Why does milk go sour?

Almost everyone has experienced the unpleasant surprise of pouring milk that has gone sour. What was once fresh and mild becomes acidic-tasting, unpleasant-smelling, and sometimes lumpy. This change is not random and it is not really the milk simply getting old on its own. Souring is a biological process, driven by tiny living organisms that are either already in the milk or that find their way into it from the environment.

The main cause of milk souring is a group of microbes known as lactic acid bacteria. These bacteria are common in nature and can easily end up in milk. Milk is an excellent food source for them because it naturally contains a sugar called lactose. When conditions allow, these bacteria begin to feed on the lactose, using it as their energy source. As they consume the sugar, they multiply and carry out a process called fermentation.

As the lactic acid bacteria ferment the lactose, they produce lactic acid as a byproduct. This acid steadily builds up in the milk, making it more and more acidic. The growing acidity is what creates the sharp, sour taste. The acid also affects the proteins in milk, especially the main protein called casein. As the milk becomes acidic enough, the casein proteins clump together and the milk curdles, which is why sour milk often looks lumpy and separated instead of smooth.

The speed at which milk sours depends heavily on temperature. Bacteria grow and multiply much faster in warmth, so milk left out at room temperature sours quickly, while milk kept cold in a refrigerator stays fresh much longer. This is why refrigeration is so important for milk storage. It is also worth knowing that this same souring process is not always unwanted. When carefully controlled with specific, chosen bacteria, the fermentation of milk is exactly how foods like yogurt and many cheeses are made.

This article was written using information from Wikipedia.