Commentaire sur Orlah 2:8

ויש בו כדי לחמץ אסור – surely it comes to tell us that we don’t say, for since that without this prohibited leaven [of Terumah or mixed seeds of the vineyard] that the started dough would leaven through the permitted leaven that fell in it first, and we shouldn’t worry about prohibited leaven.

Introduction This mishnah deals with a situation in which two batches of leaven fall into dough one being hullin (non-sacred) and one being terumah or kilayim.

If leaven of hullin has fallen into dough, and there was enough of it to cause fermentation, and after that leaven of terumah fell in or leaven of kilayim of the vineyard, and there is enough to cause fermentation, [the dough] is prohibited. In this case, the first piece of leaven to fall into the dough was enough to ferment the entire dough. When the next piece of leaven falls in, and that dough is terumah or kilayim (both would seemingly cause the dough to be prohibited), we might have thought that since the dough could be fermented even without the terumah or kilayim, the dough would be permitted. The mishnah rules that the dough is prohibited even though the prohibited leaven was not needed. The second batch of leaven, although not needed, still sped up the leavening process and therefore it causes the dough to be prohibited.