Orlah, Chapter 2, Mishnah 6. As we have mentioned, the rule that a prohibited substance is nullified in one hundred and one parts in the case of Terumah, and in two hundred and one parts in the case of orlah and kilei hakerem, applies specifically when the substance is mixed with its own kind, that is, when the prohibited item and the permitted item into which it falls are of the same type and the same species.
Two modes of nullification:
A substance mixed with its own kind: Nullification is determined by the numerical ratio - one in a hundred for Terumah, and one in two hundred for orlah and kilei hakerem.
A substance mixed with a different kind: Here we follow the taste, meaning whether the prohibited substance can be tasted or detected. The standard measure is one in sixty, and if we are dealing with something that has the power to leaven or to season - whose level of detectability is much higher - a far greater measure may be required: one in two hundred, one in three hundred, or one in four hundred. We have no precise measure for this.
The wording of the Mishnah:
The Mishnah opens: "Ulam amru" - "But they said." And what did they say? "Kol hamechametz umesabel bemeduma lehachmir" - this is what was stated in the two preceding mishnayos, and in Mishnah 4: anything that has the power to leaven or to season the mixture is treated stringently, and we do not regard it as nullified even in a ratio of one in a hundred for Terumah. And all of this applies when the substance is mixed with its own kind, when the prohibited item and the permitted item are literally the same item, for example, when both are wheat flour.
The Mishnah adds: "lekula ulechumra" - sometimes the ruling is lenient and sometimes stringent. When a substance is mixed with a different kind, where we are dealing with two different types - for example, one is barley and one is wheat - the ruling may be stringent. This matter will be explained in the next mishnah; our mishnah deals with a substance mixed with its own kind.
How does this play out in practice?
"Se'or shel chittim shenafal lesoch isas chittim veyesh bo kedei lechametz" - leaven made of wheat flour that fell into a dough of wheat, and it contains a sufficient amount and ratio to leaven the dough. Regarding this the Mishnah says: "bein sheyesh bo la'alos be'echad ume'ah uvein she'ein bo la'alos be'echad ume'ah" - the prohibition applies in either case.
It contains enough to leaven: It is forbidden whether or not there is a ratio of one in a hundred, and it is enough that the leavening was accomplished by the prohibited substance.
"Ein bo la'alos be'echad ume'ah": Even if it does not contain enough to leaven, the very fact that there is not at least one in a hundred renders it forbidden. For when a substance is mixed with its own kind, as in the case before us, the basic ratio of one in a hundred is required for Terumah (and for orlah, one in two hundred), and whenever the mixture falls short of that, there is a problem.
It emerges that with something that has the power to season or to leaven, a ratio greater than one in a hundred may be required, but the minimum threshold of one in a hundred is required in any case.
In summary: In this mishnah we learned that the rule of nullification by numerical ratio - one in a hundred for Terumah and one in two hundred for orlah and kilei hakerem - applies when a substance is mixed with its own kind, whereas when it is mixed with a different kind we follow the taste. We also learned that something that leavens and seasons is treated stringently, and therefore leaven of wheat that fell into a dough of wheat is forbidden whether or not there is enough to bring it to one in a hundred: if it contains enough to leaven - because of the leavening, and if it does not contain enough to leaven - because the ratio of one in a hundred is lacking.
In the next mishnah we will deal with a substance mixed with a different kind, and with the ways in which this leads to both leniency and stringency.