How do we kasher?

Question #1: Underwent a Grilling

Shmuel, planning a bein hazemanim camping trip, asks:

“Is it possible to kasher a treif grill?”

Question #2: Planetary Kashrus

Dawn e-mails me the following question:

“Whenever I call my rabbi about a household kitchen mixup, he asks me whether the offending vessel was used for either dairy or meat in the last twenty-four hours. Twenty-four hours is the approximate time it takes for the earth to rotate on its axis, so that every part of the planet, except for the areas near the poles, experiences day and night during that time. How does that affect whether my pot is kosher?”

Question #3: Roommate Ruckus

Chaya, who already knows that she needs to change her living arrangements as soon as practical, calls: “In a fit of anger, my roommate deliberately made one of my pots non-kosher. May I eat the food that I cooked before I found out that she treifed up the pot?”


After the Bnei Yisroel’s spectacular and miraculous victory over the nation of Midyan, they were issued instructions regarding the booty that they had just acquired: Concerning the gold and the silver; the copper, the iron, the tin and the lead: any item that was used in fire needs to be placed in fire to become pure – yet, it must also be purified in mikveh water. And that which was not used in fire must pass through water (Bamidbar 31:22-23).

This posuk introduces the concept of kashering vessels. The Torah assumes that when a utensil is used to cook food, some of the food is absorbed into the walls of the pot, and this residue continues to impart flavor. Therefore, equipment that was used to prepare non-kosher food must be kashered before it can be used. To the Bnei Yisroel in the desert, this meant that the spit or gridiron grates recently “acquired” from the Midyanites had to be kashered. For the contemporary Jew, this means that the built-in grill in the backyard of his newly purchased suburban home must be made properly kosher before he uses it.

Ke’bol’o kach polto

How does one remove the remaining residue? The instructions the halacha provides create a hierarchy of kashering, called ke’bol’o kach polto – the same way a vessel absorbed non-kosher substance is the method used to kasher it. Utensils that were placed in the fire itself must be kashered by heating them in fire. Thus, a spit, grill, or similar appliance, which absorbs food directly through fire, requires kashering by burning in fire, libun. Those utensils used to cook in water or other liquid must be kashered by boiling them in water. Therefore, a pot or similar cooking equipment may be kashered by bringing the vessel to a boil, a process called hag’alah. Hag’alah will remove completely what was absorbed by cooking, but will not remove everything that was absorbed through grilling (Taz, Yoreh Deah 121:7; see Pri Megadim, Orach Chayim 452:4 in Mishbetzos Zahav).

Libun versus hag’alah

Most late authorities understand that the dissimilarity between libun and hag’alah is because the two methods of kashering operate in different ways. Libun removes the non-kosher residue by burning it, similar to the way we destroy the last remnants of our chometz before Pesach. The spit or grill may now be used for your kosher barbecue, because the residual flavor from the non-kosher meat has now been completely consumed. (Note, however, that several rishonim appear to understand this topic differently.)

On the other hand, hag’alah, boiling, kashers by removing the residue rather than destroying it in situ. Whatever taste of non-kosher food remained in the pot has now been dissolved and nullified in the water (Shach, Yoreh Deah 121:17, quoting Re’ah; see also Pri Megadim, Orach Chayim 452:4 in Mishbetzos Zahav; Shu”t Igros Moshe, Yoreh Deah 1:60).

Therefore, libun could be used on an item requiring hag’alah, since it will satisfactorily burn up any residue. However, an appliance that requires libun cannot be kashered with hag’alah, since this does not remove completely whatever was absorbed when the non-kosher food was broiled.

Thus, the answer to Shmuel’s question whether one can kosher a treif grill is affirmative. The halacha is that one heats the grill until it glows or sparks, which is called in halacha ‘libun chamur.’

Nosein taam lifgam

To explain the background to the rest of our opening questions, we need to address a different aspect of this halacha. The Gemara quotes a dispute between tana’im, the Torah scholars of the era of the Mishnah, regarding the following question:

Often, the flavor that a food provides is nosein taam lifgam — which means that the non-kosher food provides a less than appetizing flavor into the kosher food. Is something that is nosein taam lifgam prohibited? Rabbi Meir contends that, since the Torah requires that the Bnei Yisroel kasher whatever equipment they acquired from the Midyanites, all flavor is prohibited, whether or not it is appetizing.

Rabbi Shimon disagrees, contending that the Torah prohibited only the appetizing flavor of non-kosher substances. In his opinion, the Torah did not prohibit using equipment that was once used for non-kosher when the flavor imparted does not enhance the finished product. Such a taste is called pogum (Avodah Zarah 67b).

Don’t cry over spilled vinegar

Following this latter opinion, the Mishnah (Avodah Zarah 65b) rules that if non-kosher wine vinegar spilled into hot beans, one may eat the beans even though one can taste the vinegar! This is because your favorite cookbook does not suggest adding vinegar to the beans you are cooking, and for a very good reason – the taste of cooked beans is not enhanced by vinegar.

How do we rule?

