20 Dec How protein oxidation is related to meat quality
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:
- 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.
- 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.
- Formation of cross-linking (bonds that hold different amino acid chains together), which makes the proteins more rigid.
- 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.
- 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.
Water holding capacity
Water holding capacity (WHC) is the ability to retain its own or added water throughout processing and storage, especially when external forces (force, pressure, centrifugation, or heating) are applied. A poor WHC results in a reduction of sellable meat weight and a worsening of the nutritional value, palatability and visual appeal.
The muscle contains 75% water, and the majority is held between the myofibrils (muscle fibers made of proteins) or between the muscular cells. Protein oxidation, particularly the formation of cross-linking bonds in the myofibril proteins, leads to a worsening of WHC. It can occur before or after slaughter.
Tenderness is considered the most important sensory trait. Meat becomes tender during post-mortem storage, when a group of enzymes called “calpain system” degrades the myofibrils.
Free radicals oxidate and inactivate calpain enzymes. Furthermore, the formation of cross-linking bonds in the myofibril proteins make them more resistant to degradation. Overall, protein oxidation results in tougher meat.
The oxidation of proteins impairs the nutritional quality.
The formation of carbonyl groups results in the loss of amino acids such as lysine, histidine and arginine, which are essential for humans. Phenylalanine and tryptophan are depleted by free radical attack.
On the other hand, oxidation may lead to a decrease in protein digestibility and bioavailability, and the consumption of oxidized meat may increase oxidation markers in human beings, which are linked to multiple diseases.
The use of natural antioxidants to counteract protein oxidation
Essential oils are rich in constituents with high antioxidant activity and have been proven to reduce protein oxidation in meats. In multiple field trials, PhytoShield©, our liquid antioxidant rich in essential oils, has shown to improve the taste and the tenderness of broiler meat. Furthermore, in a recent trial (not published) the meats coming from animals treated with PhytoShield© showed better Water Holding Capacity.
Products of choice
PhytoShield© is an oral emulsion that contains essential oils, electrolytes and vitamins intended to promote growth, reduce oxidative stress, increase meat and egg quality, and keep the animals hydrated.
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