Komentarz do Orla 2:9

ור"ש מתיר – in the first clause [of the Mishnah], Rabbi Shimon did not dispute since the leaven of heave-offering fell prior to the leavening of the unconsecrated produce. For it hurries to leaven through the leaven of heave-offering, but in the concluding clause [of the Mishnah], since it already became leavened in the unconsecrated produce. For when it Terumah/heave-offering falls, it does nothing other than disqualify/make it unfit, but the Rabbis hold that even though in general, when it gives it a bad taste, it is permissible, but it is different here as it makes it appropriate to leaven other pieces of started dough, and the Halakha is according to the Sages.

Introduction In yesterday’s mishnah, the terumah leaven fell into already-leavened dough before the dough had begun to ferment, and therefore, since the terumah leaven may have sped up the fermenting process, the dough was prohibited. In today’s mishnah, the leaven falls in after the dough has already been fermented.

If leaven of hullin has fallen into dough and caused it to ferment, and after that there fell in leaven of terumah or of kilayim of the vineyard, and there was enough to cause fermentation, [the dough] is prohibited. According to the first opinion, it doesn’t matter that the terumah leaven fell into the dough after the dough had already been fermented. The forbidden leaven will still increase the fermentation of the dough and therefore, the entire dough becomes prohibited.

But Rabbi Shimon permits it. Rabbi Shimon, on the other hand, holds that the dough is permitted. One explanation for Rabbi Shimon’s opinion is that he holds that extra leaven will not improve the taste of the dough, but rather make it taste worse. Since this taste is undesirable, it does not cause the dough to be prohibited. In yesterday’s mishnah, the leaven terumah improved the taste of the dough because it had not yet been fermented, therefore, Rabbi Shimon would agree that the dough is prohibited.