Protein antioxidants for the stabilization of lipid foods: current and potential applications - Oxidation in Foods and Beverages and Antioxidant Applications - page 254

2004; Diaz et al., 2003; Faraji et al., 2004; Tong et al., 2000). Protein chelators
can inhibit oxidation reactions leading to rancidity by changing the physical
location of transition metals (e.g., partioning metals away from oxidatively
labile dispersed lipids), forming insoluble metal complexes, reducing the
chemical reactivity of transition metals, and sterically hindering the interaction
of metals and dispersed lipids (Diaz et al., 2003).
Aside from the specialized metal binding proteins discussed in the preceding
section, many `non-specialized' proteins have been shown to complex transition
metals. Examples include casein (Diaz and Decker, 2004; Diaz et al., 2003),
whey proteins (Faraji et al., 2004; Elias et al., 2006; Tong et al., 2000), soy
proteins (Faraji et al., 2004), bovine serum albumin (Villiere et al., 2005), zein
(Kong and Xiong, 2006), and potato protein (Wang and Xiong, 2005). Several
amino acid residues such as histidine, cysteine, and methionine are known to
bind metals, and may serve to localize Fenton-type reactions (the one-electron
reduction of peroxides by reduced transition metals) and other metal-catalyzed
oxidations at certain sites on a protein (Davies and Dean, 1997). Iron binding by
proteins can result in site-specific oxidation reactions that can increase the
effectiveness of aqueous proteins with respect to free radical scavenging by
directing oxidation reactions away from dispersed lipids. Faraji and coworkers
reported that whey protein isolate, soy protein isolate, and sodium caseinate
(10mg/mL) were capable of binding 185, 405, and 980moles or iron, respec-
tively (Faraji et al., 2004). These proteins have been observed to inhibit lipid
oxidation in oil-in-water emulsions and meats.
As is also the case with some specialized metal-binding proteins, non-
specialized proteins have also been shown to alter the redox state of transition
metals, thus reducing their reactivity with lipid hydroperoxides. Ferrous ions are
far more effective than ferric ions catalysts in Fenton type reactions (Decker and
McClements, 2001). Furthermore, ferrous ions are 10 17 and 10 13 times more
water soluble than ferric ions at pH 7 and 3, respectively (Decker and
McClements, 2001). Some proteins have been shown to preferentially bind
ferric ions, thereby maintaining iron is its less reactive state. For example, the
polar domains of caseins ( s1 , s2 , and ) that contain phosphorylated serine
residues are capable of forming complexes with iron, and have been shown to
tightly bind ferric ions (Diaz et al., 2003; Vegarud et al., 2000). These proteins
and their phosphorylated derivatives (e.g. caseinophosphopeptides) are effective
antioxidants in oil-in-water emulsions (Diaz et al., 2003), which may be due to
their ability to retard lipid hydroperoxide decomposition by keeping iron in its
oxidized state.
While iron is thought to be the most important transition metal catalyst in
foods, copper is also capable of reducing lipid hydroperoxides and hydrogen
peroxide to reactive radical species. In fact, copper is a more effective catalyst
than iron in peroxide decomposition reactions (Halliwell and Gutteridge, 1990),
but this metal is often ignored because it is typically present at much lower
concentrations than iron. As is the case with iron, proteins are capable of
forming strong complexes with copper which may impact lipid oxidation