1.When Does Taste Absorbed into Utensils Render Their Contents Forbidden?
Though the walls of pots and other vessels appear solid and impervious, they actually absorb the taste of food cooked in them. Thus, if one cooks non-kosher meat in a pot, its flavor gets absorbed into the pot’s walls, and kosher meat subsequently cooked in the same pot becomes forbidden, as the non-kosher taste absorbed in the walls is released and absorbed by the meat. 1
However, there is a principle that anything which contributes foul taste ("noten ta’am li-fgam") does not cause other foods to become forbidden. For example, if a bit of foul-tasting non-kosher meat falls into some kosher meat, the meat may be eaten since the non-kosher taste is foul. The same applies to tastes absorbed by vessels. Moreover, the rule is that any taste absorbed by a vessel becomes foul after twenty-four hours. Therefore, if non-kosher meat is cooked in a pot, and twenty-four hours later kosher food is cooked in the same pot, the latter dish remains kosher because the taste in the pot is foul, and foul taste does not render food forbidden (SA YD 103:5).
However, it is forbidden le-khatĥila to use a pot that has absorbed non-kosher taste, even after twenty-four hours have elapsed, because the Sages were concerned that one would forget and inadvertently cook in it before twenty-four hours have elapsed. They therefore ruled that since such a pot is forbidden to begin with, it may not be used until koshered (SA 122:2). Be-di’avad, if one forgot that the pot absorbed the taste of forbidden food and cooked another food in it, if twenty-four hours passed from the time the non-kosher food was cooked, the food in question remains kosher. But if one knows that a pot had absorbed a non-kosher taste, but cooks kosher food in it anyway, the Sages penalized him by prohibiting this dish for him and his family, even though the non-kosher taste in it had already turned foul. 2
2.Ĥametz Utensils on Pesaĥ
Vessels used throughout the year with hot ĥametz foods cannot be used during Pesaĥ since heat causes vessels to absorb the taste of ĥametz. In order to use such utensils during Pesaĥ, one must first remove the taste of the ĥametz through hagala or libun.
If one cooks in such a pot on Pesaĥ knowing that it had not been koshered, even if twenty-four hours have passed and the taste of the ĥametz has been befouled, the cooked food is forbidden. As we have learned, the Sages forbade such food in order to penalize people who deliberately cook in vessels that have absorbed ĥametz and have not been koshered.
What if one errs and cooks in a pot that was not koshered for Pesaĥ? According to Shulĥan Arukh, if twenty-four hours have elapsed since the ĥametz was cooked in the pot, the food may be eaten on Pesaĥ, because the taste of the released ĥametz is foul. According to Rema, the food is forbidden even though the ĥametz is foul because the prohibition of ĥametz is especially severe, for even a drop of ĥametz causes a food to be forbidden (SA 447:10; see above 7:5).
Ĥametz utensils that one does not wish to make kosher for Pesaĥ must be cleansed of any residual ĥametz and put away in a closed place so that nobody inadvertently uses them during Pesaĥ (SA 451:1; see also above 6:4, where we learned that such utensils should not be sold).
3. Releasing through the Same Method as Absorption ("Ke-bole’o Kakh Polto"): Hagala and Heavy Libun
The most basic principle of koshering cooking utensils is that forbidden taste is released from the vessel in the same manner that it was absorbed: "ke-bole’o kakh polto." There are two principal media through which utensils absorb taste: boiling liquid and direct flame.
If a pot absorbs a forbidden food through a process of boiling – for example, if it was used to cook ĥametz food like pasta or porridge – the pot is made kosher for Pesaĥ by immersing it in boiling water, which causes the ĥametz taste to be released. The same applies to ladles and serving spoons: when used with ĥametz foods hotter than yad soledet bo (hot enough to cause the hand to recoil), they absorb the taste of the ĥametz and must be koshered through immersion in boiling water.
However, if a vessel absorbs ĥametz through direct heat of fire, without a liquid medium – such as in the case of a cake baked on a tray, dough baked on skewers, or jachnun or kugel baked in a pot – it is koshered by means of heavy libun, that is, heating the vessel by fire until it gives off sparks or becomes red hot.
Absorption through a liquid medium is relatively mild, so boiling hot water is sufficient to extract the taste of the ĥametz from the utensil. Absorption into a tray or skewer is more intense, as the heat of the fire causes the taste of the food to be absorbed deep into the very particles of the utensil. Boiling water is insufficient to remove all of the absorbed taste, and such utensils must be koshered by the same means that they absorbed – by fire. This is "heavy libun," in which the fire incinerates the taste that had been absorbed in the utensil.
To highlight the difference – hagala extracts the taste absorbed in the vessel, while libun incinerates it in situ.
Therefore, before undergoing hagala, a vessel must be cleaned of any residual food, because hagala releases the taste absorbed into a utensil but does not destroy the residual food stuck to it. There is no need, however, to clean a utensil before libun, because any food that remains will be completely incinerated in the libun process.
4.Defining the Difference between Absorption through Liquid and Absorption through Fire: the Status of a Frying Pan
Even when a baking tray is coated with oil to prevent sticking, the absorption that occurs during the cooking process is considered to be by means of fire, thus requiring heavy libun to render it kosher. Only when the oil at the bottom of the vessel sizzles and bubbles is the absorption considered to be by means of a liquid. 3
In light of this, most Rishonim maintain that a frying pan can be koshered through hagala (Rosh, Raavya). Even if the oil is used up and the food burns, hagala is sufficient since there was oil present at the beginning of the frying process, and the taste of the food absorbed into the pan was by means of the oil, i.e., the milder form of absorption. The same applies to a pot in which a non-kosher food was cooked: the pot can be koshered by hagala even if the food dries up and burns (MB 451:63; Kaf Ha-ĥayim ad loc. 137).
