Sandarbhasūkti - part 17
138. Na hi svato'satī śaktiḥ kartumanyena śakyate
It is impossible to impart some power which we don’t have in the first place. Milk has the inherent capacity of turning into curds. It is evident. The sour buttermilk is the base. If the milk didn’t have that inherent capability in the first place then it would have been impossible to turn it into curds. You can’t turn water or air into curds.