If one has two piles of untithed produce, one can separate more terumot or maaserot from one pile to satisfy the requirements of the other. The Mishnayot towards the end of the first perek of terumot discuss the limits on this process, e.g., they must be both be the same type and both tevel. One case the Mishnah (1:9) discusses is that one can take terumah from oil for olives intended pickling or wine for grapes intended for producing raisins.
The Tosfot Anshei Shem notes that it would seem we are dealing with already pickled olives and raisins, that have reached gmar melacha (all work complete). The reason is that the end of the Mishnah rules that ideally one can only separate from produce that has reached gmar melacha for other produce that has also reached that stage. Furthermore, since the Mishnah in Maaserot (1:9) teaches that gmar melacha for raisins is only once they have been formed into a pile, it must be dealing with raisins at that stage. He suggests that that is indeed the understanding of the Rambam.
The Yerushalmi however understands that the Mishnah is referring to olives that will be used for pickling and grapes that will be used for producing raisins. Consequently, the Gemara questions why one can separate from the wine for theses grapes, since one should not ideally separate from something that has reached gmar melacha for something that has not.
The Gemara cites R' Yitzchak ben Elazar who explains that the source for this rule that one should only separate from produce that has reached gmar melacha for other produce that has also reached that stage is the following pasuk: "Your gift shall be reckoned for you like grain from the threshing-floor and like the ripeness of the vat". In other words, we learn that the one must separate from products for other products that have all "reached the vat" -- gmar melacha. Importantly, the pasuk list specific cases. The Pnei Moshe explains that the pasuk only excludes those things explicitly mentioned in the pasuk, with our case left out.
If we understand the Gemara like the Pnei Moshe, that the Torah only excluded specific cases, it is difficult to understand why the Mishnah (1:8) lists those specific cases and then later (1:10) teaches this law as a general rule. (See the Radbaz, Maaser 5:18 who poses a similar question.)
R' Chaim however in his commentary explains that the pasuk excludes grapes that will be used for pickling which are "considered as if they have reached gmar melacha in this respect".
Importantly, what we derive from R' Chaim's understanding is that the term gmar melacha has different meanings in different contexts. Using the example of raisins, for terumot and maaserot, they reached gmar melacha once the raisins have finished drying and have been formed into a pile. Gmar melacha in that context -- keviut - is the point after which one can no longer eat from that food in a snack fashion without separating terumot and maaserot -- all the required work has been performed. Here however, the term gmar melacha has a different meaning and a more subtle understanding. How so?
The Chazon Ish (Maaserot 3:23) suggests that perhaps what we learn from the pasuk is that when separating from one pile for another, the Torah excludes only items that lack a substantial physical process, e.g., grapes that require pressing. Consequently, despite forming a pile being considered substantial with respect to the obligation of keviut for terumot and maaserot, it is not substantial for this law to prevent one from separating from wine for raisins. That said, he continues that if the hafrasha is performed prior to the grapes turning into raisons, as we have been explaining, then this explanation would be insufficient since it should also be considered lacking a substantial change in the grape itself.
Along a similar line to the Chazon Ish, perhaps for hafrasha, the Torah excludes not food that lack a substantial change, but food that lack a substantial change requiring human involvement -- grapes that need to be pressed, grain that needs to be threshed. Since for these grapes it is a matter of leaving them to dry, they are considered "complete" for this law.
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