Whey is the thin, translucent, pale greenish-yellow liquid that separates from the curd when milk is coagulated in cheesemaking — the watery serum left behind once the casein proteins and fat have clumped into curds. Once dismissed as a waste stream to be poured away or fed to pigs, it is now prized twice over: as a raw material re-cooked into ricotta and other whey cheeses, and, dried and refined, as the whey-protein powder that underpins a vast sports-nutrition and food-ingredient industry.