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There is an obligation for both men and women to separate Challah from any dough-like batter which they make. The specifics are discussed below.
- The Mitzvah of removing Challah is only Biblical in Israel and when majority of Bnei Yisrael reside in Israel. There is nonetheless a Rabbinic Mitzvah of Challah outside Israel and in Israel when there is a minority of Bnei Yisrael in residence there. 
- It must be stressed that the Mitzvah of Challah applies to all doughs, not just "Challah Bread"; even dough for baking pastries can be obligated in Challah .
Conditions for Being Obligated in Challah
- Any dough made from the 5 grains is obligated in the Mitzvah of Challah. 
- If the dough or a part of the dough is going to be baked one should separate Challah with a Bracha, however, if all of it is going to be cooked or fried one should separate Challah without a Bracha.
- If the dough is kneaded with eggs or fruit juice, there is some doubt as to whether one needs to separate Challah; therefore, one should knead into the dough some liquid that is considered a "משקה" [e.g. water, milk, bee's honey, wine, oil], and then one can separate Challah with a Beracha .
Who is Obligated?
- The Mitzvah of Challah applies to the owner of the Challah who is baking the dough, whether that is a man or a woman. However, the woman has preference over a man in fulfilling this Mitzvah as it is particularly relevant for women.
- The dough of a non-Jew is exempt from the mitzvah of hafrashas challah, even if the Jews bakes it into bread. 
Amount of Dough Necessary
- According to Ashkenazim, some say that the Minhag is to remove Challah with a Bracha when making a dough that is 5 pounds (2.27 kg) or more and remove Challah without a Bracha when making dough that is 3 pounds (1.35 kg) or more.  Others say that the Minhag of Yerushalyim is to take Challah off without a Bracha if the dough is 2.65 pounds (1200 grams) or more and to remove Challah with a Bracha if the dough is 3.67 pounds (1666 grams) or more. 
- According to Sephardim, one should take off Challah with a Bracha when making a dough with 3.44 pounds (1560 grams) or more of flour and it is a pious practice to remove Challah without a Bracha when making a dough with 2.65 pounds (1200 grams) or more of flour. However, some say that one should only remove Challah with a Bracha for dough that is 5.29 pounds (2400 grams) and without a Bracha for dough that is 3.67 pounds (1666 grams).
- It is forbidden to intentionally make less than the shiur challah, the minimum amount of dough which would obligate one to separate Challah. The poskim explain that this only applies if one wants to bake a large amount and is intentionally baking it in two parts separately in order to exempt oneself from the mitzvah of Challah, however, there's nothing wrong with baking a small amount if that's all one wants to bake now.
- If someone bakes Challah in order to separate them into separate loaves there is a dispute if one is obligated to take off Challah. The minhag is certainly to take off Challah. For example, if a woman kneaded and baked the shiur Challah and freezed some of the Challahs for another week there is an obligation to remove Challah at the time of the kneading.
- Similarly, some say that a person who kneads the shiur Challah and then splits it up in two parts both of which are less than the shiur Challah, and freezes one of the parts for later that there is am obligation to take Challah.
- If one makes less than the amount required in Challah in separate loaves, but then puts them back into the same vessel, the vessel combines these pieces to make a Shi'ur of Challah..
Procedure of Removing Challah
- The text of the Bracha upon removing Challah is "ברוך אתה ה, ' אלוקינו מלך העולם אשר קדשנו במצוותיו וצונו להפריש חלה". 
- After reciting the above Bracha one should remove a Kezayit of dough and burn it. The Minhag is to burn the piece of dough in the same oven that the bread is baking.
- Sephardim in the diaspora can remove Challah and give it to a child Kohen who never saw a keri or an adult who went to mikveh and isn't a Niddah, Zava, Tumat Keri, or Zav. Even Sephardim could burn the challah in the diapsora.
Forgot to Remove Challah
- If one forgot to remove Challah Erev Shabbos: if you are outside of Israel, you leave over a slice, and separate Challah from it after Shabbos. This slice must be slightly bigger than the Shi'ur Challah. .
- This is a real problem if this happens Erev Shabbos Erev Pesach .
- If Challah got cooked into other foods it makes them forbidden. If it is mixed with the same type of ingredient which is permitted everything is forbidden unless the permitted ingredient is a hundred times that amount of Challah. If it is mixed with a different type of ingredient which is permitted everything is forbidden unless the permitted ingredient is sixty times that amount of Challah.
- Rambam (Bikkurim 5:5-6) writes that The Mitzvah of removing Challah is only Biblical in Israel and when majority of Bnei Yisrael reside in Israel. There is nonetheless a Rabbinic Mitzvah of Challah outside Israel and in Israel when there is a minority of Bnei Yisrael in residence there. The Mitzvah of Challah is counted as one of the 613 mitzvot by the Rambam (Sefer HaMitzvot Asin #133) and Sefer HaChinuch (385).
