Page 189 - Natural Antioxidants, Applications in Foods of Animal Origin

168 Natural Antioxidants: Applications in Foods of Animal Origin
VetBooks.ir all, lipid oxidation products also affect protein solubility, emulsification,

water-binding capacity, texture, and the other rheological properties through
the interactions between lipid and protein oxidation products (Hall, 1987).
Cholesterol may be oxidized to form oxysterols, which are also as toxic as
fatty acid-derived hydroperoxides (Smith & Johnson, 1989). Consumption
of oxysterol-containing foods may be potentially harmful to cellular physi-
ology (Morel & Lin, 1996). Oxidation of meat lipids is a complex process
and its dynamics depend on numerous factors including chemical composi-
tion of the meat, light, and oxygen access as well as storage temperature. The
products of lipid oxidation also interfere with the absorption of protein or
folic acid and have been found to cause pathological changes in the mucous
membrane of the digestive tract.

5.2.1 LIPID OXIDATION MECHANISM

The first step in lipid oxidation is the removal of hydrogen ion from a methy-
lene carbon of a fatty acid (RH). It becomes easier as the number of double
bonds in the fatty acid increases, which is why PUFA are particularly suscep-
tible to oxidation. The initiation step can be catalyzed by OH or by certain
–
iron–oxygen complexes (e.g., ferryl or perferryl radicals).

RH + OH R + H O (5.1)
–
–
2
The fatty acyl radical (R ) reacts rapidly with O to form a peroxyl radical
–
2
(ROO ):
–
R + O ROO (5.2)
–
–
2
The rate-constant (K ) for this reaction is 3 × l0 M S . Because ROO is
–
-1
-1
8
2
more highly oxidized than the fatty acyl radical or the fatty acid itself, it will
preferentially oxidize other unsaturated fatty acids and propagate the chain
reaction:
ROO + RH ROOH + R – (5.3)
–
The rate-constant (K ) for this step is relatively low (1 × 10 M S ). Lipid
1
-1 -1
3
hydroperoxides (ROOH) formed in the propagation reaction are both prod-
ucts of oxidation and substrates for further reaction with Fe and Cu to
++
+
yield ROO and alkoxyl radicals (RO ). The ferrous (Fe ) reductively
–
++
–
cleaves ROOH (3) as follows: