There is a well-known rule that something that imparts foul taste (“noten ta’am li-fgam”) does not render a mixture forbidden. For example, if non-kosher meat falls into a pot of kosher food, and the quantity of kosher food is sixty times that of the non-kosher meat, the taste of the non-kosher meat is nullified, and it is permitted to eat the food. On the other hand, if the kosher food is not sixty times the quantity of the non-kosher meat, it is forbidden to eat the food, because the taste of the non-kosher meat is discernible in the mixture. If, however, the taste of the non-kosher meat is foul (“pagum”), since it spoils the cooked dish, it does not render it non-kosher. Therefore, as long as the kosher food constitutes the majority of the mixture, it is permitted to eat it (SA YD §103).
What about a ĥametz mixture on Pesaĥ? Some poskim (Rashbam, Rashba) say that the fact that the Sages, due to the gravity of the ĥametz prohibition, ordained that even a drop of ĥametz renders any mixture forbidden indicates that the matter does not depend upon the taste it gives to the mixture. Therefore, even when it contributes a foul taste, it is no different than a drop of ĥametz that renders its entire mixture forbidden.
The opinion of most Rishonim (Rabbeinu Tam, Ri, Rosh, and Mordechai) is that ĥametz is like other prohibited foods except with regard to bitul be-shishim. Where other forbidden foods do not render a mixture forbidden, however, like in cases of noten ta’am li-fgam, ĥametz also does not render a mixture forbidden.
In practice, SA 447:10 rules leniently whereas Rema writes that in Ashkenazic communities the custom is to follow the stringent ruling that even a drop of foul-tasting ĥametz renders an entire mixture forbidden.
Let us clarify this disagreement by way of an example: a pot in which non-kosher meat is cooked absorbs the taste of the non-kosher meat. If the same pot is then used to cook kosher food, the kosher food will absorb the taste of non-kosher meat exuded by the pot, rendering it forbidden. However, if more than twenty-four hours passed since the cooking of the non-kosher meat, the taste absorbed by the pot is pagum, and if some other food is cooked in it, it will not be rendered forbidden, because the pot is noten ta’am li-fgam.
Similarly, if one inadvertently cooks in a ĥametz pot during Pesaĥ, according to Shulĥan Arukh and most poskim since more than twenty-four hours have passed since ĥametz was cooked in the pot, the food is kosher. However, according to Ashkenazic custom, although the taste of ĥametz absorbed into the pot is foul, it renders the food forbidden, because during Pesaĥ we take the stringent position that even noten ta’am li-fgam renders a food forbidden.
. The custom of the Ashkenazic community is to be stringent, as we have said; however, MB 447:98 states (citing Terumat Ha-deshen) that where there is no established custom, one may act leniently, though one who acts stringently is commended. Sephardic custom follows SA, although some are stringent (Kaf Ha-ĥayim 447:228). Rema 447:2 states that the custom to be stringent about ĥametz pagum only applies once Pesaĥ begins, but before the onset of the holiday, the ĥametz is nullified even in less than a sixty-to-one ratio, like the normal case of foods that spoil the taste of a mixture into which they fall.
It is also important to stress that according to all opinions, one is not permitted to use a utensil that absorbed the taste of a forbidden food, even more than twenty-four hours after absorption. This is because the Sages were concerned that if they permitted the use of such utensils after twenty-four hours, people would make mistakes and use them even within twenty-four hours and would end up eating forbidden foods. Understandably, this is also the halakha on Pesaĥ. If one violated this prohibition and intentionally used a utensil that had absorbed forbidden foods more than twenty-four hours prior, most poskim maintain that he is not permitted to eat the food due to a rabbinic penalty imposed on him (MB 442:1; Knesset Ha-gedola YD §122, Hagahot Ha-Tur §26).