(Mishnah): One may take the amount for Chalah from a dough from which Chalah was not taken, to do it in Taharah, to continually separate on doughs of Demai, until [the dough from which he separate] spoils, for Chalas Demai may be taken from Tahor on Tamei, and Lo Min ha'Mukaf.

(Gemara) Question: What does it mean 'until it spoils' - [is it until it ceases to be] for human food, or until it spoils from being food for a dog?

Answer: We learn from the following [Beraisa]. If it spoiled, it has Tum'as Ochlim, and we may burn it b'Tum'ah.

It has Tum'as Ochlim, and you say that [perhaps] it is spoiled from being food for a dog?! Rather, it from being human food.

Inference: [Our Mishnah] teaches that one may be Torem [Demai] from bad on good;

It is like R. Shmuel bar Rav Nachman said in the name of R. Yonason, that one may be Torem Istpanini leaves (which are eaten only in pressed circumstances) on Istpanini, in a place where they eat them. (We explained this like SHA'AREI TORAS ERETZ YISRAEL.)

This is like [the case of] Gamliel Zuga. He forgot to tithe Istpanini [and cooked it with other matters - SEFER NIR. Do not say that he ate it. We permit separating after eating only for Terumas Chutz la'Aretz.] He asked R. Yochanan;

R. Yochanan: If there are still some of the leaves [that are normally removed], separate from those leaves.