I'm sure this has been discussed by devs already, but I think when livestock reaches Providence and the surrounding areas, purity of milk should be affected by how supple the cow's teats are. There could be some sort of balm or salve players could massage into the teat to improve this stat. Witches could use pungent oils and other medicinal liniments as I believe that mechanic is already in-game for them.
Eager to feel the hefty weight of a full milk sac in my pilgrim's calloused hands,
AFC