13 Biochemistry of Raw Meat and Poultry 307Pigs and poultry are monogastric animals that
incorporate part of the dietary fatty acids practically
unchanged into the adipose tissue and cellular mem-
branes, where desaturation and chain elongation pro-
cesses may occur (Toldrá et al. 1996, Jakobsen
1999). The extent of incorporation may vary de-
pending on the specific fatty acid and the type of
feed. Dietary oils and their effects on the proportions
in fatty acid composition have been studied. The use
of canola or linseed oils produce a substantial
increase in the content of linolenic acid (C 18:3),
which is a n-3 fatty acid. In this way, the n-6:n-3 ratio
can be reduced from 9 to 5 (Enser et al. 2000). Other
dietary oils such as soya, peanut, corn, and sunflower
increase the content of linoleic acid (C 18:2), an n-6
fatty acid. Although it increases the total PUFA con-
tent, this fatty acid does not contribute to decrease
the n-6:n-3 ratio, just the reverse. A similar trend is
observed in the case of poultry, where the feeds with
a high content of linoleic acid such as grain, corn,
plant seeds, or oils also increase the n-6:n-3 ratio
(Jakobsen 1999). As in the case of pork, the use of
feeds containing fish oils or algae, enriched in n-3
fatty acids such as eicosapentaenoic (C 22:5 n-3) and
docosahexanoic (C 22:6 n-3) acids, can enrich the
poultry meat in n-3 fatty acids and reduce the n-6:n-
3 ratio from around 8.4 to 1.7 (Jakobsen 1999).
The main problem arises from oxidation during
heating, because some volatile compounds such as
hexanal are typically generated, producing rancid
aromas (Larick et al. 1992). The rate and extent of
oxidation of muscle foods mainly depends on the
level of PUFA, but they are also influenced by early
postmortem events such as pH drop, carcass temper-
ature, ageing, and other factors. It must be pointed
out that the increased linoleic acid content is replac-
ing the oleic acid to a large extent (Monahan et al.
1992). Feeds rich in saturated fats such as tallow
yield the highest levels of palmitic, palmitoleic,
stearic, and oleic acids in pork loin (Morgan et al.
1992). Linoleic and linolenic acid content may vary
as much as 40% between the leanest and the fattest
animals (Enser et al. 1988). An example of the effect
of feed type on the fatty acid composition of subcu-
taneous adipose tissue of pigs is shown in Table
13.7. The PUFA content is especially high in phos-
pholipids, located in subcellular membranes such as
mitochondria, microsomes, and so on, making them
vulnerable to peroxidation because of the proximity
of a range of prooxidants such as myoglobin, cyto-
chromes, nonheme iron, and trace elements (Buck-
ley et al. 1995). Muscle contains several antioxidant
systems, for example, those of superoxide dismutase
and glutathione peroxidase, and ceruplasmin and
transferrin, although they are weakened during post-
mortem storage.
An alternative for effective protection against oxi-
dation consists in the addition of natural antioxidants
like vitamin E (-tocopheryl acetate); this has con-
stituted a common practice in the last decade. This
compound is added in the feed as an antioxidant and
is accumulated by animals in tissues and subcellular
structures, including membranes, substantially in-
creasing its effect. The concentration and time of
supplementation are important. Usual levels are
around 100–200 mg/kg in the feed for several weeks
prior to slaughter. The distribution of vitamin E in
the organism is variable, being higher in the muscles
of the thoracic limb, neck, and thorax and lower in
the muscles of the pelvic limb and back (O’Sullivan
et al. 1997). Dietary supplementation with this lipid-
soluble antioxidant improves the oxidative stability
of the meat. Color stability in beef, pork, and poultry
is improved by protection of myoglobin against oxi-
dation (Houben et al. 1998, Mercier et al. 1998). The
water-holding capacity in pork is improved by pro-
tecting the membrane phospholipids against oxida-
tion (Cheah et al. 1995, Dirinck et al. 1996). The
reduction in drip loss by vitamin E is observed even
in frozen pork meat, upon thawing. Oxidation of
membrane phospholipids causes a loss in membrane
integrity and affects its function as a semipermeable
barrier. As a consequence, there is an increased pas-
sage of sarcoplasmic fluid through the membrane,
known as drip loss (Monahan et al. 1992).
The fatty acid profile in ruminants is more satu-
rated than in pigs, and thus the fat is firmer (Wood et
al. 2003). The manipulation of fatty acids in beef is
more difficult due to the rumen biohydrogenation.
More than 90% of the polyunsaturated fatty acids
are hydrogenated, leaving a low margin for action to
increase the PUFA:SFA ratio above 0.1. However,
meats from ruminants are rich in conjugated linoleic
acid (CLA), mainly 9-cis,11-trans-octadecadienoic
acid, which exerts important health-promoting bio-
logical activity (Belury 2002).
In general, a good level of nutrition increases the
amount of intramuscular fat. On the other hand,