BLBS102-c15 BLBS102-Simpson March 21, 2012 13:21 Trim: 276mm X 219mm Printer Name: Yet to Come
15 Biochemistry of Raw Meat and Poultry 297Effect of the Age and SexThe content in intramuscular fat content increases with the age
of the animal. In addition, the meat tends to be more flavor-
ful and colorful, due to an increased concentration of volatiles
and myoglobin, respectively (Armero et al. 1999a). Some of the
muscle proteolytic and lipolytic enzymes are affected by age.
Muscles from heavy pigs (11 months old) are characterized by a
greater peptidase to proteinase ratio and a higher lipase, dipep-
tidylpeptidase IV, and pyroglutamyl aminopeptidase activity. On
the other hand, the enzyme activity in light pigs (7–8 months
old) shows two groups. The larger one is higher in moisture con-
tent and cathepsins B and B+L and low in peptidase activity,
while the minor one is intermediate in cathepsin B activity and
high in peptidase activity (Toldra et al. 1996a). In general, there ́
is a correlation between the moisture content and the activity
of cathepsins B and B+L (Parolari et al. 1994). So, muscles
with higher moisture content show higher levels of cathepsins
B and B+L activity. This higher cathepsin activity may produce
an excess of proteolysis in processed meat products with long
processing times (Toldr ́a 2002).
A minor effect of sex is observed. Meats from barrows contain
more fat than those from gilts. They present higher marbling,
and the subcutaneous fat layer is thicker (Armero et al. 1999a).
In the case of muscle enzymes, only very minor differences have
been found. Sometimes, meats from entire males may give some
sexual odor problems due to high contents of androstenone or
escatol.Effect of the Type of FeedA great research effort has been exerted since the 1980s for the
manipulation of the fatty acid composition of meat, to achieve
nutritional recommendations, especially an increase in the ra-
tio between PUFA and saturated fatty acids (SFA) (PUFA:SFA
ratio). More recently, nutritionists recommend that PUFA com-
position should be manipulated toward a lowern-6:n-3 ratio.
Fats with a higher content of PUFA have lower melting points
that affect the fat firmness. Softer fats may raise important prob-
lems during processing if the integrity of the muscle is disrupted
by any mechanical treatment (chopping, mincing, stuffing, etc.).
The major troubles are related to oxidation and generation of
off-flavors (rancid aromas) and color deterioration (trend toward
yellowness in the fat) (Toldra and Flores 2004). ́
Pigs and poultry are monogastric animals that incorporate part
of the dietary fatty acids practically unchanged into the adipose
tissue and cellular membranes, where desaturation and chain
elongation processes may occur (Toldr ́a et al. 1996b, Jakobsen
1999). The extent of incorporation may vary depending on the
specific fatty acid and the type of feed. Different types of cereals
as well as dietary oils and their effects on the proportions in fatty
acid composition have been studied. The use of canola or linseed
oils produces a substantial increase in the content of linolenic
acid (C 18:3), which is ann-3 fatty acid (Jim ́enez-Colmenero
et al. 2006). In this way, then-6:n-3 ratio can be reduced from 9
to 5 (Enser et al. 2000). Other dietary oils such as soy, peanut,
corn, and sunflower increase the content of linoleic acid (C18:2), ann-6 fatty acid. Although it increases the total PUFA
content, this fatty acid does not contribute to decrease then-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 then-6:n-3
ratio (Jakobsen 1999). As in the case of pork, the use of feeds
containing fish oils or algae, enriched inn-3 fatty acids such as
eicosapentaenoic (C 22:5n-3) and docosahexanoic (C 22:6n-3)
acids, can enrich the poultry meat inn-3 fatty acids and reduce
then-6:n-3 ratio from around 8.4 to 1.7 (Jakobsen 1999).
The main problem arises from oxidation during heating, be-
cause 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 temperature, ag-
ing, and other factors. It must be pointed out that the increased
linoleic acid content is replacing the oleic acid to a large ex-
tent (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 sub-
cutaneous adipose tissue of pigs is shown in Table 15.7. The
PUFA content is especially high in phospholipids, located in
subcellular membranes such as mitochondria, microsomes, and
so on, making them vulnerable to peroxidation because of the
proximity of a range of pro-oxidants such as myoglobin, cy-
tochromes, nonheme iron, and trace elements (Buckley et al.
1995). Muscle contains several antioxidant systems, for exam-
ple, those of superoxide dismutase and glutathione peroxidase,
and ceruloplasmin and transferrin, although they are weakened
during postmortem storage.
An alternative for effective protection against oxidation con-
sists of the addition of natural antioxidants like vitamin E (alpha-
tocopheryl acetate); this has constituted 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 struc-
tures, including membranes, substantially increasing 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 oxidation (Houben et al.
1998, Mercier et al. 1998). The water-holding capacity in pork
is improved by protecting the membrane phospholipids against
oxidation (Cheah et al. 1995, Dirinck et al. 1996). The reduction
in DL 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 semiperme-
able barrier. As a consequence, there is an increased passage of
sarcoplasmic fluid through the membrane, known as DL.