Define Milk as a rich source of protein?

A value of 3.5% protein is often considered as an average for milk. Milk protein has traditionally been divided into 2 classes - casein and whey protein. Casein is a heterogenous group of phosphoprotein, which can be precipitated from raw skimmed milk by acidification at pI.14.6 and 200 C. Proteins remaining in solution after casein precipitation are called ‘whey proteins' (or milk serum proteins). Casein fraction consists of about 80% of total protein content, rest is whey protein. Whey fraction mainly consists of β-lactalbumin, α-lactalbumin, immunoglobulins, bovine serum albumin etc.