Why does milk turn sour?
For grown-ups: the science
Milk turns sour when bacteria grow and make an acid that tastes sour.
Milk souring is usually caused by lactic acid bacteria metabolizing lactose into lactic acid, which lowers pH and can make milk proteins clump. Refrigeration slows bacterial growth but does not make milk sterile.
What about...?
Ask your child's own question
Transcript
A glass of milk can taste smooth and fresh. Why can it turn sour?
Milk can have tiny living things called bacteria in it. Some bacteria can grow when milk gets warm.
As they grow, the bacteria change some milk sugar into an acid. That acid gives milk its sour taste.
Cold fridge air slows the bacteria down, but does not stop them forever. That is why milk stays fresh longer in the fridge.
So milk turns sour when bacteria make acid in it. Keeping milk cold helps it last longer.