The bacteria in sour milk can lead to food poisoning and pose a danger to children who suffer from compromised immune systems, according to Healthline. The sour flavor in milk that has spoiled comes from bacteria that have spread throughout the container.
Just because the expiration date for a carton or jug of milk has passed does not mean that the milk has turned sour. The dating has more to do with the quality of the milk than it does with food safety, as stated by Healthline. If there is any concern about the milk, taking a small sip and smelling the milk before using provides information about the milk's quality. Discard the milk if it smells or tastes sour.