For milk of a kosher animal to be considered kosher, Jewish law requires that a mashgiach (supervisor) be present from the beginning of the milking to the end of processing to ensure that only milk from kosher animals is used. This milk is referred to as chalav Yisrael.1

This is sourced to a Mishnah in Tractate Avodah Zarah:

And these are items that belong to gentiles and are prohibited. . . Milk that was milked by a gentile and a Jew did not observe.2

The Talmud explains that we are not concerned about a non-Jew substituting kosher milk for non-kosher, as such milk has a different shade. Rather, the concern is that the non-Jew may dilute the kosher milk with non-kosher milk.3

Milk milked by an unsupervised non-jew is referred to as chalav Akum (milk of a non-Jew) and is forbidden.This is the halacha as cited by all major codifiers.4

Commercial Milk

In the 20th century, with the commercialization of milking, Rabbi Moshe Feinstein (1895-1986) permitted the use of unsupervised milk from North American dairy farms. He argued that since government inspections ensure that only cows' milk is present, and since a violation would result in significant financial loss, it is halachically considered as if the milk was supervised. He employed a halachic concept known as 'anan sahadi,' (“we attest”) as the basis for his assertion. This concept allows us to rely on a concrete assumption in some instances where generally we'd require the testimony of actual witnesses. The innovation of Rabbi Feinstein is that he applies this idea to the case of unsupervised milk. He adds that it is, nonetheless, proper only to consume bona fide chalav Yisrael, noting that he himself would not consume commercial non chalav Yisrael milk.5

This is similar to an earlier ruling of the Pri Chadash (17th century), which states that supervision is not required in a vicinity where there are no non-kosher milk-giving animals. The Pri Chadash felt that no supervision is required since the possibility of non-kosher milk being added is so remote. Although the thrust of this argument does, in fact, differ from that of Rabbi Feinstein, they both found ways to permit unsupervised milk in certain situations.

So it would turn out that according to the Pri Chadash no witnesses are required, but according to Rabbi Feinstein we have what is considered halachic testimony.

The Chatam Sofer (1762–1839), however, strongly disagreed with the position of the Pri Chadash. He argued that knowledge or assumptions are not enough; we must have actual witnesses attesting to the purity of the milk.6

Rabbi Yitzchok Yaakov Weiss (1902–1989), known as the Minchat Yitzchak, concurred. He stressed that the ruling of the Pri Chadash was misunderstood and that assumptions could not be relied on. He also noted that (almost) all halachic codifiers ruled that supervision was a requirement, and exceptions could not be made7

What to Do?

Practically today, with the wide availability of chalav Yisrael milk and dairy products, there is little excuse to rely on this leniency.

In addition to the strict letter of the law, Jewish tradition stresses the importance of being cognizant of possible detrimental effects that arise from specific actions.

The Sixth Rebbe, Rabbi Yosef Yitzchak Schneersohn (1880-1950), tells of an episode where an individual had doubts about his belief in G‑d. Rabbi Schneur Zalman of Liadi, the first Rebbe of Chabad, informed this individual that these doubts were due to the inadvertent consumption of gentile milk.8

The Rebbe stressed on several occasions that in the current era, with the pervasive influence of Western culture, we must be even more careful about not consuming chalav Akum. When warning of the dangers of chalav Akum, the Rebbe stressed that one must be careful even for a remote possibility that something may be chalav Akum.9

Considering the detrimental effect that chalav Akum has on the soul, it is proper to be particular only to use chalav Yisrael products, even if it is difficult to obtain. Certainly, where they are readily available, one is required by Jewish law to use these products exclusively.