Terumos, Chapter 5, Mishnah 3. This mishnah continues from the previous one and presents the other side of the coin: a se'ah of tahor Terumah that fell into a hundred of tamei chullin. Once again we have before us a single unit of Terumah that fell into a hundred units of chullin, except that here the Terumah is the tahor item and the chullin are the tamei ones.
The rule, as has been explained many times, is that since the mixture contains a hundred and one units, one unit is removed and given to a kohen. It turns out that the kohen receives Terumah back, and seemingly he may even eat it, for what fell in was Terumah, and what is removed is treated as Terumah.
Still, we cannot ignore the fact that the overwhelming majority of what is given to the kohen, even though we call it Terumah, is in fact tamei. Remarkably, we treat it as tahor, and this presents no difficulty, because what the kohen actually receives is chullin, and he is, as it were, pretending that this is Terumah and eating it as Terumah. Beneath the surface, however, the possibility of tumah remains, for most of the mixture is tamei material, and we do not want that tumah to spread to other Terumah.
For this reason the same safeguards apply here that were explained in the two previous mishnayos: to keep that se'ah of grain dry, so that it should not become susceptible to tumah; and if it is moistened, that it not be with water; and likewise to keep it in small quantities, less than a k'beitzah in any one place, since tumah does not spread in less than a k'beitzah.
The words of the mishnah:
"Se'ah Terumah tehorah shenaflah l'meah chullin temei'in" - a single unit of Terumah that was tahor, which fell into a hundred units of chullin that were tamei.
"Ta'aleh" - one single unit is removed and returned to a kohen.
"V'tei'achel nekudim" - the kohen may even eat it, provided that he takes care that the tumah not spread: he should eat it dry.
"O keliyos" - or he should roast it in fire.
"O tilosh b'mei peiros" - and he may even knead it into dough, provided that the liquid he uses is not water but fruit juice, which does not render food susceptible to tumah.
"O tis'chalek l'isos, k'dei shelo yehei b'makom echad k'beitzah" - that he divide the quantity into small portions, less than a k'beitzah in any one place.
That is, he may take that se'ah of grain which he treats as Terumah and distribute it among many batches of dough, in such a way that in no single batch does a k'beitzah of the tamei material accumulate - for in reality almost all of it is tamei. It follows that he may not put more than a k'beitzah of that se'ah into one batch of dough, lest it become a source of tumah in the measure of a k'beitzah, capable of rendering tamei the other Terumah in the mixture.
In summary: in this mishnah the Terumah that fell in is the tahor item and the chullin are the tamei ones. Since the mixture contains a hundred and one units, one unit is raised out and given to a kohen, and he may eat it as Terumah. But since most of what he receives is in fact tamei, safeguards were established to prevent the spread of tumah: eating it dry, roasting it, kneading it with fruit juice rather than water, or dividing it among many batches of dough so that there is never a k'beitzah in any one place.