26 Chapter 2
polymorphisms within the gene for bovine
leptin, a chemical messenger that affects feed
intake, fatness (fat yield and subcutaneous
fat), and tenderness.
Thomas et al. (2008) reported that beef
from medium - framed, early maturing animals
had the highest marbling scores, and had the
highest concentration of total n - 3 fatty acids,
and the lowest n - 6/n - 3 ratio. Lynch et al.
(2002) reported that meat from Hereford
cattle had higher levels of C14:0, C16:1, and
C18:0 in the phospholipid fraction than that
from Friesian and Charolais cattle.
Breed can also have signifi cant effects on
beef fl avor. Nitrogen - and sulfur - compounds,
free amino acids, alcohols, aldehydes, and
ketones in the fl avor volatiles differ in the
meat from different breeds of cattle (Sato et
al. 1995 ; Insausti et al. 2005 ). Beef from
Friesian cattle has a stronger fatty fl avor
and aftertaste, and a different volatile profi le
than that from Pirenaica cattle (Gorraiz et al.
2002 ). Enzymes, such as μ - and m - calpain,
known primarily for textural changes, can
infl uence fl avor by producing peptides that
make signifi cant fl avor contributions. Meat
from Bos taurus and Bos indicus cattle inher-
iting the CC genotype at the calpastatin
gene and the TT genotype at the μ - calpain
gene produce steaks with more intense fl avor
(Casas et al. 2006 ). These genes correlate
with increased rancid, sour, and salty fl avors,
and decreased umami fl avor (Toldr á
and Flores 2000 ). In addition, content of
several volatile compounds, such as hexane
and 2,2,4,6,6 - pentamethylheptane, differs
be tween Friesian and Pirenaica cattle (Gorraiz
et al. 2002 ). Breed also affects beef color.
Frickh and Solkner (1997) reported that beef
from Holstein cattle had higher a * values
(redness) than did Simmental and Simmental
x Limousin cattle.
Genetic differences in swine have also
resulted in pork with different quality char-
acteristics. Since 1990, producers have dra-
matically improved the nutritional profi le of
pork, producing a product that is 31% lowerof different breeds or different genotypes
of the same breed differ primarily in their
connective tissue characteristics (collagen
cross - linking and solubility), content, and
composition of intramuscular fat and/or the
characteristics of their muscle fi bers (slow -
oxidative, fast - oxidoglycolytic, fast glyco-
lytic). Mutations in the myostatin gene result
in muscle hypertrophy, producing cattle with
enlarged muscles. However, this mutation
favors glycolytic muscle fi ber metabolism
and decreases collagen and intramuscular fat
contents, favoring tenderness.
Collagen constitutes 20 – 25% of the
protein in mammals, and connective tissues
are composed mainly of collagen. It occurs
in muscle tissue, binding the fi bers together
in bundles. However, collagen is not distrib-
uted uniformly among muscle groups.
Generally, the collagen content parallels the
level of physical activity of the particular
muscle. Increasing intermolecular cross - link-
ing among collagen molecules decreases
their extensibility and their solubility (Forrest
et al. 1975 ). Those muscles that are used
extensively have higher amounts of collagen
and are generally tougher.
Smith et al. (2007a) reported that weight
at slaughter, hot carcass weight, loin muscle
area, yield grade, calpastatin enzyme activ-
ity, and carcass quality grade were relatively
highly heritable. They found moderate heri-
tability estimates for marbling score, back
fat thickness, and feedlot average daily gain.
MacNeil et al. (2001) reported that Limousin -
sired calves grew more rapidly than Hereford -
sired calves. By the fi nishing phase,
Limousin - and Hereford - sired calves had
greater average daily gains than Piedmontese -
sired calves. A clear stratifi cation of USDA
yield grade, based on differences in carcass
weight, longissimus muscle area, fat depth,
and percentage kidney, pelvic, and heart fat,
existed, depending on sire breed. Hereford -
sired calves had more marbling than progeny
of Limousin or Piedmontese sires. Schenkel
et al. (2005) reported associations between