A utensil must be cleaned properly before undergoing hagala, for although boiling water extracts the taste absorbed in the utensil, it does not clean the utensil of residual food stuck to its walls. If hagala is performed without first removing food residue, it is completely ineffective; the utensil must be cleaned, and hagala must be redone.
If the utensil has crevices containing food particles that cannot be removed, the residue can be incinerated via light libun. It is best to do so before performing hagala, but the utensil becomes kosher even if it the libun is done afterward (MB 451:25; see Kaf Ha-ĥayim 160).
If it is impractical to incinerate the food particles in these areas because it will damage the utensil, one may soak the utensil in a mixture of water and bleach or soap for a short while (thus rendering the food foul and inedible), and then perform hagala.
Pot and pan handles must also be koshered because when metal vessels are heated during cooking, the heat spreads to the handles, and if they reach the temperature of yad soledet, the taste might be transferred to them. The handles of wooden pots must also be koshered, even though they do not become very hot, because hot food often spills over or splashes onto them, and they absorb the taste. Therefore, both the utensil and its handles must undergo hagala (SA 451:12; MB ad loc. 68). However, even if the pot itself absorbs at a heat of a kli rishon over fire, its handles can be koshered through irui from a kli rishon, because they do not absorb with the same intensity as a kli rishon over fire (Rema 451:12).
Many pots have handles that are attached with small screws, and food particles get stuck in their grooves. Therefore, before performing hagala the bolts must be taken off so that all of these particles can be removed. If this is difficult to do, one may soak this area of the pot in a mixture of water and bleach or some other harsh liquid cleanser in order to render the food particles completely befouled.
. Obviously, once the food residue in the crevices absorbs the taste of the soap, it is no longer edible and thus no longer forbidden (see also Hagalat Kelim 6:4). This seems problematic, though, as MA 451:5, Taz, and many other Aĥaronim state that knives comprised of two pieces cannot be koshered due to the food residue that is stuck in the groove between the pieces. They did not raise the option of simply befouling the taste of the food stuck in between. Perhaps a distinction can be drawn between the crevices that are near the food and grooves that are further away, like where the handle connects with the utensil (see SAH 451:21, which makes this distinction when be-di’avad one forgot to kosher the handle of a utensil). In truth, even the case of the knife needs further examination, since the reason for the rabbinic prohibition against using a utensil even twenty-four hours after absorbing prohibited matter is so that one will not mistakenly use a utensil within twenty-four hours of the absorption. However, the Sages did not include the food stuck in the grooves in this prohibition, and if this food is not fit for a dog’s consumption, it is no longer considered ĥametz. Perhaps one need not take the words of the Aĥaronim literally; rather, they just meant to teach that when the utensil has nicks and grooves that may have trapped ĥametz, simply performing hagala is insufficient. This matter requires further examination.