94 Chapter 4
Instrinsic Muscle Factors
Infl uencing Postmortem
Proteolysis
In order to fully understand how calpains
contribute to postmortem proteolysis and
aging tenderization, researchers have also
looked at postmortem characteristics of
muscle that could infl uence either calpain
activation or the effi cacy of calpain - mediated
proteolysis. Differences in muscle factors
such as protein oxidation levels and sarco-
mere length have been investigated for their
potential to account for some of the variabil-
ity observed in the rate and extent of post-
mortem myofi brillar protein degradation.Effect of Oxidation on
Calpain - Mediated Proteolysis
There is increasing evidence that postmortem
proteolysis and aging tenderization are infl u-
enced by dynamic changes that occur in the
microenvironment of the muscle cells during
the conversion of muscle to meat. In addition
to a decline in pH and an increase in ionic
strength, there is a rise in the formation of
reactive oxygen species and an increase in
protein oxidation within postmortem muscle
(Martinaud et al. 1997 ; Harris et al. 2001 ;
Rowe et al. 2004a, b ). Using vitamin E sup-
plementation and irradiation to generate a
range of oxidation levels in beef longissimus
muscle, researchers have found that increased
oxidation of muscle proteins early postmor-
tem ( < 24 h) negatively impacts meat tender-
ness (Rowe et al. 2004a, b ). Similar to the
reversible inactivation of calpain that occurs
in vivo (Guttmann and Johnson 1998 ), oxida-
tive conditions in postmortem muscle can
diminish calpain activity and reduce myofi -
brillar proteolysis (Rowe et al. 2004a, b ;
Maddock et al. 2006 ) and limit tenderization
(Rowe et al. 2004a, b ). Since both μ - and
m - calpain have a cysteine residue at their
active sites and require reducing conditionsthat they may infl uence postmortem prote-
olysis (Ouali et al. 2006 ). Several recent
studies have demonstrated that HSP are up -
regulated in postmortem muscle (van Laack
et al. 1993 ; Bouley et al. 2004 ; Hwang et al.
2005 ; Jia et al. 2006a, b ; Sayd et al. 2006 ; Jin
et al. 2006 ). In a study comparing microar-
rays between high and low quality meat
groups from beef longissimus muscles, the
down - regulation of alpha β - crystallin and
HSP27 in muscle samples taken at 10 minutes
postmortem was associated with improved
tenderness, juiciness, and fl avor (Bernard et
al. 2007 ). In this study, the expression of
DNAJA1, which encodes for a member of
the large 40 kDa heat shock protein family,
was negatively correlated to tenderness mea-
surements after 14 days of aging and alone
explained 63% of the variability in sensory
assessed tenderness (Bernard et al. 2007 ).
From this it was suggested that the anti - apop-
totic activity of this gene could slow cellular
death during the conversion of muscle to
meat and lower meat tenderization. HSP27
content in fresh beef muscle and levels of
HSP27 fragments in 14 - day aged beef were
found to explain up to 91% of the variation
in sensory tenderness scores (Morzel et al.
2008 ). In pre - rigor beef, HSP20 and alpha
β - crystallin levels peak at 0.5 and 3 hours
postmortem and then decline until 22 hours
postmortem (Pulford et al. 2008 ). HSP
content postmortem is also infl uenced by
postmortem muscle pH (Pulford et al. 2008 ).
Similarly, high levels of alpha β - crystallin at
22 hours postmortem are associated with
diminished protein degradation in beef
muscle with low ultimate pH, suggesting that
HSP may shield the muscle structure from
proteolytic degradation during aging (Pulford
et al. 2009 ). Further data are needed, however,
to determine if HSP levels in postmortem
muscle are merely indicators of postmortem
proteolysis and meat tenderization or if
they play a mechanistic role in the aging
process.