Demai, Chapter Five, Mishnah Three
This mishnah returns to the subject of separating tithes from bread bought from a baker, the topic dealt with in mishnah one.
1) One who bought [bread] from a baker, he may give tithe from hot bread for cold bread or from cold bread for hot bread, even when they are of various moulds, the words of Rabbi Meir.
2) Rabbi Judah prohibits it, because I say that yesterdays wheat was bought from one man and todays wheat from another man.
3) Rabbi Shimon prohibits it in the case of terumat maaser, but permits it in the case of hallah.
Section one: According to Rabbi Meir, when one buys bread from a baker he may separate tithes for the freshly-bought hot bread from the bread that he bought the day before which is now cold, and vice versa, he may separate tithes from the older bread from the freshly bought bread. This is true even if the loaves are made in different molds. Rabbi Meir is not concerned lest the bread be made from wheat bought from several different sellers, some of whom did separate tithes and some of whom did not. If indeed the wheat had been bought from different sellers, some who tithed and some who did not, then it would be a problem to tithe from one batch for another because one cannot tithe from already tithed wheat for wheat that has not yet been tithed. We shall learn more about this in mishnah ten below.
Section two: Rabbi Judah prohibits this for the reason explained above. He fears that yesterdays wheat and todays wheat were bought from different merchants.
Section three: Rabbi Shimon agrees that this is a problem with regard to terumat maaser, because the obligation to separate terumat maaser is set at the time when the wheat is made into a pile, the same time when the obligation to tithe is set. However, the obligation to separate hallah does not come into being until the dough is made. By this point the wheat already belongs to the baker and therefore we dont have the problem of the separating hallah from stuff that belongs to two different peopleit all belongs to the baker.