The halothane gene and the
RN
−
gene are two major genes that
have a direct effect on meat quality. Both arose as mutations. In the case of
the halothane gene (
nn
), the effects are mediated
through the rate of post-mortem muscle pH decline. The
h
2
of pH
45 min
is usually in the range 0.14
to 0.37. It is difficult to predict the likely responses in pH
45 min
to genetic improvement in
growth and carcass composition in populations with an unknown prevalence of the
n
/
–
condition. In Large White
pigs, which normally have a low prevalence of
n/–
, it is thought that selection for growth rate
and fatness is more likely to raise pH
45 min
, whilst selection against fatness will have little
effect (Sonesson
et
al
., 1998).
The halothane gene has a large effect on WHC, whereas the effect is smaller for
the
RN
−
gene.
The
h
2
of most other pigmeat quality traits is generally low (Verbeke
et al
., 1999). The
h
2
of pH
ult
is about 0.21 (0.07 to 0.39),
and in
NN
and
Nn
pigs it is invariably low.
Pigmeat quality is best improved by breed substitution, rather than by
selection within breed. For example, insufficient marbling in lean pig
genotypes has been controlled by using the Duroc breed in developing meat sire
lines. This has been highly successful, and dry meat with insufficient
succulence is not such a problem as it was in the past. If this were taken too
far, such that very high levels of marbling were present in the ham, there
could be negative effects on flavour and the odour of cooked ham slices, but
these repercussions are not likely at present (Fernandez
et al
., 2000). Breeds like the
Belgian Landrace, which have heavily muscled hams and a high lean meat yield, do
not necessarily have superior eating quality. They have poorer meat flavour,
tenderness and juiciness in comparison with the Large White (Verbeke
et al
., 1999).
Breast meat has been getting paler in recent years
and the reason may be genetic. The
h
2
of meat paleness in broilers is high (0.50). It has not reached
the stage where it is a consumer complaint, but it is causing processors
inconvenience when trying to match breast meat colour in retail display packs.
Pale breast meat can indicate other problems such as excessive amounts of purge
in the pack and this adds to the negative image. In broilers, genetic selection
for growth rate and size of breast meat can lead to paler, less red breast meat
and a large abdominal fat pad (Le Bihan-Duval
et al
., 1999). Theoretically, this could be from
one of three causes. They are:
●
an increase in the PSE condition
●
lower haem pigment content in the meat, associated
with
●
either a lower capillary density and residual
haemoglobin content
●
or a lower myoglobin content
The evidence indicating which of these is most important
is conflicting. When pale broiler and turkey breast fillets were selected at a
processing plant and compared with normal fillets, it was found that the paler
meat had a lower pH
ult
and cooking
yield and produced more drip (Owens
et al
., 2000b). This implies circumstantially that paleness
is commonly due to the PSE condition. In contrast, when broilers were
genetically selected for growth rate and breast meat yield, their pectoralis
major pH
15 min
and pH
ult
were higher than normal
whilst the meat was paler, less red and had a lower iron content (Berri
et al
., 2005). This suggests that
genetic selection has increased paleness through some other mechanism, and
perhaps it is a lower haem pigmentation in the meat.
The situation in turkeys is different. Selection for
high growth rate and breast yield is linked to DFD characteristics instead of
paleness (Le Bihan- Duval
et al
., 2003). Fast-growing turkey genotypes are prone to producing
high pH
60 min
leg meat. This is not due to
pre-slaughter glycogen depletion from their greater weight-bearing (Fernandez
et al
., 2001). In fact,
faster-growing strains have greater glycogen storage capacity in their ilio tibialis
muscles. Breast meat in these lines can be darker than normal, and it is prone
to producing more drip during storage, but the difference in breast meat
quality from traditional lines is small. A more striking feature in fast-growing
turkey lines is the uniformity of their breast meat compared with traditional
strains. For example, there was less variation between birds in the pH
20 min
for the pectoralis major in a
fast-growing line. The implication is that there could be greater uniformity in
quality in the modern fast-growing type, and this could be an advantage in
retail display packs. A low pH
20 min
in
turkey breast muscle is associated with a more acceptable paler and slightly
redder colour when the meat is cured with nitrite (Fernandez
et al
., 2002b), but it also produces
more drip and has a lower processing yield. It is likely to be less preferred
in terms of flavour and texture.
There is limited information on the genetic relationships
between stress-related behaviours and meat quality. In broilers, there are
strain differences in meat pH
15 min
,
which are probably due to differences in the severity of wing flapping when they
are hung on the shackle line at processing plants (Debut
et al
., 2003). Birds from the
strain with faster growth rate flapped their wings less. A nervous temperament in
subtropical cattle breeds (measured from their shorter flight time when
released from a cattle crush) has been linked genetically to tougher meat (see
Table 12.7). This could be due to a greater prevalence of intermediate pH meat.
Table 12.7.
Some examples of genetic
correlations between production performance and meat quality traits.
Other points worth noting are:
●
in cattle, culling for dystocia could lead to indirect
benefits for meat tenderness (Splan
et al
., 1998)
●
in Australian Merino sheep, genetic selection for
wool fineness is leading to darker meat (
r
g
=
0.66;
Fogarty
et
al
., 2003)
●
selection for fast growth in rabbits could be leading
to harder, more chewy and poorer WHC meat (Ramírez
et al
., 2004)
●
selection for growth rate and leanness in ducks has
resulted in slightly darker and redder breast meat, but so far the effect on overall
acceptability has been negligible
●
triploid Atlantic salmon produce larger fibres in their
muscle. This imparts a softer texture to the meat, but it has a higher
proportion of soluble collagen and is more prone to gaping (Bjørnevik
et al
., 2004). In addition, the
meat may be redder, with a low pH
ult