy since it accepted taste of dairy. However, in terms of principles of transferring taste, there is no concern of dairy taste in the pot since it is only nat bar nat that was infused into it.
- Shulchan Aruch and Rama Y.D. 94:5. See background and more opinions in next footnote.
- Shach 94:15.
- Hagahot Smak (213 n. 1*) writes that if a person used a dairy spoon and then a meat spoon with a parve pot that was cooking water the Smak held that the pot can't be used again, while the Rabbenu Peretz argues that the pot isn't effected since the transfer of taste from the spoons to the pot is only nat bar nat. In fact, there are three steps of transfer of taste making it three nat's (dairy to spoon, spoon to water, water to pot). The Kol Bo 106 s.v. maseh ba, Orchot Chaim (Hilchot Isurei Machalot s.v. maseh ba), Isur Vheter Haaruch 35:10, Hagahot Shaarei Dura 85:15 (Bar Ilan version 85:18), and Maharshal (Isur Vheter 85:2) record this dispute as well. The Taz (Daf Acharon 94:5) writes that the Rabbenu Peretz was only lenient when he was discussing a case that the meat spoon was used after 24 hours of the dairy spoon being used with the pot. If it was a case where they were both within 24 hours he would agree that the pot is forbidden. That justifies the Shulchan Aruch 94:5 and Rama.
- The Shach 94:15 (also in Kuntres Acharon 94:5) accepts that such was the case of Rabbenu Peretz however he would be lenient in either case. This is also the contention of the Isur Vheter Haaruch 35:11. Ultimately, as long as there are three nat's even those who are strict on nat bar nat would agree to be lenient and certainly after the fact. Therefore, the pot which had the dairy and meat spoon used with it sequentially is permitted. The Taz disagrees and explains that even though it is an example of nat bar nat, nonetheless one can't cause nat bar nat. If a pot is established as meat then it isn't make forbidden because of nat bar nat. On the other hand, since it was only made forbidden because of nat bar nat to begin with, it is established as lechatchila dairy and when the meat spoon is used afterwards it is lechatchila meat and that is a contradiction. Therefore, the pot can't be used for meat or dairy.
- Gra 94:16, Pri Chadash 94:14, Chachmat Adam 48:3, and Kaf Hachaim 94:47 follow the Shach. Aruch Hashulchan 94:18 and Badei Hashulchan 94:52 follow the Shulchan Aruch, Rama, and Taz.
- Rama 94:5
- Shulchan Aruch 94:5, Aruch Hashulchan 94:19, Badei Hashulchan 94:51
- Shulchan Aruch 94:5, Badei Hashulchan 94:50
- 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.
- Shach 105:22 cites the Maharshal (Iser Vheter 51:2) that if some milk was absorbed in rice or fish and then a meat spoon was used to mix the rice or fish both the food and the spoon are non-kosher. Taz 94:14 reiterates this position. However, Taz 105:16 implies otherwise since he says that if liver with blood in it was cooked with a pot the pot is still kosher since an absorbed taste doesn’t come out without any liquids (Shulchan Aruch 105:7).
- Pri Megadim M”Z 94:14 distinguishes between when the milk taste is absorbed completely by the rice and when regular milk is mixed together with the rice and partially sticks on the outside of the kernels or there is liquid between them. If it is just a taste that is absorbed then the milk doesn’t spread. However, if the milk is stuck to the outside of the kernels it would become absorbed into the spoon. Bet Shlomo Freedman v. 2 p. 102, Chachmat Adam 57:6, and Bet Meir on taz 94:14 agree.
- Chazon Ish YD 22:7 s.v. ha distinguishes between when milk is mixed in with the rice while it is cold then it would transfer to the spoon. However, if it is cooked into the rice then it is only considered an absorption and wouldn’t transfer to the spoon. He explains that this is also the intent of the Bet Meir’s final answer.
- Chavot Daat 94:15 answers that the Maharshal holds that we say that the milk leaves even though it is absorbs since it is from meat and milk it is like something prohibited in it of itself. Mizrach Shemesh 105:7 argues it can’t be the opinion of the Maharshal as we know from Shach 94:22 and 105:17.
- Therefore if the milk was completely absorbed with cooking both the approach of the Pri Megadim and Chazon Ish would be lenient unlike the Chavot Daat.
- 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.
- Shulchan Aruch 91:4
- Shach 91:8. The Isur Vheter 29 writes that anytime one can’t do klipah it is permitted since it was only a stringency when possible. Rama 91:7 codifies the Isur Vheter. Taz 91:7 holds like the Riva that a klipah needs to be nullified in sixty and isn’t permitted whenever it is impossible to do klipah. Pitchei Teshuva 91:5 quotes the Teshuvat Chen 11 who says that if it is an unbroken stream then the klipah is forbidden and needs sixty to be nullified. However, if it is a broken stream then it is permitted after the fact. 91:8 holds that after the fact if the klipah can be recognized it still needs to be removed.
