Whey protein is the name for a collection of globular proteins that can be isolated from whey, a by-product of cheese manufacture from cow's milk. It is a mixture of beta-lactoglobulin (~65%), alpha-lactalbumin (~25%), and serum albumin (~8%), which are soluble in their native forms, independent of pH.
Native whey protein does not aggregate upon renneting or acidification of milk. Prolonged heat-treatment at sufficiently high temperatures and long duration, will denature (i.e. partly unfold) the whey protein, triggering hydrophobic interactions with other proteins, and the formation of disulphide-links between whey proteins and between whey proteins and casein micelles, leading to aggregation with other milk proteins at low pH.
Whey protein is also used as a supplement by body builders to attain optimal muscle growth. The addition of excess protein in the blood stream allows strained muscles to recuperate faster that normally. A weight lifter is usually advised to "eat his weight" of protein daily, thus making this form of protein intake quite popular.