Comment by 3eb7988a1663

American, never heard of this. Some quick searching, and I found an Australian dairy site which describes this as permeated milk. From this advert piece it might be a way of ensuring that the milk fat/protein ratios can be easily adjusted to hit some target numbers.

[0] https://www.dairy.com.au/you-ask-we-answer/why-is-milk-perme...