Proteins are important targets of oxidative reactions, since they are abundant in living organisms and react quickly with oxidizing substances. However, unlike lipid oxidation, protein oxidation processes in meats, as well as their consequences, are poorly studied.

Oxidative processes produce reversible or irreversible changes in the structure of proteins, by modifying their covalent bonds. The physical and chemical properties of proteins are altered, including their primary structure (linear sequence of amino acids), three-dimensional shape, electrostatic charge, polarity, water solubility, and biological activity.

There are four main mechanisms of protein oxidation:

1. General oxidation, produced by free radicals. It affects the peptide backbone that gives the protein its shape, as well as the side chains that determine protein charge and polarity.
2. Loss of thiol group (- SH) in sulfur-containing amino acids (methionine, cysteine, homocysteine, and taurine), which leads to a loss of biological activity of the protein. This kind of oxidation is linked mainly to oxidative stress before slaughter.
3. Formation of cross-linking (bonds that hold different amino acid chains together), which makes the proteins more rigid.
4. Formation of carbonyl groups ( C=O ) in the side chains, catalyzed by the presence of free iron. Free iron is also a key initiator of lipid oxidation. The resulting products (α-aminoadipic and γ-glutamic semialdehydes), present in oxidized meat, may be used as markets of oxidation in cooked meats.
5. Interaction between oxidized lipids and proteins, affecting amino acid side chains.

Protein oxidation impairs the nutritional, technological and sensory qualities of raw and processed meat.