Milk protein is broadly split into two groups based on how they behave when the pH drops to around 4.6. Casein, a phosphoprotein, precipitates out at this pH, while whey proteins like lactalbumin and lactoglobulin stay dissolved. Of the total protein in cow milk, casein alone makes up close to 80 percent, with whey proteins accounting for the remaining 20 percent. Because it forms the bulk of milk's total protein, casein is considered the major milk protein.