An amino acid with eight conceivable structural isomers, though only the L-forms occur in nature. Collagen, gelatin, and other animal proteins contain it, as does extensin, a plant protein. Because it is abundant in collagen but rare elsewhere, analysts measure hydroxyproline to quantify meat content and to detect the addition of collagen or gelatin to processed meat products. This makes it a key marker in routine meat authenticity testing, where undeclared extenders or excessive connective tissue would otherwise go undetected.