Milk is mostly made up of water, with smaller amounts of fat, protein, minerals, and other compounds. Fats and water don’t usually mix, but in milk the fat and water form an emulsion. It is also a suspension of a multitude of different proteins in water.
In milk, proteins cluster together to form structures called micelles. These clusters grow from small clusters of calcium phosphate, which help hold them together. There are a number of different models of these micelles, with the exact structure still being subject to scrutiny.
It’s the protein micelles which give milk its white appearance. The micelles are on average about 150 nanometres in diameter, and this very small size means they are able to scatter light that hits them. The overall effect of this scattering by the huge number of micelles in milk is that it looks white.
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