Milk will spoil at high temperatures, but it will also spoil if frozen. It is understood that milk contains three types of water with different properties: free water, bound water, and crystalline 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; it no longer dissolves other substances and does not freeze under any circumstances. The third type is crystalline 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 trapped inside, such as proteins, fats, and calcium, does not freeze. As the freezing time extends, the concentration of the internal dry matter correspondingly increases.
After the milk is thawed, the proteins in the milk are prone to precipitate and coagulate, thus spoiling. Therefore, milk should not be stored by freezing!