All utensils used for non-kosher food must be kashered to permit them for use with kosher food. If a utensil was wrongly used without having been kashered, the kosher status of the food which was prepared in it depends on several criteria.
If kosher food was prepared with a non-kosher utensil, and this utensil still had actual food remnants or greasy residue, the food is forbidden and may not be eaten.76
If kosher food was prepared with a non-kosher utensil that is ben yomo (בֶּן יוֹמוֹ), “of its day,” meaning that the utensil had been used for non-kosher food within twenty-four hours of its coming in contact with the kosher food, the food is forbidden and may not be eaten.
There are circumstances in which the kosher food will bedieved remain permitted if there was a sixty-to-one ratio [termed shishim (שִׁשִּׁים)] between it and the non-kosher residue or absorption. This leniency does not apply to food prepared on Pesach with a chametz utensil.
If kosher food was prepared with a non-kosher utensil that is eino ben yomo (אֵינוֹ בֶּן יוֹמוֹ), “not of its day,” meaning that the utensil has not been used within twenty-four hours of its coming in contact with the kosher food then, bedieved, the food remains permitted. This is because, after twenty-four hours, the non-kosher flavor imparted into the kosher food is considered to be nosein ta’am lifgam (נוֹתֵן טַעַם לִפְגָם) “imparts detrimental taste” which, bedieved, does not forbid the kosher food.77 However, an eino ben yomo utensil may not deliberately be used without kashering. [Although, when food is prepared in this utensil, the detrimental flavor will not render it as non-kosher, Chazal (Avodah Zarah 76a) nevertheless forbade using an eino ben yomo utensil to ensure that it would not inadvertently be used while still ben yomo.] If an eino ben yomo utensil was inadvertently used for kosher food, the food remains permitted.
The leniency of eino ben yomo has several exceptions:
There are circumstances in which a sharp-tasting food, called a davar charif (דָּבָר חָרִיף), remain forbidden even when prepared in an eino ben yomo utensil; see Shulchan Aruch, Yoreh Deah Chapter 96, Passages 1-2. This is because sharp-tasting food is assumed to “reawaken” the dormant flavor absorbed in the utensil.
Rema (Chapter 447, Passage 10) rules that, if chametz became absorbed into a utensil on Pesach, it forbids the Pesach foods for which this utensil is subsequently used — even when the utensil is eino ben yomo and imparts detrimental taste. This stringency is akin to the halachah discussed above: if food was prepared with a chametz utensil on Pesach, it prohibits a food — even when there is a sixty-to-one ratio between the permitted food and non-kosher absorption.
76 Shulchan Aruch, Yoreh Deah 122:3; 103:5 with Taz 8, Shach 15.
77 Shulchan Aruch, Yoreh Deah 122:2.