Q: Is butter milk Vegan?

A: No. The formation of buttermilk is based on the fermentation
by the starter bacteria which turns milk sugar (lactose) into lactic acid. As lactic acid is formed, the pH of the milk drops and it gets tart. Milk proteins, most notably casein, are no longer as soluble under acid conditions and they precipitate out, causing what we recognize as clabbering. Butter milk is not vegan.