The Mishnah (Avodah Zarah 65b) rules that nosein taam lifgam is permitted, and this is also the conclusion of the Gemara in several places (Avodah Zarah 36a, 38b, 39b, 65b). A less than appetizing flavor is not included in what the Torah prohibits. The Shulchan Aruch (Yoreh Deah 103:1-2) also rules this way, writing the following: “Any item that imparts a pogum taste does not prohibit its admixture, even if the item itself is very tasty, but it adds an ill taste to the food in which it is now mixed. This ill taste does not require that it be completely spoiled until one is disgusted to eat it. Even if the added substance creates only a slight distaste, the mixture is permitted. However, there is an opinion that this is permitted only if one mixed a small amount of prohibited food into an amount of food greater than it. But if the prohibited substance is greater, or even if the two are mixed fifty-fifty, we do not say that nosein taam lifgam is permitted, unless the food is completely spoiled and no longer fit for human consumption.”

Why is it pogum?

Why would good food impart distaste to other food? There are several ways this could happen. One is that the tastes of the two food items do not enhance one another. Although the Mishnah and Gemara choose other examples of this, think of the child who decided to squeeze a pickle into his apple sauce. Pickles taste good and apple sauce tastes good, but the combination…

Milk and orange juice

Many years ago, I happened to be at a meeting of prominent rabbonim in which one of the distinguished attendees raised the following question: How could one drink orange juice together with a fleishig meal? The same bottling equipment used to fill the containers of orange juice could be used to fill containers of milk!

A different prominent rav responded by asking the questioner when was the last time he had added milk to his orange juice? When the entire audience grimaced at the thought, the rav noted that this is exactly what the halacha calls nosein taam lifgam. Milk does not enhance the flavor of orange juice, and, therefore, even if a container of orange juice was filled using equipment that previously had bottled milk, the finished product is perfectly kosher and pareve.

Same day

Here is another application of nosein taam lifgam that is very common. The Gemara states that the absorbed residual flavor remaining in a pot provides a good taste only during the day the food was prepared, which is called ben yomo. Afterwards, this taste spoils and becomes nosein taam lifgam. Residual flavor in a pot the day after it was absorbed is called eino ben yomo. On this basis, food prepared in a non-kosher pot when it is eino ben yomo is kosher (Avodah Zarah 75b; Rambam, Hilchos Maachalos Asuros 17:2; Shulchan Aruch, Yoreh Deah 103:5; 122:6). We will discuss shortly why we are not permitted to use this pot.

Meat and milk

The rule mentioned above also applies to a mixture of dairy and meat. Mixing dairy and meat together usually provides a good flavor, and it is therefore prohibited. However, if the fleishig pot that cooked the milk was not used the same day for meat, and milk was cooked in it, the meat flavor imparted to the dairy product is nosein taam lifgam. Although the pot must be kashered, since it now contains both milk and meat residue, the milk cooked in it remains kosher.

Stories from the Talmud

The Mishnah rules that it is permitted to purchase vegetable oil or honey from a non-Jew (Avodah Zarah 35b, 39b). The Gemara asks why this is so – should we not be concerned that these foods were prepared in the gentile’s non-kosher vessel? If so, we should be concerned that the oil or the honey have absorbed non-kosher flavor.

To this question, the Gemara answers that we may assume that the non-kosher flavor in the pot will be nosein taam lifgam in the oil or honey (Avodah Zarah 36a, 39b). The rishonim explain that the reason the oil and the honey are both permitted is because the possibility exists that when the non-kosher pot was used to heat the oil or the honey, it may not have been a ben yomo. Furthermore, the possibility exists that the non-kosher food previously cooked in the vessel gave the oil or honey a less appetizing flavor (Tosafos, Avodah Zarah, 38b s.v. Iy; Shu”t Harivash #28). Therefore, there is a sefeik sefeika, meaning that there is more than one reason why something may be permitted, such that sufficient reason exists to permit it.

What is the same day?

What does the Gemara mean when it says that residue from cooking of a previous day no longer tastes good? Is there a timetable whereby we know how long it takes for the residue in the vessel to spoil? As Dawn noted in her question above, “Twenty-four hours is the approximate time it takes for the earth to rotate on its axis, so that every part of the planet, except for the areas near the poles, experiences day and night during that time. How does that affect whether my pot is kosher?”

The answer is that there is a dispute among rishonim how much time it takes for the residue to spoil. According to some opinions, spoilage happens overnight, but does not require 24 hours (for example, Rashi, Avodah Zarah 67b). According to this opinion, because of nosein taam lifgam, min haTorah a pot used for meat one day may be used for dairy the following morning.

Other rishonim contend that nosein taam lifgam occurs when the vessel or the pot was not used to cook or warm food for 24 consecutive hours since the food was prepared (Tur, Yoreh Deah 103). The Shulchan Aruch follows the latter opinion (Yoreh Deah, 103).

It dawns on Dawn

With this background, we can now answer Dawn’s question:

“Whenever I call my rabbi about a household kitchen mixup, he asks me whether the offending vessel was used for either dairy or meat in the last 24 hours… How does that affect whether my pot is kosher?”

