As we have learned, a pot absorbs the taste of the foods cooked in it. Cooking has the capacity to mix the tastes of different foods with one another, and just as cooking can cause the taste of meat to be absorbed by potatoes cooked with it, so too it can cause the taste of a food to be absorbed in the walls of the pot in which it is cooked.
There are, however, different levels of intensity in cooking, and the operative principle is ke-bole’o kakh polto. Thus, if the absorption is caused by intense cooking, the koshering process must be equally intense. However, if the cooked food never reaches the temperature of yad soledet (45ºC according to the opinion that sets yad soledet at the lowest temperature), there is no reason to be concerned about absorption, and it is not necessary to perform hagala to kosher the utensil.
The levels of koshering are as follows:
A kli rishon on the flame: The most intense form of cooking is that of a kli rishon (the vessel in which the food is cooked) on a flame, where the fire heats the mixture of food continuously, causing its different tastes to be absorbed into each other and by the walls of the pot. Ke-bole’o kakh polto: In order to kosher such a vessel, one must immerse it into the boiling water of a kli rishon on a flame. It must be emphasized that even if the water in the pot was not boiling when the non-kosher food or ĥametz was absorbed, it must boil during hagala, because the principle of ke-bole’o kakh polto relates only to the type of absorption – kli rishon or kli sheni – but whenever dealing with absorption in a kli rishon on the burner, the koshering must be carried out with boiling water.
A kli rishon removed from the flame: This refers to a vessel that was heated over a flame and then removed, or the flame was extinguished. Such a vessel still has the capacity to cook, and food placed in it will become slightly cooked from the lingering heat from the fire. Nevertheless, the heat continuously dissipates, as does its capacity to cook. Therefore, it need not be koshered in a pot of boiling water on a flame; it is sufficient to place it in a kli rishon that is no longer on a flame.
Liquid poured (“irui”) from a kli rishon: This has the capacity to cook the surface layer (“kedei klipa”) of a food item. For example, if ĥametz soup was poured into a bowl from a kli rishon, the taste of ĥametz will be absorbed by the surface layer of the bowl but will not penetrate its entire width. To kosher such a bowl, it is sufficient to pour boiling water over it from a kli rishon.
A kli sheni: This refers to hot food that was first cooked in a vessel over fire and then transferred to a different one. The poskim disagree about whether such a food can cause its taste to be absorbed into the surface layer of other foods or utensils. For example, if one places a spoon in a kli sheni, some poskim say it will not absorb the taste of the food in the vessel, and others say it will. Regarding all other forbidden foods, SA (YD 105:2) rules that although, according to the prevailing opinion, hagala is not required, it is nonetheless proper to do so le-khatĥila. Regarding Pesaĥ, however, SA (451:5) rules that hagala is required. Due to the severity of the ĥametz prohibition, the lenient position is not even mentioned (Kaf Ha-ĥayim ad loc. 20).
Some are even stringent regarding a kli shlishi and beyond, that is, utensils at least twice removed from the vessel in which the food was cooked, maintaining that as long as a food remains at the temperature of yad soledet, a vessel will absorb its taste and must be koshered – ke-bole’o kakh polto. However, most poskim are lenient in this respect. Nevertheless, because of the gravity of the ĥametz prohibition, it is customarily preferable to be strict about koshering any vessel that contained ĥametz at the temperature of yad soledet.
. A kli rishon on the flame must always be koshered in boiling water. Whether its temperature was just barely yad soledet or beyond the boiling point at the time of the absorption, all absorption in a utensil on the fire is treated uniformly and the utensil must always be koshered in boiling water. This principle is explained beautifully in R. Pfeiffer’s Kitzur SA, Basar Be-ĥalav vol. 2. However, some authorities maintain that even a utensil sitting on the fire is subject to the standard principle of ke-bole’o kakh polto, meaning that if it absorbed taste at 80ºC it releases the taste at the same temperature. See Sidur Pesaĥ Ke-hilkhato 1:4, SAH 451, and Kaf Ha-ĥayim ad loc. 82.
Regarding a kli rishon not on the flame, most poskim agree that the koshering process must be done at a temperature of yad soledet bo, as stated in MA OĤ 451:7, Pri Ĥadash 452:3, and Pri Megadim 451 (Mishbetzot Zahav 9). However, Sidur Pesaĥ Ke-hilkhato 1:4 mentions opinions that the koshering temperature must be the same as the absorption temperature, so to avoid uncertainty it is best to kosher such a utensil in boiling water. The temperature of yad soledet is uncertain, somewhere between 45ºC and 71ºC (see Peninei Halakha: Shabbat 1:10:7), so to kosher a utensil, the water must be at least 71ºC. On the other hand, if the utensil was used at a temperature of at least 45ºC, there is concern that absorption took place and therefore one should be stringent.
Most poskim maintain that a kli sheini does not absorb, but due to the stringent nature of the prohibition of ĥametz, they mandated hagala. Be-di’avad, if the utensil was not koshered and was later inserted into hot food, the food is not forbidden (MB 451:11). The vast majority of poskim maintain that a kli shlishi does not absorb at all. Even so, several authorities insist that every utensil, even ten times removed from the fire, must be koshered if its contents reached the temperature of yad soledet (Pri Ĥadash). See Hagalat Kelim 5:52. This is the le-khatĥila practice.
We should also note that the poskim disagree about the status of a solid food (“davar gush”) at the temperature of yad soledet: According to Rabbeinu Yona, Me’iri, Maharshal, Shakh, and MA, even if it is in a kli shlishi, the food has the status of a kli rishon removed from the flame, since it retains its heat. Conversely, according to Tosafot, Ran, Rema, and Gra, the food assumes the status of the utensil that contains it. Regarding hagala, the custom is to kosher such utensils in boiling water on the fire.