Avodah Zarah 35b
עבודה זרה ל״ה ב — ד"ה מַתְנִי׳ וְאֵלּוּ דְּבָרִים שֶׁל גּוֹיִם אֲסוּרִין
Avodah Zarah 35b:9
The Mishna prohibits milk milked by a gentile that a Jew did not witness, along with gentile bread and oil, though Rabbi Yehuda HaNasi's court permitted gentile oil entirely; the Gemara questions why such milk is prohibited, suggesting that if the concern is substitution with non-kosher milk it should be distinguishable by color, and if the concern is mixture, a Jew can curdle the milk to test whether it comes from a kosher animal.
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.
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.
