Gluten's a composite of the proteins gliadin and glutenin. These exist, conjoined with starch, in the endosperms of some grass-related grains, notably wheat, rye, and barley. Gliadin and glutenin compose about 80% of the protein contained in wheat seed. Being insoluble in water, they can be purified by washing away the associated starch. Worldwide, gluten's an important source of nutritional protein, both in foods prepared directly from sources containing it, and as an additive to foods otherwi… (More on Gluten)