However, several leading Rishonim maintain that a frying pan has the status of a baking tray and must be koshered via heavy libun, because people often fry with small amounts of oil, and the oil is often used up, causing the taste of the food to be absorbed via fire (Rashba). Although there is agreement that a pot can be koshered through hagala even if the food dries up and burns, there is reason to be more stringent about frying pans because only a small amount of oil is used from the outset, and it often gets used up.
In practice, frying pans should ideally be koshered via light libun (as will be explained in the next section), although if twenty-four hours have passed since the frying of the forbidden food, hagala is sufficient be-di’avad. 4
A Teflon or "non-stick" frying pan, in which food is fried without oil, cannot be koshered for Pesaĥ. In theory it is possible to kosher it via heavy libun like a baking tray, but as a practical matter this will damage the pan (as will be explained below, section 7).
A frying pan used primarily for making malawach, which is baked and heated without sizzling oil, must be koshered by heavy libun since the absorption takes place via fire. But a pan generally used for other things and infrequently used for malawach may, in extenuating circumstances, be koshered via hagala, in accordance with its primary usage (as will be explained below, section 9).
^ 1.. It is impossible to measure how much taste the walls of a pot absorb and how much they release back into the food; some vessels absorb more than others do, and some tastes are more easily absorbed than others. Since this is a persistent uncertainty with no means of resolution, the Sages determined that one should consider the entire wall of the pot to be completely filled with the taste of whatever is cooked inside of it. Thus, if one cooked non-kosher meat in a pot and then cooked kosher meat in the same pot, we assume that the walls of the pot absorbed taste from the non-kosher meat and subsequently discharged the taste back into the kosher meat. And since our pots and utensils do not hold sixty times the amount that can be absorbed into the walls of the pot, anything that was cooked in a pot that had previously absorbed the taste of a prohibited food becomes prohibited. Even if one cooked another piece of kosher meat afterward, it too would become prohibited, since perhaps not all of the absorbed forbidden taste was released during the first usage and was released during the second usage. In sum: in the case of a perpetual uncertainty where it is impossible to determine absolutely how much taste was absorbed and how much was released, we are stringent.
^ 2.. Most poskim maintain that if one intentionally cooked in a pot that required hagala, the Sages penalize him by forbidding the food for him and those for whom he cooked it (Knesset Ha-gedola YD §122; Hagahot Ha-Tur §26; Darkhei Teshuva YD 122:5; Yabi’a Omer YD 8:14; and Hagalat Kelim Le-Pesaĥ, second introduction, p. 17).
^ 3.. This is according to Pri Ĥadash 451:1, quoted by SAH 451:36 as saying: "Since the dough is not sizzling with oil or fat, it means that fire alone causes the ĥametz to be absorbed." Kaf Ha-ĥayim ad loc. 139 cites them, and this is the ruling of Yeĥaveh Da’at 1:7 (although Kaf Ha-ĥayim 451:76 states that if there was enough liquid present in the pot to wet something that touched something that was in the pot ("tofei’aĥ al menat le-hatfi’aĥ"), the pot can be koshered by hagala; this requires further investigation). Accordingly, a utensil used to cook jachnun or kugel is considered to have absorbed by means of fire, since there is no sizzling liquid (see Hagalat Kelim Le-Pesaĥ 5:23 and the supplementary material ad loc.). If a pot is generally used to cook through a liquid medium but less frequently used to make jachnun or kugel, it may be koshered by hagala in extenuating circumstances, as will be explained below in section 9.
^ 4.. BHL 451:11 concludes that most poskim maintain that one may kosher a frying pan via hagala, and I have already mentioned the leading Rishonim on this topic. SA YD 121:4 rules that a frying pan generally requires libun to be koshered from forbidden foods, but for Pesaĥ hagala is sufficient. Many Aĥaronim (Gra and Shakh ad loc.) explain that regarding Pesaĥ R. Karo combines the opinion that hagala is sufficient with the view that ĥametz before Pesaĥ is considered permissible, and thus rules leniently that one may kosher a frying pan for Pesaĥ via hagala. Rema agrees in principle but states that light libun is the proper method le-khatĥila. Later Aĥaronim also disagree about this issue: Pri Ĥadash and R. Ĥayim ibn Attar require libun since ĥametz is considered a forbidden substance even before Pesaĥ, while SAH permits using hagala even le-khatĥila. See Hagalat Kelim Le-Pesaĥ 13:202.
I have written that the preferable method for koshering a frying pan is light libun, and although apparently light libun is ineffective according to those who require heavy libun, there are in fact those who maintain that light libun is as effective as heavy libun (this may be the opinion of R. Avigdor, quoted in the Hagahot Maimoniyot). Additionally, some authorities explain that the principle of ke-bole’o kakh polto also applies to the temperature of the libun, as will be explained in the next section. Consequently, light libun is sufficient, since it takes place at least at the same temperature as the initial absorption. On the other hand, it is better to use light libun since sometimes there are small grooves or cracks in the pan which are difficult to clean, in which case even those who maintain that hagala is enough to kosher a frying pan would require libun to eradicate the residue in the cracks. However, one may rely, be-di’avad, on the majority of poskim who maintain that one may kosher a frying pan via hagala, especially since once twenty-four hours have elapsed since its last use, any uncertainty pertains to a rabbinic prohibition.