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.
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.
. 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.