The main ingredient used to make cheese is milk. Cow's milk is commonly used, though goat, sheep or buffalo can also be used, as could the milk of any mammal in theory. Starter cultures are typically added to aid in the culturing stage of the process. Rennet is added to the cheese milk to promote the separation into cheese curd and whey. Ultimately, salt is added to halt the production of acid later in the process. Finally, mould spores are introduced to assist in ripening. This can be added to the cheese milk early on in the process, or just prior to maturing.