Curds are a dairy product obtained by curdling (coagulating) milk with rennet or an edible acidic substance such as lemon juice orvinegar, and then draining off the liquid portion. The increased acidity causes the milk proteins (casein) to tangle into solid masses, orcurds. The remaining liquid, which contains only whey proteins, is the whey. In cow's milk, 80% of the proteins are caseins. Milk that has been left to sour (raw milk alone or pasteurized milk with added lactic acid bacteria or yeast) will also naturally produce curds, andsour milk cheese is produced this way.