Is milk souring a physical or chemical process?

Milk nourishment is a chemical process, since the essence of chemical reactions is that from the original substance, regardless of whether it is one or several, there is a transformation into another substance. In this case, the nuclei of atoms do not change, only their redistribution occurs (as well as the redistribution of electrons), forming new chemical substances. Chemical reactions are widely used to obtain substances found in nature in limited quantities, or do not occur at all.