The answer is that once twenty-four hours have passed since the last use of this pot, the flavor that is absorbed is nosein taam lifgam and does not prohibit the food that is prepared subsequently in the pot.


There is one major exception – in which a food item that would otherwise be considered pogeim, now adds a positive taste, and therefore the food prepared is prohibited. A pungent item, davar charif, absorbs and transmits flavor in ways that more bland items do not. When a davar charif herb or spice is prepared, the pungency of the food transforms the nosein taam lifgam absorbed in the vessel into a good-tasting product. For this reason, the Mishnah (Avodah Zarah 35b) prohibits using a slice of a highly pungent food item called chiltis obtained from a non-Jew, since it may have been sliced with a gentile’s non-kosher knife. The sefeik sefeika mentioned above regarding vegetable oil and honey does not apply, since the pungency of chiltis enhances the taste of the non-kosher residue.

What is chiltis?

Rashi explains it to be laser. The Latin name for this very strongly-tasting herb is laserpitium, which was also sometimes called silphion or silphium.

I have seen other commentaries identify chiltis with a different ancient herb called assa foetida, also called “devil’s dung,” because of its extremely powerful odor. Neither silphium nor assa foetida are commonly used in flavoring today, because the modern palate prefers the tastes of other spices.  Nevertheless, many spices and herbs are considered devarim charifim, but we will leave that discussion for a different time.

Non-kosher pots

We concluded above that if someone cooked food in a pot that had been used more than 24 hours ago to cook non-kosher food, the food that is now cooked remains kosher.  At this point, one could ask the following question: If we determine that a pot has not been used for the last 24 hours, should we not be able to use it to cook kosher food, even if we know it was used to ccok non-kosher food previously?  After all, once 24 hours has passed, whatever non-kosher food was absorbed becomes nosein taam lifgam.

The answer is that Chazal forbade using this pot because of a concern that one may make a mistake and use it within 24 hours of it being used for non-kosher. The Gemara calls this gezeirah kedeirah eino ben yomo mishum kedeirah ben yomo, a decree on a pot last used a day earlier because it will lead to using a pot that is ben yomo (Avodah Zarah 76a).

Intentional misuse

What is the halacha if someone knew that Chazal prohibited using an eino ben yomo treif pot, but decided to cook in it, anyway? Is the food that he produced not kosher?

Let me explain the question. In many situations, if a non-kosher substance became mixed into kosher food, the finished product may be used because of a halachic principle called bitul, which means that the non-kosher product was nullified or neutralized.

On the other hand, there is a halachic principle that ein mevatlin issur lechatchilah, one is not permitted to take a non-kosher substance and nullify it. Bitul is a concept that is applied only after-the-fact, be’dei’evid.

What if someone used an eino ben yomo treif pot without kashering it? Is the food prohibited because of the principle of ein mevatlin issur lechatchilah? Similarly, we can ask regarding someone who intentionally used an eino ben yomo dairy pot to cook meat or an eino ben yomo meat pot for dairy. The pot certainly will now require kashering. The question is whether the food is permitted.

The question is: When Chazal prohibited use of an eino ben yomo, did they only prohibit using the pot, or did they rule that the residue in the pot is considered a prohibited substance. If they ruled the former, then food prepared in such a pot is permitted. If they ruled the latter, then someone who intentionally used the pot, knowing that it was prohibited to do so, was attempting to nullify a prohibited substance, and, because of ein mevatlin issur lechatchilah, the product is prohibited.

This matter is a the subject of a dispute both among the rishonim and among the acharonim.  According to the Rashba (Toras Habayis, 4:3, 4:4) and the Chamudei Daniel (quoted in Darchei Teshuvah 122:26), the food is prohibited. The Rashba states that product cooked intentionally in an eino ben yomo treif pot is prohibited because of ein mevatlin issur lechatchilah.  However, the Rambam and the Shulchan Aruch imply that the food is permitted, and this is the way Rav Moshe Feinstein rules (Shu”t Igros Moshe, Yoreh Deah 2:41).

Roommate ruckus

At this point, we can discuss Chaya’s question. “In a fit of anger, my roommate deliberately made one of my pots non-kosher. May I eat the food that I cooked before I found out that she treifed up the pot.”

The answer is that provided the pot was not ben yomo, Chaya may eat the food and serve it to guests. Even according to the Rashba, who rules that someone who intentionally used a non-kosher pot makes the food non-kosher, this would not apply to Chaya, who was not trying to violate any ruling of Chazal. Unfortunately, if her roommate really made the pot non-kosher, Chaya will need to kasher it before she can use it again. Although someone suggested that she boil her roommate in the pot, there are probably more effective ways for Chaya to deal with her difficult living situation.


This article has provided a small introduction to some of the ideas of kashering, particularly to the concepts of libun and hag’alah. We have not yet dealt with several other types of kashering, including iruy, kli rishon, and miluy ve’iruy, all of which we will need to leave for a future time. We should always hope and pray that the food we eat fulfills all the halachos that the Torah commands us.