What Happens When Milk Curdles?

Written by fred decker
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What Happens When Milk Curdles?
Cheese is a common form of curdled milk. (Creatas Images/Creatas/Getty Images)

Milk is one of the most perishable foodstuffs in common usage, able to go from freshly harvested to undrinkable in as little as a few hours in hot weather. This process of breaking down, or curdling, changes sweet-tasting fresh milk into a sour, lumpy substance with an unpleasant aroma. Paradoxically, the same process is harnessed to preserve milk in several forms.

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As milk ages, naturally occurring microorganisms begin to change its make-up. The lactose in milk, its natural sugar, is consumed by some bacteria, which digest it and produce lactic acid. Other microorganisms consume the lactic acid, converting it into more potent substances such a acetic acid. As the acidity of the milk increases and pH lowers, the molecules of casein, an important milk protein, begin to bond together. This is why milk curdles naturally with age, producing sour flavours and lumps of tart protein.

Cultured Milks

This process has been deliberately harnessed by humans over the centuries to create a whole range of cultured dairy products, which manage the souring process by deliberately introducing colonies of microorganisms to the milk. These cultivated forms of souring produce milk products with desirable flavours and characteristics, like sour cream, yoghurt and kefir. In much of Europe, fresh cream is cultured to a slight tang before it is churned into butter.


The curdling of milk is also the fundamental stage in making cheese. Most cheeses are produced by deliberately curdling the milk with acidity or rennet, an enzyme found in the stomach lining of calves. Rennet has the advantage of producing curds with a better texture, and rennet cheeses grow more complex with ageing. In each case, once the curds are formed, the whey is drained away and the curds are pressed together to form a cheese.


Whey is the watery substance left behind when milk is curdled. It may or may not be sour in flavour, depending whether the milk curdled from age or whether it was curdled deliberately. Although thin and unprepossessing in appearance, whey contains a significant amount of nutrition, much of it in the form of non-casein proteins. These can be cooked out with a combination of heat and acidity, creating fine-grained cheeses such as ricotta. Whey is also commercially dried to a powder and used as a dietary supplement.

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