The Shulchan Aruch in Yoreh Deah 115:1 deals with the kashrus of various non-Jewish dairy products. The three main products discussed are gevinah (cheese), chemah (butter) and cholov (milk). The general rule is that gevinah is the most strict, chemah is the most lenient and cholov falls somewhere in the middle.
In the first paragraph of YD 115, the Shulchan Aruch writes: "Cholov that was milked by a non-Jew and a Jew did not see the milking, is forbidden, perhaps he mixed in cholov tamei (non-kosher milk)".
What kind of milk is non-kosher? The Mishnah in Bechoros teaches us that anything that comes out of a non-kosher creature is non-kosher as well. So therefore milk from animals such as camels and horses is forbidden on a Torah level for us to drink, since those animals themselves are non-kosher.
Just like the gezeiros (decrees) of pas akum and bishul akum that we've learnt about before, the gezeirah of cholov akum is another gezeirah where Chazal saw the need to make a protective fence for the Jewish people. However, in the case of pas akum and bishul akum, Chazal were focused on preventing chasnus (intermarriage). Therefore even if you could prove that these items were 100% kosher, they would still be forbidden because of the concern of chasnus. With cholov though, the concern is purely about kashrus, i.e. is the milk kosher or not?
What is the source in the Gemara for the gezeirah of Cholov akum? It is mentioned in Avodah Zara 35b, where the Gemara lists a number of items that Chazal forbade. Among these are "Cholov she'chol'va ovdei kochavim v'ein Yisroel roeihu" (milk that a non-Jew milked and a Jew did not witness the milking). The Gemara tries to clarify the specific case that we are concerned could happen. If we are worried that the non-Jew switched the cow's milk with non-kosher milk, we would recognize this immediately. The Gemara explains that milk from kosher animals is white, while milk from non-kosher animals has a yellowish color.
Then the Gemara suggests that perhaps we are concerned that some non-kosher milk was mixed in with a majority of kosher milk. We wouldn't be able to use the color test because the mixture would still be white. The Gemara says that you could test it by making it into cheese. Only kosher milk can be made into cheese, non-kosher milk cannot.
The Gemara says that if your original plan was to make cheese, then obviously this test would help. But what if you didn't want to turn the milk into cheese? The Gemara says that you could take a sample of the milk and try to make the sample into cheese. If the sample doesn't become cheese, then you know that non-kosher milk has been added. However, the Gemara then concedes that even with 100% kosher milk, not all the milk turns into cheese. In the cheese-making process, a liquid called whey gets separated from the rest of the milk and sits above the cheese. So if there was non-kosher milk present, it would just get mixed in with the whey and you would have no way of knowing if it was present or not. So without supervision, we have no way of knowing the status of the milk and this is the reason Chazal saw it necessary to make the gezeirah.
In the coming issues we will look at the different ways in which the Rishonim and Acharonim understood the exact nature of the gezeirah of cholov akum. We will also look at the famous heter of R' Moshe Feinstein and how this heter applies to us today.