Page 42 - Natural Antioxidants, Applications in Foods of Animal Origin

Mechanism of Oxidation in Foods of Animal Origin 21
VetBooks.ir 2006b). Heme is the nomenclature used to describe the porphyrin ring

containing ferrous (Fe ) iron, while hemin describes the porphyrin ring
2+
containing ferric (Fe ) iron. Ferrous myoglobin are typically either liganded
3+
with O or no ligand is present (e.g., deoxymyoglobin). The problem in under-
2
standing the pathway by which heme proteins promote lipid oxidation is that
heme protein autoxidation, ferryl radical formation, heme dissociation, heme
destruction, and iron release can all occur in a very short time sequence and
simultaneously so that the most relevant step related to lipid oxidation is
obscured ring (Grunwald & Richards, 2006a, 2006b). Heme-initiated lipid
oxidation, especially myoglobin, has been extensively reported in meats
(Ledward, 1987; Love & Pearson, 1974; Richards & Hultin, 2002). The
interrelationship between lipid and myoglobin oxidations in muscle foods
has been reported by Chaijan (2008). Ohshima et al. (1988) proposed that the
lipid oxidation in fish muscle was promoted by autoxidation of myoglobin.
Moreover, O’Grady et al. (2001) reported a relationship between oxymyo-
globin oxidation and lipid oxidation in bovine muscle. There are numerous
potential mechanisms by which myoglobin can promote lipid oxidation in
muscle foods. The process by which ferrous myoglobin (or hemoglobin) is
converted to ferric metmyoglobin is called autoxidation. Superoxide anion
radical (O ) or •OOH is liberated in this process depending on whether
•−
2
deoxy or oxy heme protein undergoes autoxidation (Brantley et al., 1993).
O and •OOH can readily be converted to hydrogen peroxide (H O ), which
•−
2
2
2
enhances the ability of heme proteins to promote lipid oxidation. Moreover,
metmyoglobin can react with H O or lipid hydroperoxides to generate ferryl
2
2
heme protein radicals, which can abstract hydrogen from PUFA and hence
initiate lipid oxidation. Alternatively, displaced hemin or released iron can
stimulate lipid oxidation. Metmyoglobin is an effective prooxidant at acidic
pH and in the presence of hydroperoxides. Morey et al. (1973) found that
H O acting as an oxidizing agent caused changes in the oxidation state of the
2
2
iron in myoglobin. The reaction between H O and metmyoglobin results in
2
2
the formation of red pigment, ferrylmyoglobin (MbFe(IV)=O). Under physi-
ological conditions (pH 7.4), ferrylmyoglobin is a strong prooxidant, which
is able to abstract a hydrogen atom from fatty acids with subsequent stereo-
specific addition of oxygen (Rao et al., 1994). The prooxidative activity of
ferrylmyoglobin is independent of pH and of lipid concentration (Chan et
al., 1997a, 1997b; Kanner et al., 1987).
The concentration of ferrous iron and its ability to be active in the lipid
oxidation reaction will be a key factor causing differences in lipid oxida-
tion among species. In general, dark meats tend to have more reactive iron.
Chaijan et al. (2004) reported that lipid and myoglobin contents were higher