Halacha requires that one use separate pots for the preparation of meat and dairy foods. The pots should be carefully marked to ensure that a meat pot is not used for the preparation of dairy food, and vice versa.
If nevertheless a mistake is made, and one used a meat pot for cooking a dairy food, what is the status of the food and the pot?
The Halacha in such a case depends on whether or not the pot had been used for cooking meat within the previous twenty-four hours. If the pot had been used for meat during this period, then the dairy food cooked in that pot is forbidden for consumption, since during the process of cooking the dairy food absorbs the meat taste from the walls of the pot. By the same token, the walls of the pot, which already contain the taste of meat, now absorb the taste of the dairy food, and therefore the pot becomes forbidden for use until it undergoes Hag'ala (immersion in boiling water).
If, however, the pot had not been used with meat within the last twenty-four hours, then the dairy food does not become forbidden for consumption. There is a Halachic principle known as "Notein Ta'am Li'fgam," which says that taste absorbed by the walls of a pot becomes spoiled after twenty-four hours, at which point it no longer has the capacity to render other foods forbidden. In our case, then, the taste of meat embedded within the walls of the pot has no effect on the dairy food. The pot, however, indeed becomes forbidden for use until it undergoes Hag'ala.
It must be emphasized that this Halacha applies only if this occurred due to an error or oversight. If, however, one purposely used a meat pot for the preparation of a dairy food, then even if twenty-four hours had passed since the pot's previous use, the food may not be eaten. The Sages imposed a penalty against a person who intentionally uses a pot in this fashion, and forbade the food for consumption, despite the fact that the taste of meat in the walls of the pot cannot technically render the dairy food forbidden.
These Halachot apply just the same to kosher food cooked in a pot that had been used for non-kosher food. If, for example, a person mistakenly cooked kosher food in a pot that had been used by a gentile and had not undergone Hag'ala, then if the pot had not been used for non-kosher food within the previous twenty-four hours, the food is permissible. The pot, however, still requires Hag'ala. But if the pot had been used for non-kosher food within the previous twenty-four hours, the food may not be eaten. If one used this pot while fully aware that it had been used for non-kosher food, then the food he cooks is forbidden even if the pot had not been used during the previous twenty-four hours.
(These Halachot are based on Halichot Olam, vol. 7, page 70.)
Summary: If one cooks dairy food in a meat pot that had been used for meat within the last twenty-four hours, then the food is forbidden for consumption and the pot may not be used until it undergoes Hag'ala. If the pot had not been used for meat within the previous twenty-fours, then the food is permitted for consumption, provided that this occurred by mistake, and not intentionally. Even in such a case, however, the pot requires Hag'ala. These laws apply as well to a case of kosher food that was cooked in a pot that had been used for non-kosher food.
