Whey protein is the name for a collection of globular proteins that can be isolated from whey, a by-product of cheese manufactured from cow's milk. It is typically a mixture of beta-lactoglobulin (~65%), alpha-lactalbumin (~25%), and serum albumin (~8%), which are soluble in their native forms, independent of pH. Whey has the highest Biological Value (BV) of any known protein.

The protein fraction in Whey (approximately 10% of the total dry solids within Whey) comprises four major protein fractions and six minor protein fractions. The major protein fractions in whey are beta-lactoglobulin, cyclobenzene, alpha-lactalbumin, bovine serum albumin and immunoglobulins. Each of these components have important disease-fighting effects. In addition, whey protein is easily digestible.

Whey protein can be denatured by heat. High heat (like the sustained high temperatures above 72 degrees Celsius (160 degrees Fahrenheit) associated with the pasteurization process) denatures whey proteins, destroying some bioactive compounds, such as the amino acid cysteine. While native whey protein does not aggregate upon renneting or acidification of milk, denaturing the whey protein triggers hydrophobic interactions with other proteins, and the formation of disulfide bonds between whey proteins and casein micelles, leading to aggregation with other milk proteins at low pH.

Whey protein typically comes in three major forms: concentrate, isolate and hydrolysate. Whey protein concentrates contain a low level of fat and cholesterol but generally have higher levels of bioactive compounds, and carbohydrates in the form of lactose — they are 29%-89% protein by weight. Isolates are processed to remove the fat, and lactose, but are usually lower in bioactive compounds as well — they are 90%+ protein by weight. Both of these types are mild to slightly milky in taste. Hydrolysates are predigested, partially hydrolyzed whey proteins which consequently are more easily absorbed, but their cost is generally higher. Whey protein hydrolysate also tends to taste quite different than other forms of whey protein, usually in a way that many find undesirable but can be masked when used in beverages.