(Mishnah): One may (be hired by an idolater to) tread on grapes in a winepress together with an idolater, but one may not pick grapes with him (as they will later become tameh).

If a Jew is working in a state of tumah, one may not tread or harvest grapes with him (since he is assisting the Jew to bring tumah onto the produce).

But he may bring with him barrels to the winepress as well as carry them back with him from the winepress.

(Gemara) (R. Yona): (When the Mishnah permits treading on grapes with a gentile) it is when the gentile has already tread on them lengthways and widthways (thereby making all of them impure), but if not, and he has just begun treading on them, the Jew may not (as he is causing non-holy items in Eretz Yisrael to become tamei).

Question (R. Yosi): If the gentile already tread on them lengthways and widthways, meaning that it is no longer the beginning of the pressing, since the grapes are already tamei, a Jew many now begin to tread with him. Is this true even according to the later teaching (below)....?

(Tosefta) (R. Meir): Originally they said that one may not harvest grapes with a gentile nor tread with a Jew who is working in a state of tumah, but one may tread with a gentile.

They later said that one may not tread with a gentile nor harvest with a Jew who is working in a state of tumah, but one may harvest with a gentile.

(Chachamim): One may transfer both new and even old barrels (that are already saturated with Yayin Nesech) together with a gentile.

One may help him as long as the Jew can see him, but if not, it becomes Yayin Nesech. (So when it first taught that one may not tread with a Jew, that refers to the beginning of the treading, as he is causing non-holy items in Eretz Yisrael to become Tamei. And according to the later teaching, he is causing tumah to tevel (untithed produce of Eretz Yisrael, since it contains produce that will become Terumah).

Question: What is the reasoning of R. Yosi?

Answer: They already became Rabbinically Tameh at the time of harvest.

Question: What is the reasoning of R. Yosi?

Answer: The Torah says that they are Tahor, but you made a Rabbinic decree that they are Tameh.

(Proof of R. Yosi): R. Yosi said in the name of R. Hila - it should have been that the Torah allows a man to make his Tevel Tamei, as the pasuk states (Bamidbar 18:5). "Behold I have given you the guarding of My Terumos" - Terumah needs guarding but Tevel does not. But the later pasuk states (Bamidbar 18:25), "and you shall give from it Hash-m's Terumah to Aharon HaKohen"? Be careful to keep Tevel Tahor, so that you can give it to Aharon HaKohen (and he can eat it in Taharah). However, here (in the Mishnah in Maseches Challah, when there is a doubt as to the Taharah of a certain dough), since its Terumah cannot be eaten, it is permitted to make it Tamei when Tevel. (Note: The Pnei Moshe makes significant changes to the text of this entry, based on Maseches Challah Yerushalmi.)