Kashering a Pot Used to Cook Teruma
Question
I separated terumot and ma'aserot from my potatoes, and accidentally cooked the potatoes that I set aside. How do I kasher the pot?
Answer
The Gemara in Zevachim (96b) quotes the following Tosefta (Terumot 8,16):
With regard to a pot in which one cooked meat, one may not cook milk in it; and if one cooked [milk in it, the meat absorbed in the pot renders the milk forbidden, if it] imparts flavor to it. Similarly, if one cooked teruma in a pot, one may not cook chullin (non-sacred food) in it; and if one cooked [chullin in it, the teruma renders the mixture sacred if it] imparts flavor.
This Gemara implies that the same rule applies equally to milk and meat mixtures and to teruma and non-sacred mixtures, and that the pot should not be used until kashered. The Rambam (terumot 15,19) quotes this Tosefta, but adds: "and it if one rinses the pot with water or wine, it will be permitted to cook in it." Accordingly, the pot would not require hagala and rinsing would suffice. Many were confounded by this ruling, and the Rambam wrote to the sages of Lunel (Shut harambam 344) that his ruling was based on the Mishna in Terumot (11:8):
"One who pours from jar to jar and drips three drops, may place in it non-sacred food. But if he tilted the jar and gathered [the drops], it is teruma." The Rambam's understanding of the Mishna (pirush hamishna) is that if one pours a teruma substance from a vessel, and upon finishing pouring, waits for the remaining drops to pour out as well, it is unnecessary to wipe down the vessel so no moisture remains; rather, one is allowed to use the vessel for chullin—despite the fact that if one were to tilt the pot, one would see drops inside. From this Mishna the Rambam learns a priori that teruma wine remaining in a vessel does not impart teruma sanctity, even though it is substantial (if one would tilt the pot to gather the drops, these drops would be considered teruma). Therefore, any teruma wine absorbed by the walls of the pot would certainly be nullified by a simple rinsing.
In explaining the Rambam, the Aruch HaShulchan (YD 93, 3-12) adds that one can be lenient when a vessel absorbs a permitted substance (heteira bala) such as teruma. If this were a forbidden substance, hagala would be required to kasher it.
However, most poskim disagree with the Rambam's ruling and write that it is necessary to purge teruma cooked in a pot through hagala, and this is the halacha in practice.
For the full answer including footnotes, see here.