Two major groups of non-protein components effects the degradability of proteins. The first one includes the major nutrients and their degradation products e.g. of carbohydrates and fats, which may undergo the typical "MAILLARD" reaction with proteins. The second one includes those minor components represented e.g. by the indigenous group of secondary plant metabolites. This paper focuses on protein degradability as effected by interactions of proteins with secondary plant metabolites giving special attention to the group of phenolic compounds.