What protein is commonly found in wheat, rye, and barley?

Gluten is the protein commonly found in wheat, rye, and barley. It serves as an essential component of the structure of these grains, providing elasticity and strength to dough, which is why it plays a crucial role in baking and cooking. Gluten is a composite of two main proteins: gliadin and glutenin. When flour made from these grains is mixed with water, gluten proteins form a network that holds gas bubbles produced by yeast or other leavening agents, allowing bread to rise and gain that characteristic chewy texture.

The other options are not related to the grains mentioned in the question. Casein is a protein found in milk, whey is another milk-derived protein, and lactose is a sugar found in milk, none of which are relevant to wheat, rye, or barley. Thus, gluten is specifically associated with the unique properties of these grains and their role in cooking and nutrition.