Halacha forbids partaking of "Bishul Akum" food cooked by a gentile, even if the food is kosher and was prepared in kosher utensils. The Ben Ish Hai (Rabbi Yosef Haim of Baghdad, 1833-1909), amidst his discussion of this prohibition (Parashat Hukat, Shana Sheniya, 24), addresses the question of whether a utensil becomes forbidden for use if a gentile uses it for cooking. If, for example, a gentile cooks food over the stove in a pot, and that food meets the criteria of "Bishul Akum," does the pot now become non-kosher, such that food cooked subsequently in that pot will be forbidden?
The Ben Ish Hai rules that the pot indeed becomes non-kosher, as it has absorbed the taste of forbidden food. It therefore requires "kashering" through immersion in hot water in order to be suitable for use in the preparation of kosher food. The Ben Ish Hai emphasizes that the pot may not be used even after twenty-four hours have passed since the gentile used it. Even though absorbed taste becomes "Pagum" (spoiled) after twenty-four hours, Halacha forbids preparing food with even this kind of taste of forbidden food. Thus, a pot used by a gentile for cooking may not be used before it is "kashered."
Certain kinds of utensils such as earthenware and porcelain do not generally have the possibility of "kashering," because the absorbed taste cannot be extracted even though the process of immersion in hot water. Thus, if this kind of utensil absorbed non-kosher food, it may never again be used by a Jew. However, the Ben Ish Hai rules that with regard to "Bishul Akum," such a utensil may be used after it is immersed in hot water three times. Therefore, if a non-Jewish housekeeper prepared kosher food in an earthenware utensil, the Jew should immerse the utensil in hot water three times and may then use it. If the utensil would be ruined as a result of immersion in hot water, then one may use the utensil after twenty-four hours have passed since the gentile used it for cooking.
Summary: A utensil used by a gentile for cooking even kosher food must be "kashered" before it may be used by a Jew for cooking.