- Hagahot Shaarei Dura 22:3 clarifies that roasting in this discussion isn’t specific and it is the same as being cooked in any other way as we know that food is softened by cooking more than roasting. Rama 91:7 codifies this.
- The Chagurat Shmuel 91:24 and Yad Yehuda Pirush Haaruch 91:37 cited by Darkei Teshuva 91:88 and 90 hold that the discussion is only with meat that fell into a liquid but not two solids that touch. See, however, Chavot Daat.
- The Gemara Chullin 112a concludes that if a piece of chicken fell into a dairy liquid if the chicken is raw it can simply be washed off. If it is roasted then it is forbidden up to a klipah unless it has cracks or spiced in which case it is completely forbidden. There are three approaches in the rishonim:
- The Rashba and Ran understand that the case of the roasted chicken is only an issue if it is still hot. Otherwise it would never absorb taste and could simply be washed off.
- The Rosh, Tur, and Sefer Hatrumah hold that it is true even if the roasted chicken is cold. Additionally, the gemara’s condition of having cracks is true even if it is raw.
- The Or Zaruah also maintains the gemara is relevant when the chicken is cold but the condition of having cracks is only relevant if it is roasted and not if it is raw.
- Shulchan Aruch 91:7 follows the Rashba and Ran. The Rama 91:7 follows the Or Zaruah but in a case of great loss is lenient to accept the Rashba. Shach 91:21 isn’t sure if we should follow the Rashba even for a great loss.
- 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 and advises being strict. Yavetz (1:93) is lenient and is adamant about this issue.
- Minchat Yitzchak 5:20 and Chelkat Yakov 2:136 are strict. Chut Shani Pesach 10:2 writes that one should kosher an oven between meat and milk by waiting 24 hours and heating it up to its highest temperature which is certainly libun kal.
- 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 issue 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. Torat Chatat 35:6 implies like the Pri Megadim. See Minchat Yakov 35:10 who is bothered because of Shulchan Aruch OC 451:15 which assumes steam even from dry dishes. He answers that it causes a transfer of a tiny bit and on pesach that is an issue, it is used many times, and when doing kashering we should do so lechatchila.
- Rav Hershel Schachter cited by Laws of Kashrut by Rabbi Pinchas Cohen p. 30 fnt. 7.
- 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, Chut Shani Pesach 10:2. He explains that although preheating it isn't going to be libun chamur it is sufficient to kosher it with libun kal since it is only hetera baala (Shulchan Aruch O.C. 509:5). 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 (Sh"t Peni Yehoshua 13, Yesodei Yeshurun v. 6 p. 158), 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.
- Rosh responsa 20:26, Shulchan Aruch 92:8. The source for zeyia is either from Mashkin 5:11 (Rosh) or Chullin 98b regarding pot covers (Gra 92:39). This is against the Mishkenot Yakov who denies the concept that zeyia can transfer taste.
- Yabia Omer 5:7:6 suggests this based on Sh"t Peni Yehoshua 13, Yesodei Yeshurun v. 6 p. 158
- Horah Brurah 92:8
- Bach Chadashot 24, Aruch Hashulchan 92:55, Yaskil Avdi 7:4
- Trumat Hadeshen Ketavim Upesakim 103, Rama 92:8
- Horah Brurah 92:8 s.v. im is concerned that zeyia gets into the salt that is above the pot and becomes meat or dairy like the food in the pot. He says that it is an issue even if the zeyia isn't Yad Soledet Bo since there is a bit of transfer though it could be cleaned off. However, since salt or a spice can't be cleaned off it is an issue. Even though the amount transferred would be nullified it is an issue of bitul lechatchila to use that salt for milk if it was used over a meat pot.
- Rosh 20:26 isn't sure if the zeyia of a bottom pot affects a top pot if the top pot itself is steaming. His reason is that in the laws of tumah having two hot pots doesn't transfer liquids. Aruch Hashulchan 92:55 considers this possibility. Maharsham 3:208 and Horah Brurah 92:8 s.v. hakol write that this possibility can be used as a factor in certain cases.
- Bet Shlomo YD 164 disregards this idea since the Rosh rejected it.
- Rama in Torat Chatat 35:6 writes that foods don't have zeyia. This can be explained in two ways: (1) The Pri Megadim (seder v’hanahagot hashoel im hanishal b’isur v’heter seder 2 no. 37) says that zeiya is only an issue from liquids and not dry foods based on the laws of tumat mashkin. (2) 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. This is implied in the language of Rama YD 108:1.
