Hametz on Pesach differs from other forbidden foods in that it is not subject to "Bittul" – nullification in a mixture. Generally, if a small quantity of forbidden food falls into permissible food, such as a drop of milk that falls into meat, the food remains permissible if there is a 60:1 proportion of permitted food to forbidden food. (Of course, one may not knowingly pour a small amount of milk into meat; the law of "Bittul" applies in situations where this occurs accidentally.) Hametz, however, renders a mixture forbidden for consumption in any proportion; even the slightest amount of Hametz suffices to forbid food with which it comes in contact.
The Rishonim (Medieval authorities) debate the question of whether this rule affects the status of Hametz with respect to the Halachic principle of "Notein Ta'am Lifgam." Generally speaking, although one may not cook with a pot that had been used for non-kosher food, if one did cook with such a pot, the food is nevertheless permissible if the pot had not been used with non-kosher food within the previous twenty-four hours. Any taste particles in the walls of a pot become spoiled after twenty-four hours; therefore, if a person uses a pot in which non-kosher food had been cooked over twenty-four hours earlier, the food he cooked is deemed kosher. This Halacha is known as "Notein Ta'am Lifgam Mutar" (literally, "that which lends a foul taste is permissible").
Would this Halacha apply to the use of a Hametz pot on Pesach? Consider, for example, a case of somebody who cooks rice on Pesach (according to the Sephardic custom permitting the consumption of rice on Pesach) and then realizes that the pot had been used several days earlier for cooking spaghetti. Do we apply to the prohibition of Hametz the rule of "Notein Ta'am Lifgam," and therefore allow the rice for consumption, or does this rule not apply to Hametz, just as the provision of "Bittul" does not apply to Hametz?
The Rishonim debate this issue, and Rabb Yosef Karo writes in his Beit Yosef (447) that we should follow the majority of Rishonim, who allow eating food cooked in this kind of pot on Pesach. This is indeed his ruling in the Shulchan Aruch.
Therefore, if a person prepared on Pesach rice or some other food in a pot that had been used for cooking Hametz over twenty-four hours earlier, the food is permissible for consumption.
It must be emphasized, however, that this applies only Be'di'avad, after the fact. Needless to say, one must either put away or kosher all his Hametz pots and one certainly may not use them to prepare food for Pesach. The question we addressed here involves a case of someone who mistakenly used a Hametz pot in preparing food for Pesach, in which case one may partake of the food if the pot had not been used within the previous twenty-four hours.
