Talmud Bavli, Avodah Zarah
Avodah Zarah 35b
The Talmud establishes the original rabbinic decree (gezeirah) against non-Jewish milk, citing the concern that it may be mixed with milk from a non-kosher animal. This is the foundational sugya for the entire chalav yisrael requirement.
מַתְנִי׳ וְאֵלּוּ דְּבָרִים שֶׁל גּוֹיִם אֲסוּרִין וְאֵין אִיסּוּרָן אִיסּוּר הֲנָאָה: חָלָב שֶׁחֲלָבוֹ גּוֹי וְאֵין יִשְׂרָאֵל רוֹאֵהוּ, וְהַפַּת וְהַשֶּׁמֶן שֶׁלָּהֶן. רַבִּי וּבֵית דִּינוֹ הִתִּירוּ הַשֶּׁמֶן. גְּמָ׳ חָלָב לְמַאי נֵיחוּשׁ לַהּ? אִי מִשּׁוּם אִיחַלּוֹפֵי — טָהוֹר חִיוָּר, טָמֵא יָרוֹק! וְאִי מִשּׁוּם אִיעָרוֹבֵי — נֵיקוּם! דְּאָמַר מָר: חָלָב טָהוֹר עוֹמֵד, חָלָב טָמֵא אֵינוֹ עוֹמֵד. אִי דְּקָא בָעֵי לִגְבִינָה, הָכִי נָמֵי. הָכָא בְּמַאי עָסְקִינַן? דְּקָא בָעֵי לֵיהּ לְכַמְכָּא. וְנִשְׁקוֹל מִינֵּיהּ קַלִּי וְנֵיקוּם! כֵּיוָן דִּבְטָהוֹר נָמֵי אִיכָּא נַסְיוּבֵי דְּלָא קָיְימִי, לֵיכָּא לְמֵיקַם עֲלַהּ דְּמִילְּתָא.
MISHNA: This mishna lists items belonging to gentiles which it is prohibited to consume, but from which it is permitted to derive benefit. And these are items that belong to gentiles and are prohibited, but their prohibition is not that of an item from which deriving benefit is prohibited: Milk that was milked by a gentile and a Jew did not see him performing this action, and their bread and oil. The mishna notes that Rabbi Yehuda HaNasi and his court permitted the oil of gentiles entirely. GEMARA: The Gemara asks: Concerning milk, with regard to what need we be concerned? Why is the milk prohibited? If it is due to the concern that a gentile might exchange the milk of a kosher animal with the milk of a non-kosher animal, this concern is unfounded, as kosher milk is white whereas non-kosher milk has a green tinge to it, and therefore they are easily distinguishable. And if it is prohibited due to the concern that it might be mixed with non-kosher milk, let the Jew curdle the milk obtained from the gentile, as the Master said: Milk from a kosher animal curdles, but milk from a non-kosher animal does not curdle. The Gemara answers: If one desires to eat it as cheese, indeed, one can simply curdle it, as the milk of non-kosher animals does not curdle. What are we dealing with here? We are dealing with a case where one desires to use the milk in kamkha, also known as kutaḥ, a food item that contains milk. The Gemara raises a difficulty: But in that case, let him take a bit of milk and curdle it, to test whether or not it has been mixed with the milk of a non-kosher animal: If it curdles completely, it is kosher; if some milk is left over, it is not. The Gemara explains: Since there is also whey in kosher milk, which does not curdle, there is no way to establish the halakhic matter with regard to it. Even kosher milk will not curdle completely, and therefore this is not a reliable method to determine the halakhic status of the milk.