Elastin is one of the animal proteins of mammalian connective tissue and thus a component of meat and meat products, particularly rich in glycine residues with high levels of proline, alanine, and valine that give it rubber-like extensibility. Unlike collagen it does not gelatinize on cooking but remains tough and chewy, which is why cuts rich in elastin, such as those containing ligaments, are graded lower for tenderness. Its cross-linked structure resists most proteolytic enzymes, limiting its nutritional availability. Meat scientists consider elastin content when explaining tenderness variation between muscles and when developing tenderization treatments, since no ordinary cooking method fully softens this resilient protein.