Protein antioxidants for the stabilization of lipid foods: current and potential applications - Oxidation in Foods and Beverages and Antioxidant Applications - page 263

and Um, 1992; Zamora and Hidalgo, 2005). Maillard reaction products represent
a wide range of structures, which include furans, reductones, Schiff bases, and
aldehydes. Many of these products have been observed to inhibit lipid oxidation
reactions through radical scavenging mechanisms and metal chelation. Protein-
sugar Maillard reaction products are often water soluble or produced on the
surface of protein-stabilized emulsion droplets. Augustin et al. (2006) found that
heating casein-stabilized fish oil-in-water emulsions in the presence of glucose
yielded Maillard reaction products with good antioxidant activity.
11.5.3 Potential problems with protein antioxidants in foods
Despite their proven antioxidant activity in many food systems, proteins and
peptides are not suitable replacements for traditional, or synthetic, antioxidants
in all food applications. For example, proteins and peptides are potential
allergens, which should be avoided among consumers with pre-established
hypersensitivy. Dairy, soy, nut, and egg protein pose the greatest risk. From a
sensory and consumer acceptability perspective, small peptides are often
perceived as bitter, and can be perceived as such even at the concentrations used
to inhibit lipid oxidation reactions. Furthermore, the incorporation of proteins
and peptides to foods may cause unwanted rheological changes (e.g., increased
viscosity of aqueous solutions or gelation) under some conditions. Maillard
reaction products that form between proteins and sugars may have good
antioxidant activity in lipid foods, but the inevitable color changes (e.g.,
browning reactions) that occur may not be desirable in all foods.
11.6 Consequences of protein oxidation
Protein oxidation most often results in amino acid R-group modification. Protein
backbone (i.e., peptide bond) fragmentation is also possible with some oxidants,
as are polymerization reactions. Dissolved oxygen concentration has also been
observed to affect the course of protein oxidation in model systems. For
instance, the hydroxyl radical-mediated oxidation of BSA results in extensive
cross-linking under anoxic conditions; however, only limited cross-linking is
seen in the presence of oxygen (Dean et al., 1997).
The chemistry of free amino acid R-groups oxidation is similar to that
which occurs on peptide and protein residues (Davies and Dean, 1997).
However, a key difference is that radical transfer reactions (i.e., the transfer of
free radicals from one amino acid to another) are known to occur in proteins,
meaning that the initial site of protein oxidation may be difficult to assess by
conventional methods in some cases. The intramolecular transfer of radicals
within a protein is affected by its physical structure. For example, little
evidence of radical transfer is observed between Trp25 and Tyr29 on the
neurotoxin erabutoxin b, despite the fact that these two amino acids are a mere
1.3 nm apart (Butler et al., 1982; Prutz et al., 1982). This appears to be a