Found this article for using sour milk. Of course, if there is no longer refrigeration, (because the aliens have taken control of the power grid in an attempt to squash the zombie uprising) you will need to act fast on sour milk acquisition. On the other hand, if, as you travel across the country to your safe harbor you come across an abandoned home with some milk, it’s good to know how to utilize this source of protein.
It’s always good to test things out before an emergency. If you want to wait for it to naturally spoil, I’ve found that letting it expire slightly in the fridge works. To test if it is ready, simply try adding it to your tea in the morning. If it curdles, it’s good to go! If not, happy tea drinking.