Casein is a protein that is found in milk and used independently in many foods as a binding agent. Technically, it is part of a group called phosphoproteins, collections of proteins bound to something containing phosphoric acid. Casein may also be called caseinogen, particularly in European foods.

Casein is a salt, meaning it has no net ionic charge, of the element calcium. It has a number of interesting properties that make it useful in foods and cooking. Many people believe proteins are healthier if consumed when not denatured – one of the major lines of reasoning used in supporting a raw food diet. Denaturing occurs when a protein loses its inherent structure, due to high heat or acid for example, at which point it no longer acts in the ordinary manner. Casein, because of its structure, is not susceptible to denaturing.

Casein can be found in two main types: edible and technical. Edible casein is widely used in both medicine and food, both for nutritional value and as a binder. Technical casein is used in an enormous range of products, including paints, cosmetics, and many types of adhesives. A not-insubstantial number of people have a casein allergy and may find themselves experiencing negative reactions both to casein-containing food products and to products such as nail polish that contain casein.

Many people who are either allergic to casein and have linked it to milk, or who are vegan and therefore avoid animal products altogether, are not aware of the prevalence of casein in foods. For these people, it is important to note that, although a product may be labeled Lactose Free, it may easily still contain casein for other reasons. Soy cheeses, for example, often contain casein derived from milk, which may stimulate allergic reactions in people who assume that they are dairy free.

Casein has also been linked to negative effects in people with autism. While in most people, casein is easily broken down by the digestive system into peptides known as casomorphins, and then further processed into basic amino acids, some evidence suggests that in autistics, this process does not occur fully. The resulting casomorphins, which fail to break down completely, may have an effect on the body similar to that of morphine or other opiates. For this reason, some experts on autism recommend that people suffering from autism avoid casein in their diets.