Orlah, Chapter 2, Mishnah 4. This mishnah comes to teach an important principle: even though Terumah is nullified in one hundred parts, and orlah and kilei hakerem (mixed species of the vineyard) are nullified in two hundred parts, nevertheless, if the forbidden substance - whether kilei hakerem, orlah or Terumah - affects the permitted food it fell into by way of leavening (as a fermenting agent) or by way of seasoning, and it is noticeable in the mixture even in a proportion greater than one in a hundred or one in two hundred, the mixture is forbidden. Since the forbidden substance is noticeable, it is not nullified. This is the subject our mishnah describes.
The text of the mishnah:
"Kol hamachmitz umetabel vehamedama beTerumah uve'orlah uvechilei hakerem - asur" - anything that leavens, seasons or creates a forbidden mixture, in the case of Terumah, orlah and kilei hakerem, is forbidden. Three cases are listed here:
"Hamachmitz" - anything that causes leavening, that is, fermentation.
"Umetabel" - anything that seasons, imparting taste to the mixture.
"Vehamedama" - anything that creates a forbidden mixture. This word is difficult, and some read it as "medama," since the meaning of "medama" is simply a mixture, and so it is not obvious how to read it in the mishnah.
In any event, the mishnah means that if one of these mixtures came about by way of leavening or by way of seasoning - "beTerumah uve'orlah uvechilei hakerem" - in Terumah, in orlah and in kilei hakerem - the mixture is forbidden, even beyond the measure of one in a hundred for Terumah and one in two hundred for orlah and kilei hakerem.
The dispute between Beis Shammai and Beis Hillel regarding tumah:
"UVeis Shammai omrim: af metamei" - and Beis Shammai say: it even conveys tumah. For example, a pure dough that was leavened with impure yeast, or that was seasoned with an impure spice. Generally we say that food does not convey tumah to other food unless it has the volume of an egg; but in the case of something that leavens or seasons, Beis Shammai hold that even a tiny amount conveys tumah, since its effect is noticeable.
"UVeis Hillel omrim: le'olam eino metamei ad sheyehei bo kabeitzah" - and Beis Hillel say: it never conveys tumah unless it has the volume of an egg. When it comes to tumah, it makes no difference whether the forbidden substance is noticeable or not. This is a law we learn from the Torah regarding the tumah of foods: one food cannot convey tumah to another food unless the impure food has the volume of an egg; less than that, and it has no power to convey tumah.
In summary: in this mishnah we learned that a forbidden substance that serves as a leavening agent, a seasoning or a mixing agent in Terumah, orlah and kilei hakerem is not nullified, even beyond one in a hundred and one in two hundred, because its effect is noticeable in the mixture. Regarding tumah the two houses disagree: Beis Shammai hold that it conveys tumah even in a tiny amount, while Beis Hillel hold that the laws of the tumah of foods do not depend on whether the substance is noticeable, and it never conveys tumah unless it has the volume of an egg.