- R. Shechter, http://www.yutorah.org/lectures/lecture.cfm/802895/Rabbi_Hershel_Schachter/Inyonei_Challah
- Kitzur Shulchan Aruch 35:1
- Kitzur Shulchan Aruch 35:6
- Kitzur Shulchan Aruch 35:7
- Mishna (Shabbat 2:6), Kitzur Shulchan Aruch 35:8, Yalkut Yosef YD 328:12
- Shulchan Aruch YD 330:1
- Shulchan Aruch YD 324:1 and Kitzur Shulchan Aruch 35:1 write that the amount necessary in order to be obligated in Challah is 43.2 KeBaytzim of flour.Rabbi Hershel Schachter on yutorah.org (Shiur on Parshat Shalach, min 50) quotes Rav Eliyahu Yosef Henkin (Edut LeYisrael page 138) who says to remove Challah with a Bracha if the dough is 5 pounds or more and without a Bracha if the dough is 3 pounds or more.
- Halichot Shlomo (Moadim vol 2 pg 338 note 54) writes that this was the Minhag in Rav Shlomo Zalman’s house and that of Yerushalyim.
- Yalkut Yosef YD 324:6
- Rav Modechai Eliyahu (comments on Kitzur Shulchan Aruch 35:2
- Rambam Bikkurim 6:15, Shulchan Aruch Y.D. 324:14, Yalkut Yosef YD 324:7. Shulchan Aruch Y.D. 324:14 writes that it is forbidden to make Challah less than the requisite amount to remove Challah in order to absolve oneself of the Mitzvah. The Gra 324:23 points out that this is based on Yerushalmi Challah 3:1 and Pesachim 48b.
- Shach 324:25 and Taz 324:17 write that someone who only has a small piece of dough is nonetheless allowed to bake it and doesn't have to worry about this prohibition since one's intention isn't absolve oneself from the mitzvah. Aruch Hashulchan 324:41 explains further that the entire prohibition is only for someone who wants to bake a shiur challah and intentionally absolves himself from the mitzvah by baking it in halves. Mishna Halachot 8:228 and 19:112 echoes this sentiment explicitly that there's nothing wrong at all with someone who only wants to bake a small amount less than the shiur Challah.
- The Yerushalmi Challah 1:5 cites Rabbi Yochanan who establishes that a woman who bakes Challah in order to separate it to other people in quantities less than the shiur Challah is not obligated to remove Challah. Rambam Bikkurim 6:19 codifies this. Although Raavad disagrees, Shulchan Aruch Y.D. 326:2 accepts the Rambam. Tosfot Brachot 37b s.v. lechem quotes Rabbenu Yechiel's doubt whether there is an obligation of Challah for a woman who makes one dough that is then cut into small pieces to make noodles. Pitchei Teshuva 326:2 writes that the Bach, Perisha, and Levush hold that a woman making Challah to separate it into loaves is obligated to take off Challah. She's only exempt if she gives it other people. With this they defend the minhag of women to take off Challah even though they separate it into loaves before baking. However, the Bet Efraim YD 69 disagrees based on Tosfot above. Instead he distinguishes between someone who makes loaves that are meant to remain separate in which case they are exempt and loaves which are baked together which would be obligated in Challah. Yalkut Yosef (Otzer Dinim Lisha p. 551) writes that we can rely on the minhag and remove Challah even though the dough is separated into loaves. Aruch Hashulchan 326:16 writes that it is obvious to him that Challah must be removed from dough that is kneaded and separated into loaves as is normally done. He explains that the Tosfot Brachot isn't relevant since it is referring to cooking.
- Shach 324:25 writes that the exemption of separating dough from the obligation of Challah applies only at the time of kneading but not if one is only going to divide it once it is baked. Rabbi Heber (Kashur Kurrents Fall 2004 Section VI. 1.) writes that if a woman kneaded and bakes the shiur Challah and then freezes some Challahs for next week the dough is obligated in Challah at the time of the kneading.
- Mayan Omer v. 5 p. 116 5:1 cites Rav Ovadia Yosef that a woman who makes a dough with a shiur Challah and then separates that dough into parts that have less than a shiur Challah to be bake at separate times that she is still obligated in Challah. Yet, Otzer Piskei Challah siman 23 by R' Michael Peretz argues that one should remove Challah without a bracha. He explains that in this case one is careful that the parts don't combine and according to many achronim it is exempt since it is made to be divided.
- Mishna Challah 2:4, Kitzur Shulchan Aruch 35:3; See Pesachim 46a-b
- Kitzur Shulchan Aruch 35:1
- Kitzur Shulchan Aruch 35:1
- Shulchan Aruch Y.D. 322:5 based on Bechorot 27a
- Rabbi Mansour on dailyhalacha.com
- Kitzur Shulchan Aruch 35:9
- Kitzur Shulchan Aruch 35:9
- Ran Nedarim 52a citing the Yerushalmi, Tosfot Chullin 99a s.v. ein, Rama YD 323:1 all hold that min b'sheino mino is batel one in sixty by Challah. However, the Rambam (Machalot Asurot 15:30) holds that it isn't batel unless there is one hundred even for a case of sheino mino. See Rabbi Akiva Eiger (Frankel Rambam) who asks on this Rambam.