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 cooked dish. If the kosher food is not sixty times the quantity of the non-kosher meat, since its taste is discernible, the cooked dish is forbidden. If, however, the taste of the non-kosher meat is foul (“pagum”), since it spoils the cooked dish, then as long as the kosher food constitutes the majority of the mixture, it is permitted to eat it (SA YD 103:1).
What about a ḥametz mixture on Pesaḥ? Some say that the fact that the Sages, due to the gravity of the ḥametz prohibition, ordained that even a drop of ḥametz 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 ḥametz that renders its entire mixture forbidden (Rashbam, Rashba).
The opinion of most Rishonim is that ḥametz is like other prohibited foods except with regard to bitul be-shishim. Thus, in cases like noten ta’am li-fgam, where other forbidden foods would not render a mixture forbidden, ḥametz also does not render a mixture forbidden (Rabbeinu Tam, Ri, Rosh, and Mordechai).
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 ḥametz renders an entire mixture forbidden.
The custom of most Ashkenazic communities is to be stringent, but MA and MB 447:98 state (based on Terumat Ha-deshen §128) that where there is no established custom, one should rule that one who acts leniently has done nothing wrong, though one who acts stringently is commendable. The prevailing Sephardic custom follows SA, as stated by Pri Ḥadash, Ḥida, Mahari Ayash, and others, for in addition to the fact that most Rishonim are lenient, this dispute is about a rabbinic enactment, in which halakha follows the lenient view. Even so, some accept the stringent practice, as stated by Kaf Ha-ḥayim 447:228. Rema 447:2 states that the custom to be stringent about ḥametz pagum only applies once Pesaḥ begins, but before the onset of the holiday, the ḥametz is nullified even in less than a sixty-to-one ratio, like any other prohibited food that befouls a mixture.