Milk is nutritious, easy to digest and absorb, affordable, and convenient, making it the "closest to a perfect food".
Milk spoils at high temperatures, but it also spoils when frozen. It is understood that milk contains three types of water with different properties: free water, bound water, and crystallization water. Among them, free water is the most abundant; it does not combine with other substances and only acts as a solvent. Bound water is water that is combined with proteins, lactose, and salts, no longer dissolves other substances, and does not freeze under any circumstances. The third type is crystallization water, which exists together with lactose crystals.
When milk freezes, the free water freezes first, and the milk gradually freezes from the outside in. The dry matter inside, including proteins, fats, calcium, etc., does not freeze. As the freezing time extends, the concentration of the dry matter inside correspondingly increases.
When the milk is thawed, the proteins in it are prone to precipitate and coagulate, thus spoiling. Therefore, milk should not be stored frozen!