What Is Whey?
Whey is the watery liquid left behind after milk is curdled and strained during cheesemaking, it's the part that separates from the solid curds.
More detail
It contains proteins, lactose, vitamins, and minerals from the original milk. "Sweet whey" comes from rennet-based cheesemaking (most hard cheeses); "acid whey" comes from acid-set cheeses like ricotta and paneer, and has a more sour flavor and different mineral profile.
Where it's used
Historically often discarded or fed to livestock as a cheesemaking byproduct. Today it's commercially dried and concentrated into whey protein powder (a major fitness and supplement product), and used in some traditional dishes like Norwegian brunost (whey cheese) and ricotta, which is itself made by cooking whey further.
Substitutes
There isn't a real substitute for whey's specific role, in recipes that call for leftover whey (like using it as a liquid in baking), buttermilk or milk work as a reasonable stand-in.
FAQ
Is whey protein powder the same as the liquid whey from cheesemaking?
It starts from the same liquid, but whey protein powder is filtered, concentrated, and dried into a much more protein-dense product, not something you'd get by just draining and drying leftover cheesemaking whey at home.
Can I use leftover whey from making ricotta or yogurt?
Yes, it works well as a liquid in bread dough, pancake batter, or smoothies, adding a mild tang and some extra protein rather than being wasted.
