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The Sages prohibited Jews drinking milk milked by a non-Jew, due to the concern they might have added the milk of a non-kosher animal. However, as long as a Jew supervises the milking, the milk is kosher. It is not necessary for the Jew to see the whole process; it is good enough if the non-Jew is aware the Jew might come in at any time and see them add non-kosher milk. Many rabbis feel that large dairy companies do not require supervision, for two reasons. First, non-kosher milk is more expensive, so there is absolutely no motivation to mix it in with cow’s milk. Second, in Israel and other countries (such as the United States), it is illegal to adulterate cow’s milk without informing consumers. Nevertheless, some rabbis are stringent and insist on a Jewish supervisor or a security camera. In Israel, regular kosher certification relies on the lenient opinion, while mehadrin certification follows the more stringent position. (See section 34 below.)
The Sages also prohibited cheese made by non-Jews. If a Jew supervises the production of the cheese, it is kosher. This is also true for soft cheese and other dairy products such as yogurt.
