(Mishnah): One may not separate from olives for oil or from grapes for wine. If he did, Beis Shammai say that they contain their own Terumah (but not the Terumah of the oil or wine). Beis Hillel say that the Terumah is not valid.

(Gemara) Question: The Mishnah (at the end of this Perek) teaches that one may not separate from something which is completed for something which is not completed (implying that if it was done, it takes effect). But here the Mishnah taught that it is not valid...?

Answer (R. Ila citing R. Yochanan): In our Mishnah, there will be a loss to the Kohanim (lit. theft from the tribe).

(R. Chananya explains): R. Yochanan was concerned for the extra effort that the Kohen must make to crush the olives or the grapes.

Question: If rice was separated as Terumah in its skins (even though extra effort is needed by the Kohen to crush it in order to remove the skins, it is still valid)?

Answer: A person is happy to do some crushing.

Question: If a person separated Terumah from ears of wheat (which involves a lot of effort to process, is it valid)?

This was the way that R. Chanina questioned the reasoning of R. Yochanan, but R. Mana didn't understand that it was about making extra effort. Rather, the problem is that he should be separating from the olives according to the quantity of oil that is extracted from them (rather than according to the volume of the olives. As a result, he gives less Terumah than is required and is actually stealing from the Kohanim.)

Question: But if he did separate correctly according to the oil, why wouldn't that Terumah be valid?

Answer: We are concerned that on another occasion, he might separate according to the olives. And not only that, but even if he separated olives according to the oil, an onlooker might think that he is only doing this as an extra stringency, but it isn't a requirement.

(Chizkiyah): Beis Shammai only spoke about separating from olives for oil or from grapes for wine; but for other cases of separating from that which is exempt for that which is obligated, it isn't valid Terumah at all.

(R. Yochanan): They don't differentiate at all between olives and grapes and other things.