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1. A Levi may process his Ma'aser Rishon in certain ways before he separates from it Terumas Ma'aser.
2. The Gemara explains the reason for this law (#1).
3. Both Terumah and Terumas Ma'aser may be separated by estimating an amount, and by mere thought.
4. The Mishnah states that one may take one barleycorn out of its chaff and eat it raw without separating Ma'aser.
5. One is allowed to take away the chaff surrounding a wheat kernel on Shabbos by rolling the kernel with his fingers.
A BIT MORE
1. For example, if he receives grapes he may turn them into wine and then separate Terumas Ma'aser from the wine.
2. Just as one is obligated to separate Terumah only "from the silo," meaning after the produce has been fully processed, one is obligated to separate Terumas Ma'aser only after the produce has been fully processed. Accordingly, if a Levi wants to turn the grapes into wine and they have not yet been fully processed, he may wait until after they have been turned into wine to separate Terumas Ma'aser.
3. One is allowed to think (estimate) in his head that a specific portion of the grain is Terumah or Terumas Ma'aser without actually separating it, and the rest of the grain may be eaten.
4. This is because it is considered a temporary type of eating that does not obligate the food to have Ma'aser separated from it. However, one may not even place the barleycorn in his other hand before he eats it, because that would be considered an established (and not temporary) form of eating.
5. However, he may not separate the chaff in the normal fashion, because doing so would appear to be the Melachah of Dash (threshing).
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