Aging/Tenderization Mechanisms 91substantially diminishes early postmortem,
and (2) the fact that many of the conclusions
regarding the calpain role in tenderization are
based on indirect evidence. Within the pH
range of 7.4 to 5.8, both μ - calpain and m -
calpain retain enzymatic activity, but as
muscle pH drops more autolysis of μ - calpain
occurs and proteolytic activity diminishes
(Koohmaraie 1992 ). Over the fi rst 24 hours
postmortem, bovine μ - calpain retains < 20%
of its activity when assayed at pH 7.0 and
5 ° C (Koohmaraie 1992 ). Even though
Koohmaraie (1996) demonstrated that μ -
calpain in muscle retains 5% to 10% of its
original activity even after 14 days storage,
the question still remains whether this level
of activity is suffi cient to explain the protein
degradation observed in muscle beyond 24
to 48 hours postmortem. Direct evidence
that μ - calpain is involved in the postmortem
proteolysis associated with tenderization
was strongly provided by two studies in
which postmortem proteolysis of myofi bril-
lar proteins was severely diminished in μ -
calpain knockout mice (Geesink et al. 2006 )
and in mice over - expressing calpastatin
(Kent et al. 2004 ). Despite such strong evi-
dence for the role of μ - calpain, some post-
mortem proteolysis was still detected in both
these studies, suggesting that μ - calpain does
not account for postmortem proteolysis in
its entirety.
Many recent studies on meat tenderness
have confi rmed the importance of the calpain
system but have further indicated that aging
tenderization is a highly complex process
that stretches beyond the explanation pro-
vided by the current calpain theory of post-
mortem tenderization. A growing body of
evidence suggests that multiple enzymes and
interdependent muscle factors may be neces-
sary to fully explain postmortem proteolysis
and its link to tenderization. The remainder
of this chapter will focus on recent novel
fi ndings that contribute to a more complete
understanding of the underlying mechanismsCurrent Enzymatic Model of
Postmortem Proteolysis and
Aging Tenderization
In order to identify enzymes responsible for
the postmortem aging of meat, researchers
have used the criteria that candidate enzyme
systems must: (1) be endogenous to skeletal
muscle and have access to substrates, and (2)
have the ability to degrade the same proteins
that are degraded during the postmortem
storage of muscle (Goll et al. 1983, 2003,
2008 ; Koohmaraie 1996 ). Of the three major
enzyme systems investigated, only calpains
meet both criteria. Calpains have access to
substrates and have been shown to have
limited proteolytic capabilities (Goll et al.
2003 ), cleaving myofi brillar proteins at a
specifi c number of sites to produce large
polypeptide fragments similar to those
observed after the postmortem storage of
muscle. In contrast, lysosomal cathepsins and
proteasomes are capable of exhaustively
degrading proteins into small peptides or
short amino acid segments but cannot disas-
semble the myofi bril and do not generate the
same degradation patterns of myofi brillar
proteins observed during meat aging.
Furthermore, the location of cathepsins in
lysosomes is thought to restrict their access
to substrates. Thus, the prevailing theory is
that the calpain/calpastatin system is the pre-
dominant driver of postmortem proteolysis
and aging tenderization.
While it is widely accepted that proteoly-
sis of key myofi brillar proteins by the calpain
enzyme system is primarily responsible for
increased tenderness during postmortem
storage (Koohmaraie et al. 1991 ; Uytterhaegen
et al. 1994 ; Goll et al. 2003 ; Koohmaraie and
Geesink 2006 ), it can be argued that calpains
alone are not suffi cient to fully explain post-
mortem proteolysis and meat tenderization.
Questions regarding the role of calpains in
postmortem tenderization initially centered
on: (1) the observation that calpain activity