In general, the gross composition of cow's milk in the U.S. is 87.7% water, 4.9% lactose (carbohydrate), 3.4% fat, 3.3% protein, and 0.7% minerals (referred to as ash). Milk composition varies depending on the species (cow, goat, sheep), breed (Holstein, Jersey), the animal's feed, and the stage of lactation. Although there are minor variations in milk composition, the milk from different cows is stored together in bulk tanks and provides a relatively consistent composition of milk year round in the U.S.
It rises when it boils because when heat is applied, the following processes take place:
1. The protein coagulates
2. The carbohydrate linkage solidifies
3. The fat wants to rise to the top
In pure water, the only thing that occurs is the escape of dissolved carbon dioxide (this you see), then the rest of the heat is used to add energy to the vibrating molecules of water to bring it from the liquid phase into the gas phase.