Before us is Mishnah 10 of Maseches Challah. The Mishnah deals with a piece of sourdough starter taken from a dough of wheat flour that became mixed into a dough made of rice flour: "Im yeish bo ta'am dagan - chayeves ba'challah, ve'im lav - peturah" - if there is a taste of grain in it, the dough is obligated in challah, and if not, it is exempt. In other words, if the taste of the wheat is discernible in the rice dough, the dough is obligated in challah, and if its taste is not discernible, it is exempt.
The foundation of the matter, as we have already mentioned: rice is exempt from challah, while wheat is obligated in it. For that reason, in the case before us the obligation depends entirely on the question of whether the taste of the wheat is discernible in the dough.
The novelty of the Mishnah:
Generally, when we deal with sourdough starter, the measure by which it is judged is its leavening power - whether it is enough to cause the dough to rise. That quantity is smaller than the ratio required for imparting taste, which is usually one in sixty; the starter can be effective even at a ratio of one in a hundred, one in two hundred, and even less than that. Accordingly, there would have been room to determine the law of nullification here as well according to the leavening power.
However, the mixture before us is a rice dough, and rice is not subject at all to the concept of leavening - it rots rather than leavens. For that reason leavening cannot serve as the measure here, and the ruling is determined by taste alone: if the taste of the wheat is discernible in the dough, it is obligated, and if not, it is exempt.
"Im kein, lamah amru ha'tevel oser kol shehu?" - If so, why did they say that tevel prohibits in any amount at all?:
Once we have heard that the mixture is judged according to taste, the Mishnah raises a difficulty: a general rule was stated that produce from which Terumah was not separated - and likewise dough from which challah was not separated - prohibits the mixture in any amount at all, even in the smallest quantity. Here we do not say so, but rather we make the law depend on the imparting of taste.
The Mishnah answers that this rule was stated specifically where the two kinds are the same:
"Min be'mino - kol shehu" - a kind mixed with its own kind, in any amount at all: dough from which challah was not separated, which is tevel, that became mixed with dough from which challah was already separated, both of them made from wheat flour - this is a kind mixed with its own kind, and in such a case the tevel prohibits even in the smallest amount.
"Ve'shelo be'mino - be'nosein ta'am" - and a kind mixed with a different kind, by imparting taste: wheat flour that became mixed into a rice dough, which are two different kinds - here the ruling goes by the imparting of taste, that is, by whether one can discern the taste of the one within the other.
In summary: we have learned that wheat starter that fell into a rice dough is not judged according to its leavening power, since rice is not capable of leavening at all, but rather according to the taste of the grain in it. We have also learned the distinction between a kind mixed with its own kind, where the tevel prohibits in any amount at all, and a kind mixed with a different kind, where the law is determined by the imparting of taste.