Page 168 - Natural Antioxidants, Applications in Foods of Animal Origin

Natural Antioxidants: Control of Oxidation in Fish and Fish Products 147
VetBooks.ir texture of the product. Metal catalyzed reactions oxidize amino acids such

as arginine, proline, and lysine into carbonyl residues (Lund et al., 2011).

Amino acids such as cysteine or methionine react in cross-linking or sulfur
derivatives. Furthermore, H O reacts with metmyoglobin (Mb (Fe )) which
3+
2
2
accumulates in the muscle after slaughter, leading to formation of perfer-
rylmyoglobin and ferrylmyoglobin. Lund et al. (2011) went further to state

that protein oxidation can also be catalyzed by non-heme iron and other
transition metals in the presence of H O . Myofibrillar proteins are oxidized
2
2
in the presence of ferric iron (Fe ) and H O , resulting in formation of a
3+
2
2
semialdehyde and ferrous iron (Fe ). Ferrous iron in the presence of H O
2+
2
2
continues the oxidative reaction degrading amino acids into hydroxyl radi-
cals. Also, reaction between H O and the thio groups of amino acids, such
2
2
as cysteine leads to the formation of sulfhydryl groups. The results of the
oxidation of thio groups are sulfenic acid (RSOH), sulfinic acid (RSOOH),
and RSSR(Lund et al., 2011).

The formation of carbonyl during protein oxidation can alter the tertiary
structure of protein and lead to various degrees of irreversible and irrepa-
rable protein unfolding (Aldini et al., 2007). Protein oxidation causes the

loss of normal functions, such as enzymatic activity, channel-forming prop-
erties, and the proteins become more susceptible to proteolytic degradation.
However, with alteration in the tertiary structure and increased hydropho-
bicity by oxidation, the rate of protein degradation is reduced (Matsuishi &
Okitani, 1997).
4.3 OXIDATION MECHANISM

Fish and fish products are made up of several compounds especially lipids
that can easily undergo oxidation due to its high tendency to lose electrons.
The centerpiece of this reaction is the molecular species known as free radi-
cals. Free radicals are molecules or atoms that have unpaired electrons and
can vary greatly in their energy. Auto-oxidation of lipids triggered by expo-
sure to light, ionizing radiations, metalloprotein catalyst, or heat can have a
deteriorating effect on color, texture, flavor, quality, and safety of fish prod-
ucts (Urquiaga & Leighton, 2000). Two major components are involved in

lipid oxidation; unsaturated fatty acids and oxygen. In the oxidation process,
oxygen from the atmosphere is added to fatty acids particularly oleate, lino-
leate, and linolenate, creating unstable intermediates which break down to
form unpleasant compounds. This process involves primary auto-oxidative
reactions which are further accompanied by various secondary reactions