Whey is the fluid that remains of the milk of any animal after cheese making. Cheese is made by clotting milk. After clotting the fluid is separated from the curd. This fluid is called whey. It contains approximately 50% of the lactose of the milk, as well as proteins, vitamins and minerals. The proteins are often purified and marketed as whey protein. Dried whey is called whey powder.

Whey obtained as a by-product in the manufacture of hard, semi-hard or soft cheese and rennet casein is known as sweet whey and has a pH of 5.9 – 6.6. Manufacture of mineral-acid-precipitated casein yields acid whey with a pH of 4.3 – 4.6.

Although whey contains valuable nutrients, it is only in recent years that new commercial processes are being developed for the manufacture of high quality whey products such as proteins and vitamins.