The Basket

The Mishna Challah (3:1) says that the obligation of challah kicks in when the dough is needed with water.  The Mishna in Challah (2:4) brings the opinion of R' Eliezer that when doughs are combined together in a basket they combine to the shiur to obligate the dough in challlah.  The halacha follows this view as concluded in Pesachim 48b.  The Rishonim (stems from Rav Achai Gaon Shiltos Tzav #73) explain that the possuk says באכלתם מלחם הארץ it tells us that there is an obligation that comes about even after it is already bread.  The Achronim ask why is it different from other scenarios where the dough is exempt at the time of placing water in the dough then it is exempt forever such as a dough of a gentile or hekdesh (Mishna 3:3 and 3:6)? 

According to the Rashba (Teshuva 461) it is not difficult for he explains that that it in the other scenarios as well if the dough is put in a basket afterward it will become obligated in challah.  He explains the lomdus of the obligation of the basket is that putting the dough the is like doing a new act of kneading the dough.  I don't know why he needs that lomdus.

The Toras Zeraim brings from Rav Chayim that the shiur for challah of a kav and a quarter isn't a regular shiur of a minimal amount to cause the obligation to kick in rather less than that amount isn't called dough - doesn't have a שם עיסה.  Based upon this he explain a dough that has the shiur of challah, it had a שם עיסה at the time of the chiuv, during the kneading, it is permanently exempt. But a dough that didn't have the shiur at the time of the kneading, the time of the chiuv never knicked in since it isn't considered a שם עיסה and its obligation is only when it obtains a שם עיסה which is when its combined in the basket with other dough.

The Toras Zeraim brings a few proofs to this principal of Rav Chayim.  One of them is the Gemorah (4:2) compares separating challah before the time of the obligation to separating terumah before the time of the obligation.  What's the comparison, its not challah taken off too early, its lacking in the shiur?  Because lacking in the shiur defines it as before the time of the obligation for the שם עיסה was never chal.  Rav Shlomo Zalman Aurerbach (Minchas Shlomo volume 1 #68) disagrees with this Rav Chayim.  He understands the Yerushalmi there is just saying that less than the shiur isn't considered akin to terumah before the final stage of מירוח where one can separate terumah, rather since it is lacking in the shiur the challah can't be chal. Additional proofs are disussed there but it is beyond the scope of this elucidation.