יורה דעה שכ״זYoreh De'ah 327

Se’if 1א׳

הַמַּפְרִישׁ חַלָּתוֹ קֶמַח, אֵינָהּ חַלָּה, וְגֶזֶל בְּיַד כֹּהֵן, וּשְׁאָר הָעִסָה חַיֶּבֶת בְּחַלָּה. וְאוֹתוֹ הַקֶּמַח שֶׁהִפְרִישׁ לְשֵׁם חַלָּה, אִם יֵשׁ בּוֹ עֹמֶר וְעָשָׂהוּ עִסָה, הֲרֵי זֶה מַפְרִישׁ מִמֶּנָּהּ חַלָּה. וְדַוְקָא בְּאוֹמֵר שֶׁיָּחוּל עָלֶיהָ שֵׁם חַלָּה בְּעוֹדוֹ קֶמַח, אֲבָל הַמַּפְרִישׁ קֶמַח וְאוֹמֵר: כְּשֶׁתֵּעָשָׂה עִסָה יָחוּל עָלֶיהָ שֵׁם חַלָּה, דְּבָרָיו קַיָּמִים.

One who separates his challah (the portion of dough given to a priest) as flour — it is not challah, and it is robbery in the priest's hand, and the rest of the dough is liable to challah. And that flour which he separated for the sake of challah, if it holds an omer and he made it into dough, he separates challah from it. And this is specifically where he says that the name of challah shall take effect upon it while it is still flour; but one who separates flour and says: when it is made into dough, let the name of challah take effect upon it — his words stand.

Se’if 2ב׳

אֵימָתַי מַפְרִישִׁין חַלָּה, כְּשֶׁיִּתֵּן הַמַּיִם וִיעָרֵב הַקֶּמַח בַּמַּיִם. וְהוּא שֶׁלֹּא יִשָּׁאֵר שָׁם בָּעֲרֵבָה קֶמַח שֶׁלֹּא נִתְעָרֵב בַּמַּיִם שִׁעוּר עֹמֶר. וְאִם אָמַר: הֲרֵי זוֹ חַלָּה עַל הָעִסָה וְעַל הַשְּׂאוֹר וְעַל הַקֶּמַח שֶׁנִּשְׁתַּיֵּר, וְלִכְשֶׁתֵּעָשֶׂה כֻּלָּהּ עִסָה אַחַת תִּתְקַדֵּשׁ זוֹ שֶׁבְּיָדִי לְשֵׁם חַלָּה, הֲרֵי זֶה מֻתָּר. וְכֵן אִם מַתְנֶה שֶׁתָּחוּל הַחַלָּה אַף עַל הַקֶּמַח שֶׁיִּתְעָרֵב אַחַר כָּךְ בִּשְׁעַת עֲרִיכָה. וְטוֹב לְלַמֵּד לַנָּשִׁים לְהַתְנוֹת כֵּן אִם יֵשׁ לָחוּשׁ שֶׁיִּתְעָרְבוּ שִׁעוּר חַלָּה אַחַר כָּךְ (בֵּית יוֹסֵף בְּשֵׁם סְמַ"ג). וְצָרִיךְ לִזָּהֵר שֶׁלֹּא לִשְׂרֹף הַחַלָּה עַד אַחַר כָּל הָעֲרִיכָה.

When do they separate challah? When he puts in the water and mixes the flour with the water; provided that there does not remain there in the kneading trough flour that has not mixed with the water, to the measure of an omer. And if he said: this is challah for the dough, and for the se'or (the sourdough leaven that makes the dough ferment), and for the flour that is left over, and when it has all become one dough let this in my hand be consecrated for the sake of challah — this is permitted. And likewise if he stipulates that the challah shall take effect also upon the flour that will mix in afterwards at the time of the kneading. And it is good to teach the women to stipulate so, if there is ground for concern that the measure of challah will be mixed in afterwards (Beit Yosef in the name of the Semag). And he must be careful not to burn the challah until after all the kneading.

Se’if 3ג׳

הָאִדָּנָא טוֹב לְהַמְתִּין מִלְּהַפְרִישׁ חַלָּה עַד אַחֵר גְּמַר לִישָׁה, שֶׁתֵּעָשָׂה כָּל הָעִסָה גּוּף אֶחָד.

Nowadays it is good to wait before separating challah until after the kneading is finished, so that the whole dough becomes one body.