Where should the apostrophe be placed in "Goats Milk"?
I think it's useful to substitute "child" and "painting" because child is a word where the plurals and possessive forms are more obviously different.
What would be the correct label for a picture drawn by several kids?
- Children Picture
- Child's Picture
- Children's Picture
I think the third - the possessive plural. This suggests that "Goats' Milk" is the correct label for the milk.
Why not simply say "goat milk"?
David Marsh - Production Editor at the Guardian newspaper and contributor to that paper's "Mind Your Language" column - would prefer "Goat's Milk". Here is his rationale:
Phrases such as butcher's hook, collector's item, cow's milk, goat's cheese and writer's cramp are best treated as singular. We either don't know or don't care whether one cow, or many, are involved.
That, to me, is not an example of sound reasoning: we may well care, and in the case of industrially processed milk from any but the smallest dairy we can also be pretty sure, that more than one animal was involved in producing the contents of any given container of milk that has reached a retail shelf. Nevertheless, it is a considered view from a reasonable professional.
I agree with @tobyink that "Goats' Milk" is the most comprehensible of the three options, for the reasons he gives.
In answer to your actual question, it is clear that reasonable people can disagree about where to place this apostrophe, and therefore it seems unlikely that consistency will emerge.