Whey is the liquid that separates from the curds in cheese making. It’s a useful byproduct that’s high in protein, so don’t throw it out!
You can make more cheese with it, like ricotta. (Fun fact for the day: “ricotta” means “recooked.”) It’s easy, says lisa13:
Heat the whey to 200F. If the whey is acidic enough, you’ll see flecks of albumin when it gets to about 200F. If that doesn’t happen, add a tablespoon or two of white vinegar. When the resulting albumin starts to separate, maintain the heat for a few minutes to allow it to set up. To drain, pour through very fine cheesecloth in a collander; it usually takes several hours to drain completely. Salt, if you like.
The yield is 1-1 1/2 cup of ricotta from the whey left over from cheese made from a gallon of milk.
Whey freezes well, so you can freeze it and save up enough to make a bigger batch of ricotta.
JGrey recommends the book Nourishing Traditions, by Sally Fallon. She uses whey in nearly everything, from the liquid used in bread baking, to making sauerkraut and pickles.