Whey protein is a mixture of proteins isolated from whey, which is the liquid part of milk that separates during cheese production. Milk actually contains two main types of protein: casein (80%) and whey (20%). Whey is found in the watery portion of milk. When cheese is produced, the fatty parts of the milk coagulate and the whey is separated from it as a byproduct. If you’ve ever opened a yogurt container to see liquid floating on top — that’s whey. Cheesemakers used to discard it before they discovered its commercial value. After being separated during cheese production, whey goes through various processing steps to become what people generally recognize as whey protein — a powder that is added to shakes, meal replacements and protein bars. Whey protein doesn’t taste very good on its own, which is why it’s usually flavored. Chocolate-, vanilla- and strawberry-flavored powders are popular.