Why milk turns sour — Chemistry, 4–6 years
Tiny living things can change food as they use it. Milk becoming sour is a familiar chemical change that happens faster when it is warm.
Tiny helpers change milk
Some bacteria use the sugar in milk and make new substances, including acids. The milk then tastes sour and may become thicker, so it is no longer the same food it was before.
Why keep milk cool?
Bacteria usually work faster in warmth, so warm milk can sour sooner than milk in a refrigerator. Cooling does not remove every bacterium; it slows their activity and gives us more time to use the milk safely.
A simple comparison
Pour the same amount of milk into two clean cups. Leave one in the refrigerator and one in a warm place, without tasting either. Check them after a day: the warm cup may smell sour or look lumpy sooner because its bacteria had better conditions.
It is not simply ‘going bad’
It is reasonable to say that milk has gone bad, because the taste and smell warn us not to drink it. But chemistry gives a closer picture: bacteria changed some milk substances into others, and that change is why the milk is different.
Yoghurt and cheese
People use carefully chosen bacteria to change milk into foods such as yoghurt and some cheeses. The same broad kind of change can be useful when controlled, but milk left warm by accident is not automatically safe to eat.
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