Page 40 - Natural Antioxidants, Applications in Foods of Animal Origin

Mechanism of Oxidation in Foods of Animal Origin 19
VetBooks.ir and mackerel, and higher contents of both constituents were found in sardine

muscle than mackerel muscle.
Myoglobin is made up of a single polypeptide chain, globin, consisting
of 153 amino acids and a prosthetic heme group, an iron (II) protoporphyrin-
IX complex (Hayashi et al., 1998; Pegg & Shahidi, 1997). This heme group
gives myoglobin and its derivatives their distinctive color (Dunn et al., 1999;
Pegg & Shahidi, 1997). The structure and chemistry of the iron atom have
an impact on the reactions and color changes that myoglobin undergoes
(Livingston & Brown, 1981). The oxidation of ferrous-oxymyoglobin (Fe )
2+
to ferric-metmyoglobin (Fe ) is responsible for discoloration of meat during
3+
storage. Ferrous iron (Fe ) can react with molecular oxygen to produce
2+
superoxide anion (O ) with concomitant oxidation to ferric iron (Fe ).
•-
3+
2
Hydrogen peroxide (H O ), which may be produced by dismutation of O ,
•-
2
2
2
can react with Fe to produce hydroxyl radical (OH ) (Hultin, 1992). This
2+
•
reaction termed as Fenton reaction is the principal mechanism for myoglobin
oxidation (Fig. 1.2).
Fe + O Fe + O 2 •-
2+
3+
2
2O + 2H H O + O
•-
+
2 2 2 2
Fe + H O Fe + OH + OH •
3+
2+
-
2 2
FIGURE 1.2 Reactive oxygen species generated by the Fenton reaction.
In general, fish myoglobins are more readily oxidized than the mamma-
lian counterpart (Haard, 1992). Discoloration of tuna meat during frozen
storage is associated with the formation of metmyoglobin (Haard, 1992).
This phenomenon can be influenced by many factors such as pH, tempera-
ture, ionic strength, and oxygen consumption reaction (Renerre & Labas,
1987). Metmyoglobin formation is positively correlated with lipid oxida-
tion (Chan et al., 1997a, 1997b; Lee et al., 2003a, 2003b). Benjakul and
Bauer (2001) suggested that the freeze-thaw process caused damage of cell
and heme-proteins, resulting in the release of prooxidants. Haard (1992)
also reported that fish myoglobins are at least 2.5 times more sensitive to
autoxidation than mammalian myoglobins. Autoxidation of myoglobin
becomes greater as temperature increased and pH decreased (Livingston
et al., 1981; Chaijan et al., 2007). Chaijan et al. (2007) demonstrated that
sardine myoglobin was prone to oxidation and denaturation at tempera-
ture above 40 °C and at very acidic or alkaline pHs as evidenced by the