Aging/Tenderization Mechanisms 97logical advances during that time frame have
helped uncover new pieces to the puzzle.
Traditionally, most of the research on post-
mortem proteolysis and meat aging was
done using classic SDS - PAGE and western
blotting techniques to document protein deg-
radation in either tissue sampled from aged
intact muscle cuts or from protein extracts
following the in vitro digestion of isolated
myofi brils and muscle proteins. Researchers
are increasingly taking a proteomics approach
to understanding protein changes related to
meat quality by utilizing two - dimensional
electrophoresis (2DE) combined with protein
identifi cation by mass spectrometry (MS).
Rather than just investigating a few proteins
at a time, this powerful tool allows resear-
chers to simultaneously and effi ciently sepa-
rate a wide range of proteins expressed in
muscle tissue and to identify numerous
protein changes that occur in postmortem
muscle.
Lametsch and Bendixen (2001) fi rst dem-
onstrated in porcine muscle the use of pro-
teome analysis to determine postmortem
protein changes. Using 2DE to separate pro-
teins from 5 – 200 kDa with pIs ranging from
pH 4 – 9, 15 signifi cant changes were observed
in the proteome patterns of porcine longis-
simus muscle between slaughter and 48 hours
postmortem. A subsequent study using
matrix - assisted laser desorption/ionization
time - of - fl ight mass spectrometry (MALDI -
TOF MS) to identify proteins demonstrated
that peptide fragments from three structural
proteins (actin, myosin heavy chain, and
troponin - T) and six metabolic proteins (gly-
cogen phosphorylase, creatine kinase, phos-
phopyruvate hydratase, myokinase, pyruvate
kinase, and dihydrolipoamide succinyltrans-
ferease) accumulated in porcine longissimus
muscle between slaughter and 48 hours post-
mortem (Lametsch et al. 2002 ).
Several studies have investigated protein
changes in porcine longissimus muscle
between 0 and 72 hours postmortem
(Lametsch et al. 2003 ; Morzel et al. 2004 ).pletion of rigor (0.24 – 0.30 M) could be high
enough to induce partial dissociation of the
myofi brillar structure and increase proteo-
lytic susceptibility of myofi brillar proteins.
The high ionic strength in postmortem muscle
(0.3 M) was shown to be responsible for
the solubilization of structural proteins
(C - protein, M line protein, troponoin T,
actin, tropomyosin, and α - actinin; Wu and
Smith 1985, 1987 ) and changes in myofi bril-
lar ATPase activity with aging (Ouali 1992 ).
This was further supported by the fact that
the highest osmotic pressure values coin-
cided with the contraction speed of muscles
(i.e., fast - twitch white muscles tenderize
faster than slow - twitch red muscles; Geesink
et. al. 1992 ; Ouali et al. 1991, 1992 ). From
these studies it was concluded that elevated
osmotic pressure, in addition to proteolytic
enzymes, has a physico - chemical impact on
myofi brillar proteins that could be associated
with improvements in tenderness.
Results so far do not support a synergistic
role of elevated ionic strength with proteoly-
sis. The pH/ionic strength conditions in post-
mortem muscle induce conformational
changes in the substrate proteins, conse-
quently altering their susceptibility by ren-
dering specifi c cleavage sites inaccessible to
proteolytic attack. Secondly, an increase in
ionic strength was also shown to inhibit the
activity of μ - and m - calpain (Huff - Lonergan
et al. 1995 ; Geesink and Koohmaraie 2000 ;
Li et al. 2004 ; Maddock et al. 2005 ). Increased
ionic strength, similar to the fall in pH, is an
important variable to examine in determining
the relative contribution of proteolytic
enzymes to postmortem tenderization.
Emerging Use of Proteomic
Approaches to Study Aging
Tenderization
While the basic understanding of the mecha-
nisms that control postmortem proteolysis
and aging tenderization have not changed
substantially over the last decade, techno-