*2.1.2.1 Bioactive peptides with antioxidant activity*

Oxidizing compounds can cause damage to proteins, lipids, or DNA. These damages are related to the development of various diseases and to aging. Antioxidant peptides present in dietary proteins can limit oxidative damage, both in food and in the oxidation of body cells when they are ingested in the diet [16]. There are dairy peptides with antioxidant activity, and caseins and whey proteins are considered as precursors of these peptides [4]. Antioxidant peptides derived from milk are formed from 5 to 11 hydrophobic amino acids, including proline, histidine, tyrosine, and tryptophan, in sequence, that are widely distributed among the caseins, which can work by eliminating or preventing the formation of radicals as well as inhibiting enzymatic and nonenzymatic lipid peroxidation [15].

1,1-Diphenyl-2-picrylhydrazyl (DPPH) is known as a stable free radical due to the delocalization of an unpaired electron over the entire molecule. The delocalization of the electron intensifies the violet color of the radical, which it absorbs in methanol at 517 nm. When the DPPH solution reacts with an antioxidant substrate that can donate a hydrogen atom, the violet color fades. The antioxidant activity cannot be measured directly, but it can be determined by the effects of the antioxidant compound in a controlled oxidation process [17].
