Challah, Chapter 2, Mishnah 6. This Mishnah deals with the quantity that is subject to challah. We have already touched on this matter earlier, and now the Mishnah states it explicitly.
The quantity that is subject to challah:
"Chameishes reva'im kemach chayavim be'challah" - five quarters of a kav, that is, a kav and a quarter of flour, are subject to the separation of challah. Once a person has such a quantity of flour and makes it into dough, the obligation to separate rests upon him.
What is included in the reckoning of the flour:
The Mishnah continues and specifies: "Hem u'se'oran u'suvanan u'mursanan chameishes reva'im chayav" - four components combine together to make up the quantity:
"Hem" - the flour itself.
"Se'oran" - the leaven, which ferments the dough and causes it to rise.
"Suvanan" - the bran, meaning the fine, small bran.
"Mursanan" - the thicker, coarser bran.
When this entire mixture - the flour, the leaven, the fine bran and the coarse bran - amounts to five quarters of a kav, it is subject to challah.
Bran that was removed and returned:
The Mishnah then continues with a qualification: "Natal mursan mitocham v'chazar l'tocham - harei eilu patur". If one removed the coarse bran from the mixture and afterward returned it, it is no longer an original part of it, and it does not combine to complete the quantity of five quarters. As long as the bran was in the mixture from the outset, it is counted along with it; once it has been taken out, it is no longer counted, even though it was afterward put back in.
In summary: in this Mishnah we learned that the quantity that obligates in challah is five quarters of a kav of flour, and that the flour, the leaven, the fine bran and the coarse bran all combine toward this quantity - provided that they were in the mixture from the start. Bran that was removed from the mixture and returned to it no longer combines, and such mixtures are exempt.