- Those who hold that there's no zeyia for solid foods: Bet Meir OC 461, Masat Moshe YD 1:4, Igrot Moshe 1:40, and Minchat Shlomo 2:51. Horah Brurah 92:86 and Rav Moshe Heinemann (Guide to Halachos p. 58) agree with Rav Moshe. For this purpose, Rav Moshe Heinemann (Guide to Halachos p. 58) writes that pizza and lasagna are not liquids.
- Minchat Yakov 35:10 who is bothered with the Rama's statement denying zeyia of solid foods because of Tur and Shulchan Aruch OC 451:15 which assumes steam even from dry dishes. He answers three answers: (1) zeyia of solids causes a transfer of a tiny bit and on pesach that is an issue, (2) food zeyia is minimal but if the utensils are used for zeyia frequently it adds up, (3) when doing kashering we should do so in the most ideal form and remove the small amount of zeyia from solids. Based on these Torat Chatat's answers it sounds like there is zeyia from solid foods to be concerned about lechatchila going forward. This is also the opinion of the Shoel Umeishiv 5:4, Tzur Yakov 68, Bet Shlomo YD 164, Yabia Omer 5:7:5, and Or Letzion 3:10:2. Aruch Hashulchan 92:54 writes that it doesn't depend on whether it is solid or liquid but if it is fatty.
- Even Yikara 3:18 cited by Yabia Omer 5:7 holds that zeyia is burnt up in an oven if it has an exposed fire in the oven but not if it has a fire behind a wall (heseko mbchutz). Rav Ben Tzion Wosner (Or Yisrael 5763 year 8 no. 4:34 pp. 92-102) holds that zeyia is burnt up in an oven. Sheilat Yavetz (1:93) holds that the zeyia is burnt up in the ovens. Maharsham 3:208 applies this reason to be lenient even if the fire is behind the wall of the oven. He isn't ready to be lenient initially.
- Igrot Moshe 1:40, Minchat Yitzchak 5:20, Chelkat Yakov 2:136
- Aruch Hashulchan 92:55 writes that there's no concern of zeyia in an open area as he proves from Mishna Machshirin 5:10. Therefore he writes that he isn't concerned for zeyia in ovens where there is a lot of airspace.
- Trumat Hadeshen Pesakim Uketavim 103, Rama 92:8
- Rama 92:8
- Rama 92:8
- Shulchan Aruch Y.D. 108:1.
Gemara Pesachim 76b records a dispute between Rav and Levi whether two foods cooked together transfer a smell from one to another. Rif Chullin 32b and Rashi Pesachim 76b s.v. amar hold like Levi and understand that reycha is permitted specifically after the fact. Ramban a"z 66b s.v. vkayma, Rashba a"z 66b s.v. vkatav, Chiddushei Haran a"z 66b s.v. amar concur, and Nemukei Yosef a"z 66b agree with the Rif. Tosfot 76b s.v. asra citing Rabbenu Tam, and Tosfot Avoda Zara 66b s.v. rava hold like Rav who is strict even after the fact.
- Rif Chullin 32b presents two approaches as to why it is only permitted after the fact. One is that even Levi only permitted it after the fact and that is how he addresses the gemara's which seem to be strict for reycha. The other is that it is considered dvar sheyesh lo matirin, something which could be permitted, since it is possible to avoid this question by eating the bread with meat as opposed to dairy. Ramban a"z 66b s.v. vyesh, Rashba a"z 66b s.v. vkatav, and Ran Chullin 32a question the Rif's answers and entertain the possibility that reycha is permitted even initially and there is a specific prohibition to bake bread in the oven at the same time as meat is cooking. The Ran poses a question for the Rif; why should reycha be forbidden initially if nat bar nat is permitted initially (after it is cooked according to the Shach). The Gra 108:9 answers that nat bar nat is considered as though nothing is mixed in and reycha is considered as though a minute amount of prohibited ingredient is mixed in. Further proof of that can be seen in Magen Avraham 447:24.
- Rama Y.D. 108:1. Tosfot avoda zara 66b s.v. rava forbids smell regarding chametz since it certainly transfers a little bit.
- 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
- Rama 108:1
- Mateh Reuven 249 as a limud zechut for people who put bread in the oven at the same time as dairy bread. He says to consider the milk in the bread as an infusion of taste (based on Rashi a"z 67b unlike Tosfot) and by the time the dairy taste gets to the parve bread it is nat bar nat. He explains that this assumes that there is nat bar nat from food to food unlike the Pri Megadim that needs to go through a utensil. See there at length.
- 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). This is also reflected in this OU article and is the opinion of the Star-K.
- 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. Rabbi Lebowitz in another shiur (min 44) mentions that one should use separate trays or make sure it is clean. He isn't concerned about zeyia if all the foods are covered. (Microwave turntables can become yad soledet bo (subzero.com)).
- Yad Yehuda Pirush Hakatzar 92:72 writes that if the zeyia is Yad Soledet Bo but the pot isn't it will make the pot absorb the zeyia but not the food inside it, however, if the pot itself is also hot it'll go into the food as well. See Pirush Haaruch 55 at length. like nizchal in 92:7.
