A curd is the solid, lumpy part that forms when milk separates. When you add something acidic (like lemon juice or vinegar) to milk, or when certain bacteria grow in it, the milk proteins clump together into soft white chunks. Those chunks are curds, and the liquid left behind is called whey.

Cheesemakers use curds to make cheese. They heat milk, add special cultures or enzymes, and collect the curds that form. Then they press and age these curds into cheddar, mozzarella, or hundreds of other cheeses. Different cheeses start with different types of curds, which is why cottage cheese still looks lumpy while parmesan becomes hard and smooth.

You might have heard the nursery rhyme about Little Miss Muffet eating her “curds and whey.” That's essentially cottage cheese, which is just curds with some whey mixed back in. In some parts of the world, people eat fresh curds as a simple, tangy food before they become cheese.

The word also appears in “lemon curd” or “fruit curd,” but that's a different meaning: a smooth, creamy spread made with fruit juice, sugar, and eggs. Despite sharing the name, fruit curds don't involve the same separation process as milk curds do.