cooked meat palatability or those factors
that are thought to influence meat palata-
bility. These include cooked meat sensory
panel tenderness, juiciness and flavour, as
well as fresh meat quality factors such as
colour, firmness, marbling and water-
holding capacity of the muscle. When con-
sidering effects on meat quality, it must be
remembered that reductions in carcass fat
either through genetic selection or nutri-
tional manipulation generally result in
decreased tenderness and juiciness of the
resulting edible meat (Wood and Warriss,
1992).Tenderness
Phenethanolamines have the potential to
reduce meat tenderness. These observed
effects on meat quality appear to be
dependent on dosage and the specific
phenethanolamine under investigation, as
well as the species of livestock studied
(Merkel, 1988). An increase in the shear
force required to cut through a standard
cooked strip of muscle has been reported
for phenethanolamines in poultry, cattle,
sheep and pigs in response to specific
phenethanolamines and experimental
conditions (examples included in Table
4.3). Some effects of phenethanolamines on
tenderness appear to be the result of
decreased fat content, but some are clearly
greater than that which would be expected
for increased leanness alone. Early studies
with 2-selective adrenergic receptor
agonists in sheep and cattle gave significant
reductions in tenderness. This reduced
tenderness is thought to be related to an
effect on the calpain protease system, and
specifically to an increase in the level of
calpastatin, an inhibitor of muscle protein
degradation (Koohmaraie, 1992). It has
been demonstrated that the mechanism of
action of these 2-selective compounds is
primarily through a reduction in the rate of
muscle protein degradation in vivo (Reeds
et al., 1986; Bohorov et al., 1987) and is
therefore consistent with an increase in
calpastatin.
A reasonable criterion for any
phenethanolamine that is to be success-
fully developed for market is to affecttenderness no more than expected from the
direct effect of increasing leanness. The
effects of ractopamine and zilpaterol on
meat tenderness appear to be less severe
than those of clenbuterol, cimaterol and
L-644,969 in sheep and cattle. Although
increases in shear force values have been
reported in pigs fed ractopamine (Aalhus et
al., 1990; Uttaro et al., 1993), other shear
testing and taste panel studies detected no
difference in meat tenderness due to
ractopamine treatment (Merkel et al., 1990;
Stites et al., 1994; Jeremiah et al., 1994a,b).
Zilpaterol also appears to have minimal
effects on meat tenderness in cattle. Casey
et al.(1997a) observed greater shear force
values in steers treated with zilpaterol
compared with controls, but shear force
values for treated steers were not different
from untreated steers with similar fat con-
tent. In another study, no significant effect
of zilpaterol treatment on tenderness was
observed (Casey et al., 1997b). The increase
in shear force values reported in animals
treated with ractopamine and zilpaterol
may be a reflection of differences in fatness
of these animals. For example, Uttaro et al.
(1993) found a greater difference in shear
force value and leanness between gilts and
barrows than between ractopamine-treated
and control pigs. The minimal effect that
ractopamine has on tenderness may also be
related to its mechanism of action on
muscle protein accretion. Ractopamine has
been shown to increase muscle protein
synthesis in swine, with no apparent
decrease of protein degradation (Bergen et
al., 1989). It has also been shown that
ractopamine has no effect on muscle
calpastatin levels in swine (see Mode of
Action, Skeletal Muscle). These apparent
differences in mechanism may be related
to the primary 1-adrenergic receptor
selectivity of ractopamine compared with
the 2-adrenergic receptor selectivity of
clenbuterol, cimaterol and L-644,969.
Therefore, tenderness effects of the
phenethanolamines currently in use or
under development (zilpaterol and
ractopamine) appear to be similar to
qualitative effects normally associated
with leanness.Phenethanolamine Repartitioning Agents 7104 Farm Animal Metabolism 04 20/4/00 12:02 pm Page 71