Page 191 - Natural Antioxidants, Applications in Foods of Animal Origin

170 Natural Antioxidants: Applications in Foods of Animal Origin
VetBooks.ir Post-slaughter changes which predispose muscle foods to oxidation are:

a) Stunning and bleeding—circulation of blood ceases.
b) Anaerobic metabolism—lactic acid accumulates and pH declines to
approximately 5.5.
c) Circulation of nutrients rapidly ceases.
d) Preventative antioxidant enzyme system namely superoxide dismutase,
catalase, glutathione peroxidase, glutathione reductase etc., unlikely
to function.
e) Acute phase proteins which scavenge Fe aeruloplasmin, transferrin,
+
haptoglobin—unlikely to be activated.
f) Sarcoplasmic reticulum loses its calcium accumulating ability.
g) Calcium dependent proteinases degrade muscle proteins.
h) Some destruction of cell compartmentalization.
i) Low molecular weight chelatable iron is released.
j) Iron-catalyzed chain reactions.
k) Membranal lipid oxidation initiated.

5.2.3 CATALYSTS OF LIPID OXIDATION

Some confusion prevails about the nature of the initiation process in lipid
oxidation, but spontaneous lipid radical formation or direct reaction of unsat-
urated fatty acids with molecular oxygen is thermodynamically unfavor-
able. Most researchers believe that the presence of transition metals, notably
iron, is pivotal in the generation of species capable of abstracting a proton
from an unsaturated fatty acid. During handling, processing, cooking, and
storage, iron is released from high molecular weight sources (e.g., hemo-
globin, myoglobin, ferritin, hemosiderin) may directly cause oxidation or
made available to low molecular weight compounds such as amino acids,
nucleotides, and phosphates with which it is believed to form chelates. These
chelates are thought to be responsible for the catalysis of lipid oxidation in
biological tissues. However, the relative contributions of the different forms
of iron have not been clearly defined. Much of the information pertaining to
lipid oxidation in meat deals with hydroperoxide dependent lipid oxidation.
Pure lipid hydroperoxides are fairly stable at physiological temperatures,
but in the presence of transition metal complexes, especially iron salts, their
decomposition is greatly accelerated.