Handbook of Meat Processing

Aging/Tenderization Mechanisms 93

have observed cross - talk between the calpain

and caspase systems during apoptosis, in

which calpain activity is indirectly up - regu-

lated by caspase enzymes cleaving calpastatin

(Wang et al. 1998 ; Porn - Ares et al. 1998 ;

Neumar et al. 2003 ). Calpains have also been

shown to impact caspase activity during

apoptosis (Nakagawa and Yuan 2000 ; Chua

et al. 2000 ; Neumar et al. 2003 ). There is

relatively little data, however, on the interac-

tion between caspases and the calpain enzyme

system in skeletal muscle as it relates to post-

mortem proteolysis. A negative relationship

between peak caspase 3/7 activity at 8 hours

postmortem and calpastatin activity at 0 and

2 days postmortem has been observed in the

muscles of normal lambs but not in callipyge

lambs (Kemp et al. 2009 ). Thus, while there

is no direct evidence that caspases contribute

signifi cantly to postmortem tenderization,

data suggest that they may play an indirect

role by degrading calpastatin. More data is

needed, however, to determine defi nitively

the direct and indirect contribution of the

caspase system to postmortem proteolysis

and meat tenderization.

Heat Shock Proteins

Due to their anti - apoptotic functions in living

tissue, small heat shock proteins (HSP) are

increasingly being investigated as potential

factors infl uencing the conversion of muscle

to meat and meat quality. In living muscle

tissue, HSP such as alpha β - crystallin,

HSP20, and HSP27 have a homeostatic func-

tion in which they stabilize unfolded pro-

teins, help refold denatured proteins, and

prevent protein aggregation (Liu and

Steinacker 2001 ). Due to their abilities to

protect cellular proteins from denaturation

and loss of function, HSP expression is up -

regulated in living tissues in response to

stress. It is speculated that HSP expression

could be stimulated after slaughter in

response to the muscle cell stress and death

during the conversion of muscle to meat and

resulted in the degradation of desmin, tropo-
nin I, actin, troponin T, and myosin light
chains under in vitro conditions similar to
those found in muscle during postmortem
aging (Kemp and Parr 2008 ). One study
investigated the in vivo behavior of the
caspase system by measuring caspase 3/7 and
caspase 9 activities and the degradation of
caspase substrates, alpha II spectrin, and poly
(ADP - ribose) polymerase (PARP) in porcine
longissimus muscle between 0 and 8 days
postmortem (Kemp et al. 2006b ). In this
study, caspases were found to be most active
early postmortem ( < 4 hr), and caspase activ-
ity diminished with postmortem time. It was
also observed that caspase activity (caspase
3/7 and caspase 9) and the abundance of
alpha II spectrin degradation products were
negatively correlated to Warner - Bratzler
shear force measurements. This led to the
conclusion that the changes in caspase
activity and caspase - mediated cleavage of
muscle proteins observed during postmortem
aging may be associated with meat
tenderization.
Other research, however, contends that
caspases are not likely to play a major role in
the postmortem proteolysis associated with
meat tenderization. In beef muscle, it was
observed that caspase 3 activity is present
immediately after slaughter but that it
decreases with time postmortem (Underwood
et al. 2008 ). In this study, pro - caspase 3 was
not activated during postmortem storage and
caspase 3 activity was not correlated with
Warner - Bratzler shear force in beef longis-
simus. The data from one study using muscle
from callipyge and normal lambs indicated
that caspase 3/7 and caspase 9 activities
decreased between 1 and 21 days postmor-
tem but did not directly support or reject the
involvement of the caspase system in meat
tenderization (Kemp et al. 2009 ).
There is some speculation that caspases
may infl uence postmortem proteolysis
through their interaction with the calpain/
calpastatin enzyme system. Numerous studies