Orlah, chapter two, mishnah 7. This mishnah is a continuation of the previous one. In the previous mishnah we dealt with a mixture of a substance with its own kind - two things of the same type - and there we learned that the forbidden item is not nullified unless there is one hundred times as much permitted food against it; and if the forbidden item is what causes the dough to leaven or what spices the mixture, it has the power to prohibit even beyond that measure.
A substance mixed with a different kind - the criterion is taste:
Our mishnah discusses a substance mixed with a different kind, where the two kinds are not similar to one another: Terumah wheat, for example, that fell into barley or into some other kind. In such a case we do not follow the measure of one in a hundred, but rather the taste - whether the taste of the forbidden item can be detected, which is generally a measure of one in sixty.
The words of the mishnah: "l'hakel ul'hachmir min b'sheino mino" - the law of a substance mixed with a different kind sometimes tends toward leniency and sometimes toward stringency, and we will immediately see how this is expressed.
The example the mishnah gives: "k'gon g'risin shenisbashlu im adashim" - split grain made from wheat kernels, which is Terumah, that was cooked together with lentils. These are two different kinds, and therefore this is a substance mixed with a different kind.
The stringent side:
"im yesh bahem b'nosen ta'am" - when the taste of the Terumah can be detected, then "bein sheyesh bahem l'ha'alos b'echad u'meah u'vein she'ein bahem l'ha'alos b'echad u'meah - asur". That is, even if the mixture contains one hundred times as much and even more than that, as long as the taste is noticeable - it is forbidden. Once the taste can be detected, the measure of one in a hundred is not taken into account at all. Generally, when the mixture does not contain one hundred times as much, the taste can indeed be detected, and therefore taste is the only criterion here, while the measure of one in a hundred is the criterion for a substance mixed with its own kind alone.
The lenient side:
"ein bahem b'nosen ta'am" - when the taste is not noticeable, then "bein sheyesh bahem l'ha'alos b'echad u'meah u'vein she'ein bahem l'ha'alos b'echad u'meah - mutar". Even if the mixture does not contain one hundred times as much, since the taste is not noticeable - it is permitted. The measure of one in a hundred for Terumah, and likewise the measure of one in two hundred for orlah and kilayim, was stated only regarding a substance mixed with its own kind, and not regarding a substance mixed with a different kind, which is our case. This is the leniency in this law.
In summary: when a substance is mixed with a different kind we do not follow the measures of nullification - one in a hundred for Terumah and one in two hundred for orlah and kilayim - but rather taste alone: if it imparts taste, it is forbidden even if there is one hundred times as much against it (a stringency); if it does not impart taste, it is permitted even if there is not one hundred times as much against it (a leniency).