Why does milk go sour?
Milk goes sour because bacteria feed on lactose, the natural sugar in milk, and produce lactic acid. The acid gives sour milk its tangy taste and makes it thicken and curdle.
Even pasteurized milk contains some bacteria, and more can enter after the carton is opened. As they multiply, they break down lactose and release lactic acid. This lowers the pH of the milk. When it becomes acidic enough, milk proteins called casein clump together, causing lumps.
Keeping milk cold slows bacterial growth, which is why refrigerated milk lasts much longer. Leaving milk out at room temperature for a few hours can let bacteria multiply quickly. Ultra-high-temperature milk, heated to about 135 degrees Celsius, can last for months unopened without refrigeration.
Useful sourness
People have used controlled souring for thousands of years to make foods like yogurt, buttermilk, kefir and many cheeses. These use specific friendly bacteria. However, milk that has gone sour on its own may contain unwanted bacteria and should not be drunk if it smells bad.
Did You Know?
Raw milk that sours naturally was once used in baking, because its acid reacts with baking soda to help cakes and breads rise.