What is the origin of the idiom "go sour" and when to use it?

The idiom "go sour" is thought to have started in the early 1500s from the way people saw milk turn sour, which happens naturally because of bacteria. Milk that is sour is not good anymore and it tastes bad, so people started using this idea to describe other things that were not good or that got worse. Over time, the phrase became common in informal contexts and is now used to mean that something has changed for the worse.