Why milk curdles: a scientific explanation

Bylim Olena

Why milk curdles: a scientific explanation

Milk curdles as a result of milk protein coagulation. Coagulation is the process by which proteins dissolve in water and form a gel-like or clotted mass.

Milk contains two main types of proteins: casein and whey proteins. Casein makes up about 80% of the protein in milk. It is a complex protein that consists of several subunits. Whey proteins make up about 20% of the protein in milk. They are simple proteins that do not form clots.

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As explained by experts from Science Notes, fresh milk consists of fat and protein particles floating in a water-based solution. The colloidal suspension scatters light, which is why milk turns white. The protein molecules, mainly casein, repel each other, so they naturally spread out evenly in the liquid.

When the pH is lowered further by adding another acidic ingredient, the protein molecules stop repelling each other. This allows them to stick together or clump together. The watery liquid that remains is called whey.

How sour milk curdles

Milk becomes sour when acids produced by bacteria lower the pH of the milk and the proteins begin to "clump". The bacteria living in milk naturally produce lactic acid by digesting lactose so that they can grow and multiply. This happens regardless of whether the milk is fresh or pasteurized.

Cooling milk slows down the growth of bacteria. Likewise, warm milk promotes bacterial growth and also accelerates the "clumping" reaction of proteins.

How to turn fresh milk into soured milk

It is noted that adding lemon juice or vinegar to fresh milk is an easy way to make homemade soured milk. Hot milk sours faster, but cold milk takes longer.

You can also add a spoonful of yogurt, kefir, or sour cream to the milk to make it ferment faster.

Earlier, scientists explained why mosquito bites itch and how to stop itching quickly.

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