Whey protein is a dairy by-product, the protein contained within the watery portion left over when all of the solid material is removed from milk. Whey is produced principally by the cheese-making industry, which uses milk solids called curds to make cheese.
When cheese is made, milk is separated into two types of protein. One type, casein, responds to the cheese-making process by coagulating into soft, thick masses. The other type of protein, whey protein, remains within the watery liquid solution. When cheese is pressed, the casein remains as curds and the whey protein is washed away with the rest of the liquid, leaving a relatively dry young cheese. Whey protein can be processed from whey into a nutritious powder that is used as a food additive.