There is a Mitzva to take a piece of dough from the batter when one bakes and give the piece to a Kohen (Bamidbar 15:19-20). Nowadays, this Mitzva is binding only on the level of rabbinic obligation. This obligation entails taking a piece of dough when one bakes and declaring, "Harei Zu Hallah" ("This hereby is Hallah").
The Mitzva of Hallah applies only to dough produced from the five principal grains: wheat, barley, oats, rye and spelt. Furthermore, the obligation takes effect only if one bakes dough that could potentially become bread meaning, that was kneaded with water. If one kneaded dough with only eggs, for example, the obligation does not apply. Nevertheless, even in such a case one should take Hallah, only without reciting the Beracha.
One takes Hallah with a Beracha only if he prepares dough of an amount of 1,660.5 grams, or approximately 3-3.5 lbs. Although the Ben Ish Chai (Rabbi Yosef Chayim of Baghdad, 1835-1909) indicates that one must separate Hallah with a Beracha only if he kneads a larger amount approximately 5 pounds nevertheless, Chacham Ovadya Yosef rules that the obligation applies even if one kneads only 3-3.5 lbs.
In the past, there was a specific amount of dough that one had to separate to properly fulfill this Mitzva. But nowadays, when the obligation applies only on the level of rabbinic enactment, one need take only a minimal amount. And since today we are all including Kohanim presumed Tamei (ritually impure), the Hallah is burned, rather than given to a Kohen.
