It is a mitzvah from the Torah to dedicate the first portion taken from a dough intended for baking to God. When the Temple stood, this portion—the challah offering—was given to the kohanim, who would eat it in a state of ritual purity.
Today, since we are all considered ritually impure due to contact with the dead, no one may eat the challah portion, and therefore, it is customary to burn it.
In practice, one who makes dough using more than 1.2 kg of flour separates challah without a blessing, and when using more than 1.6 kg (some say 2.25 kg), challah is separated with a blessing.
The Blessing for Separating Challah (Ashkenazim).
ברוך אתה ה’ אלוהינו מלך העולם אשר קדשנו במצוותיו וצוונו להפריש חלה מן העיסה. Take a small piece from the dough, lift it up, and say: “This is challah.”
The Blessing for Separating Challah
The blessing should only be recited after the flour and water have been mixed, and preferably at the end of kneading the dough.
After reciting the blessing, take (separate) a small piece of dough and say: “This is challah”. The piece that was separated should be burned, or alternatively, wrapped in two coverings and discarded in the trash.
The blessing for separating challah – Ashkenazi version:
ברוך אתה ה’ אלוהינו מלך העולם אשר קדשנו במצוותיו וצוונו להפריש חלה מן העיסה
Nachman Harris. Kosher Expert.
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