We’ve all done it: You find that carton of milk in the fridge, only to discover that it expired yesterday. I In order to see if it’s still safe to use, we give it the smell test: Open it up, take a whiff, and judge accordingly. When milk smells “off,” we can tell right away (thanks to thousands of years of evolution), and down the drain it goes. But why exactly does bad milk smell bad?
It all comes down to bacteria. As milk gets older, the bacteria inside the milk begins to multiply and break down the lactose. The offensive odor is a side effect of this chemical breakdown.
As a guide, it’s safe to say that milk can survive for about a week after it’s opened (even after this stretches a few days past the expiration date), provided that it’s kept in a cold fridge and isn’t opened and closed too often. If it smells or tastes bad to you, throw it out. If it doesn’t smell or taste bad, it’s okay to use.