[Celt.] The thickened part of milk. If a weak acid be added to milk, solid whitish lumps of curd separate from a watery liquid called whey. If, instead of a weak acid, an acid fluid obtained by soaking the stomach of a young calf in salt water, which is called rennet, is used, it quickly and completely coagulates the curd and separates it from the whey. When curds are analyzed, they are found to be composed of a substance known as casein, which contains nitrogen, and is classed with the nitrogenous or flesh-forming food-stuffs.