- 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.
- Laws of Kashrut p. 366, Rabbi Mansour in the name of R' Shmuel Pinchasi at dailyhalacha.com. 
- Rav Schachter in OU document K-36 is lenient on using a steam jacket with steam that was used for forbidden foods because steam isn't the same as vapor. Steam is a gas and water vapor are droplets in the air (Quora.com). Since steam isn't a liquid it can't absorb taste. He adds that since the metal pots don't let beliyot through the pot perhaps that can be used as a factor to be lenient.
- 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. Yalkut Yosef Isur Vheter v. 3 p. 474 holds that strictly speaking it is permitted to use one sink for meat and milk dishes one after another since the water isn't usually Yad Soledet Bo and there's soap. However, initially one should have two sinks.
- 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 Y.D. 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
- Badei Hashulchan 95:77
- Shach 95:20
- Pri Chadash (cited by Badei Hashulchan 95:77)
- Shach 95:20 isn't certain whether we assume that an iruy cooks so much to infuse the taste of the meat into the dairy dish and then extract the non-kosher taste from the dairy dish and infuse it back into the meat dish. He concludes that we should be strict unless there's a large loss. Rabbi Akiva Eiger on the Shach 95:20 is strict even if there's a large loss if there was more than one pouring.
- 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
- Yalkut Yosef (v. 3 p. 174) writes nine factors to be lenient including: iruy doesn't cook according to Rashbam, since his intention isn't to cook the meat and milk it isn't derech bishul, and the meat and milk were already cooked..
- Badei Hashulchan 95:81.
- Rav Moshe Feinstein (Igrot Moshe OC 1:104, OC 3:58, YD 1:28-29, and YD 3:10-11) is lenient to use a dishwasher for meat and milk dishes consecutively with different racks. His reasoning is that the only concern of taste from the dirty dishes is Nat Bar Nat and therefore doesn't render the dishwasher meat or milk. He explains that it is permitted even initially and isn't considered like creating Nat Bar Nat. Also, he says that there is probably sixty times the beeyn in the actual pieces of leftovers on the plate. Yet, he says that a person should use different racks for meat and milk out of a concern that a piece of meat or milk got stuck onto the rack and taste directly got absorbed into the racks.
- Yalkut Yosef (Otzar Dinim L'isha p. 618), Yalkut Yosef (Isur Vheter v. 3 p. 485) only allows consecutive cycles. Yabia Omer YD 10:4 allows meat and milk dishes together. Rav Ovadia himself in the new edition of Yabia Omer 10:4 fnt. 6 added that it is better not to follow his teshuva to do it at the same time. Rabbi Assayag and Rabbi Wiesenfeld in Kashrut in the Kitchen Q&A p. 91 rule like Yalkut Yosef.
- Shaarei Shalom on Piskei Ben Ish Chai Basar Bchalav p. 143-5 is strict for all dishwashers even one after another. If the first cycle is with soap he is concerned for the fact that noten taam lifgam is only after the fact. He quotes Tzemech Tzedek 61, Minchat Yakov 57:26, and Shulchan Gavoha 95:7 who only allow the case of using ashes to clean dishes (Shulchan Aruch 95:4) after the fact. Also, if the first cycle is without soap there is an issue because if the dishes are dirty everything becomes forbidden and if they're clean we don't rely on Shulchan Aruch 95:3 initially as Shach points out.
- Horah Brurah 95:46 discusses whether the kula of 95:4 is permitted initially but concludes like Rav Ovadia that it is permitted even initially.
- Whether the beliyot can be koshered with ashes in the water is a major dispute: Kaf Hachaim 95:59 quotes the Bet Dovid YD 41 quoting Mahariku and Bet Yehuda 114c, 2:77 hold that it is effective, while the Dvar Moshe 10, Dvar Shmuel 88, Kol Eliyahu YD 1:11, and Erech Hashulchan 95:17 hold that it isn't effective.
- Igrot Moshe OC 3:58, Star-K
- Shulchan Aruch 91:1, Rama 92:8, 93:1, 95:3. See R' Sultan's piece about a boiling meat pot placed with boiling eggs in water placed in a dairy sink.
- Shulchan Aruch YD 91:8, Shach 87:2
- Shulchan Aruch YD 105:1
- Badei Hashulchan 105:2 based on Rosh responsa 20
- Kaf HaChaim 105:6
- Taz 105:1, Shach 98:13, Badei Hashulchan 105:3, Kaf HaChaim 105:1. Taz thinks that the absorption is only as deep as a klipa, while the Shach thinks it is completely absorbed.
- Yad Yehuda 105:1 is lenient on metal and glass not absorbing taste through soaking. See Darkei Teshuva 105:11 for discussion.
- 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