14 Biochemistry of Processing Meat and Poultry 331consequence of further oxidative reactions. Oleic,
linoleic, estearic, and palmitic acids are those accu-
mulated in higher amounts because they are present
in great amounts in the triacylglycerols and have a
better stability against oxidation.
OXIDATIVE REACTIONS
The lipolysis and generation of free polyunsaturated
fatty acids, susceptible to oxidation, constitute a key
stage in flavor generation. The susceptibility of fatty
acids to oxidation and the rate of oxidation depends
on their unsaturation (Shahidi 1998a). So, linolenic
acid (C 18:3) is more susceptible than linoleic acid
(C 18:2), which is more susceptible than oleic acid
(C 18:1). The animal species have different suscepti-
bility to autoxidation in the following order: poultry
pork beef lamb (Tichivangana and Morrisey
1985). Oxidation has three consecutive stages (Sha-
hidi 1998b). The first stage, initiation, consists in the
formation of a free radical. This reaction can be
enzymatically catalyzed by muscle lypoxygenase or
chemically catalyzed by light, moisture, heat, and/or
metallic cations. The second stage, propagation,
consists in the formation of peroxide radicals by
reaction of the free radicals with oxygen. When per-
oxide radicals react with double bonds, they form
primary oxidation products, or hydroxyperoxides,
that are very unstable. Their breakdown produces
many types of secondary oxidation products by a
free radical mechanism. Some of these secondary
oxidation products are potent flavor-active com-
pounds that can impart off-flavor to meat products
during cooking or storage. The oxidative reactions
finish by inactivation of free radicals when they
react with each other (last stage). Thus, the result of
these oxidative reactions consists in the generation
of volatile compounds responsible for final product
aroma. It is important to have a good control of these
reactions because sometimes, oxidation may give
undesirable volatile compounds with unpleasant off-
flavors.
OXIDATION TOVOLATILECOMPOUNDS
As mentioned above, some oxidation is needed to
generate volatile compounds with desirable flavor
properties. For instance, a characteristic aroma of
dry-cured meat products is correlated with the initi-
ation of lipid oxidation (Buscailhon et al. 1994,
Toldrá and Flores 1998). However, an excess of oxi-
dation may lead to off-flavors, rancidity, and yellow
colors in fat.
The primary oxidation products, or hydroperox-
ides, are flavorless, but the secondary oxidation pro-
ducts have a clear contribution to flavor. There are a
wide variety of volatile compounds formed by oxi-
dation of the unsaturated fatty acids. The most im-
portant are (1) aliphatic hydrocarbons that result
from autoxidation of the lipids; (2) alcohols, mainly
originated by oxidative decomposition of certain
lipids; (3) aldehydes, which can react with other
components to produce flavor compounds; and (4)
ketones produced through either -keto acid decar-
boxylation or fatty acid -oxidation. Other com-
pounds, like esters, may contribute to characteristic
aromas (Shahidi et al. 1998a).
Oxidation rates may vary depending on the type
of product or the processing conditions. For in-
stance, TBA (thiobarbituric acid), a chemical index
used as an indication of oxidation, increases more
markedly in products such as Spanish chorizo than
in French saucisson or Italian salami (Chasco et al.
1993). On the other hand, processing conditions such
as curing or smoking also give a characteristic flavor
to the product.ANTIOXIDANTSThe use of spices such as paprika and garlic, which
are rich in natural antioxidants, protects the product
from certain oxidations. The same applies to antiox-
idants such as vitamin E that are added in the feed to
prevent undesirable oxidative reactions in polyun-
saturated fatty acids. Nitrite constitutes a typical
curing agent that generally retards the formation of
off-flavor volatiles that can mask the flavor of the
product, and allows extended storage of the product
(Shahidi 1998). Nitrite acts against lipid oxidation
through different mechanisms: (1) binding of heme
and prevention of the release of the catalytic iron,
(2) binding of heme and nonheme iron and inhib-
ition of catalysis, and (3) stabilization of lipids
against oxidation. Smoking also contains some anti-
oxidant compounds such as phenols that protect the
external part of the product against undesirable oxi-
dations. The muscle antioxidative enzymes also
exert some contribution to the lipid stability against
oxidation. In the case of fermented meats, the mi-
crobial enzyme catalase degrades the peroxides