Terumos, Chapter 10, Mishnah 2. In the previous Mishnah we established the basic principle: when the taste of the forbidden food is present in a mixture, the presence of the forbidden food itself is considered to remain intact. Therefore, in a mixture of issur and heter - forbidden and permitted foods together - as long as the forbidden component can be tasted, the entire mixture remains forbidden.
Our Mishnah presents two exceptions to this principle:
Ma'amid: the taste of the forbidden food is no longer present, and nevertheless the mixture remains forbidden.
Nosein ta'am lifgam: the taste is still present, and nevertheless the mixture is permitted.
The first exception - 'ma'amid':
The Mishnah discusses an apple of Terumah, which of course is a forbidden item that non-kohanim may not eat: "sherisko" - he crushed and ground it until it became a sort of pulp and apple juice, "unesano lesoch isah vechimtzah" - he placed it into a dough of chullin, and the presence of the apple and its juice caused the dough to ferment and rise.
The loaf that comes out of that dough is now forbidden, and it has the status of Terumah, which non-kohanim may not eat. The reason is not because the taste of the apple remains in the dough - for the apple juice is no longer detectable in it at all - but because the very rising of the dough announces the presence of the apple juice, and for that reason one cannot say that it is nullified and gone.
This is the rule of 'ma'amid'. 'Ma'amid' is the causative form of 'omeid', meaning that which causes something to stand: in our case, it is the apple that made the dough rise, and therefore its presence in the mixture is evident to everyone. For this reason, even though it is impossible to taste the apple, it renders the mixture forbidden. We have already seen a similar halachah in this tractate, regarding sourdough used as a leavening agent: if it was Terumah, the entire mixture becomes forbidden, as we discussed in connection with challah.
The second exception - 'nosein ta'am lifgam':
The second case in the Mishnah is an exception to the rule of "ta'am ke'ikar", and it is called 'nosein ta'am lifgam': the taste is indeed present, but it spoils and detracts from the good taste of the food. In such a situation, even though the taste exists, it does not meet the rabbinic requirements to forbid the mixture, since this added taste makes the food less good rather than better, and therefore the mixture remains permitted.
The Mishnah discusses barley that fell into a cistern containing water, and teaches that even though the water soured - and this does not mean actual fermentation, but rather that it rotted and spoiled - the water remains permitted. But this wording as it stands is difficult: it is not a case of "even though" the water spoiled, for it is precisely because of the barley that fell in that the water spoiled.
The simplest explanation is that of the Yerushalmi: at the moment the barley first falls into the water, the taste improves; but afterward the barley begins to rot, and the taste is ruined and becomes worse than it was at the outset. This is the novelty of the Mishnah, which chose specifically the case of barley: even though at first it imparted a favorable taste and temporarily improved the taste of the water, now that it has ruined and worsened the taste, its presence in the mixture falls under the category of 'nosein ta'am lifgam' - an added taste that spoils.
Therefore, even though the barley in this Mishnah is Terumah, the water remains permitted to non-kohanim, because the Terumah barley imparts a spoiling taste: it ruins the water rather than improving it. And this is the exception to the rule of "ta'am ke'ikar".
In summary: In this Mishnah we learned two exceptions to the principle of "taste is like substance." A "ma'amid" (a substance that sets or activates the mixture) - an apple of Terumah that was mashed and placed into a dough and caused it to ferment: even though its taste is no longer discernible, the very fact that it sets the dough and causes it to ferment renders the mixture forbidden. "Nosen ta'am lifgam" (imparting a taste that spoils) - barley of Terumah that fell into a cistern of water and ruined its taste: even though the taste is present, it does not render the mixture forbidden, and the water remains permitted to non-kohanim.