How is it todays milk considered kosher? In the past chazal were matir milk because one could assume that the majority of cows are kosher and therefore the milk is kosher because in each cup of milk one could rely on rov but today the milk on large dairy farms are all compiled by 100’s of cows into 1 big vat and there for sure is not 60 times the amount of kosher milk to the treif milk in the vat. What is the heter that the world is relying upon?
This is not a simple question, and would require a lengthy article to do it justice. Rather than giving a full answer, I will mention a number of points that poskim have raised concerning the question of permitting milk for drinking.
First of all, the assumption that must be made is that there is a ruba deleisa kaman of kosher cows. Some questoin the fact of this majority of kosher cows today, based on findings of milking cows that were later slaughtered. However, other findings indicate that there is still a majority of kosher cows. Tiferes Tzvi (Yoreh De’ah 13, brought in Pischei Teshuvah 81:4 and Darchei Teshuvah 18) adds that one cannot necessarily extrapolate from slaughtered cows to living cows, and also expounds on the question of using a chazakah based on the majority for each individual cow — a question we will not enter here. In addition, several acharonim have noted that it is not clear that the lung “sirchos” which are to blame for virtually all cases of modern-day tereifos are in fact true tereifos (see Shaagas Aryeh and other authorities, as brought in Darchei Teshuvah, loc. cit.; see also Yabia Omer Yoreh Deah, vol. 5, no. 3, sec. 3).
Yet, even assuming a majority of kosher cows, the presence of a significant minority is also a problem, seeing as though a X60 majority of kosher cows is required for the milk in the vats to be kosher. It is hard to believe that fewer than 1.63% or so of cows are tereifos. The question would now be an issue of how to define the power of rov (decision based on the majority)–does statistical unlikelihood hamper the decision of a rov, or not? In other words, can we use the power of rov to decide that each individual cow that is milked is kosher, and ignore the statistical reality, or not?
This question has been discussed in Kovetz Beis Aharon Veyisroel, 5763 (Kislev-Teves, pp. 66-76, Nissan-Iyar 124-136), and by several authorities, and the simple reason for which we continue to drink milk is that yes: we apply the power of rov even when statistical reality suggests that the result is untrue.
I will beli neder try to post a lengthier article on this topic on the site.