🧪 Casein

Casein is the main protein in milk: in cow's milk it makes up 80% of all the protein. It forms tiny particles called micelles, held together by calcium; these are what make milk white and opaque. The rest of the protein is whey protein, which stays in the liquid.

Casein is the basis of cheese. When rennet is added to milk or the milk sours, the micelles clump together into a curd and the whey separates. Casein curdles in the same way in the stomach. Milk from different animals behaves differently here: in cow's and buffalo milk the ratio of casein to whey proteins is 82 to 18, while human milk contains several times less casein. That is why breast milk forms a soft, fine curd in a baby's stomach, whereas cow's milk forms a dense one.

Casein is also produced on its own, from skimmed milk. The Codex Alimentarius distinguishes edible acid casein, rennet casein and caseinates; acid casein has to contain at least 90% milk protein in the dry matter and no more than 12% moisture. Casein is added to foods, and in winemaking it is used to clarify wine.

Casein or whey protein? Both come from milk, but they behave differently. Casein curdles into a dense clot, which is how cheese forms, while whey proteins stay dissolved in the whey that drains off. Cow's milk holds them in a ratio of 82 to 18, and human milk has several times less casein, so a baby's stomach gets a softer curd. Casein products are made in different ways too: the Codex Alimentarius separates acid casein, rennet casein and caseinates, and acid casein must be at least 90% milk protein in the dry matter.

On ingredient lists look for the words "casein" or "caseinate", both of which mean milk protein. In the EU, allergens such as milk must be emphasised in the ingredients, usually in bold. On wine labels the wording is "milk casein" when traces remain after fining. Casein turns up in processed foods, so it is worth checking packs at the supermarket, and not only at a dairy or cheese counter.

Why it matters to you. Caseins are among the main milk allergens, and heating does not neutralise them: according to the World Allergy Organization (WAO) guidelines, milk allergens remain active after boiling, pasteurisation and UHT treatment. So if you are allergic to milk protein, read labels carefully: the word "casein" or "caseinate" means milk protein. In the EU, even wine is labelled "milk casein" if traces remain after clarification.