The obligation of "Halla" requires separating a small portion of dough from the batter when a person kneads a certain quantity of flour. In ancient times, the separated dough was given to a Kohen, whereas nowadays it is burned.
How much flour must one knead for the obligation of Halla to take effect?
It is commonly assumed that one does not separate Halla unless he kneads 5 lbs of flour, and this is indeed the position of the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909). However, Hacham Ovadia Yosef, in his Halichot Olam, disagrees, and rules that the minimum quantity is approximately 3.5 lbs, or 1560 grams. According to Hacham Ovadia, one who kneads this amount of flour must separate Halla and recite the Beracha, and this is the ruling that one should follow.
Another debate between the Ben Ish Hai and Hacham Ovadia Yosef concerns the status of flour kneaded with milk, instead of water. According to the Ben Ish Hai, a person who kneads flour with milk and without water must separate Halla but does not recite a Beracha. Hacham Ovadia, however, rules that since milk is included among the seven Halachic liquids, flour kneaded with milk is no different from flour kneaded with water. Therefore, one who kneads 3.5 lbs or more of flour with milk would be required to separate Halla and recite the Beracha. According to all opinions, however, one does not separate Halla from flour kneaded with only fruit juice, such as apple juice. Fruit juice is not included among the seven Halachic liquids, and therefore flour kneaded with fruit juice does not fall under the obligation of Halla, unless it is kneaded also with water or milk.
Summary: One who kneads 3.5 lbs of flour or more with water or milk must separate Halla (a small piece of dough which is then burned) and recite the accompanying Beracha. One who kneads less than this quantity of dough, or who uses fruit juice, instead of water or milk, does not separate Halla.
