96 Chapter 4
animal species, evidence was presented that
the rate of free calcium increase paralleled
the respective tenderizing rates for different
species in the order of chicken (fastest),
rabbit, pork, and beef (slowest) muscles
(Yamanoue et al. 1994 ; Ji and Takahashi
2006 ). A prevalent argument that proteolysis
could not be a factor is that meat during aging
is under nonphysiological conditions, so the
activity levels of the highly pH and tempera-
ture dependent proteolytic systems are too
low or inactive (Kanawa et al. 2002 ) in a
post mortem cellular environment (ultimate
pH 5.5 – 5.8; 2 ° – 5 ° C temperature) to elicit the
postmortem changes observed. One concern
regarding all these studies is the absence of
objective measurements of tenderness to
further support the calcium theory of tender-
ization. One report (Geesink et al. 2001 )
observed a rise in sarcoplasmic Ca 2+ in post-
mortem muscle and correlated it to the myo-
fi brillar fragmentation index (MFI) and shear
force, which seems to support the calcium
theory, but provided alternative interpreta-
tions of these results that contradicted the
calcium theory of tenderization.Osmotic Pressure
One of the most extensively investigated
factors during the development of rigor
mortis is the postmortem fall in pH. The
intracellular osmotic pressure (i.e., ionic
strength) increases nearly twofold and has a
close relationship with pH (r = 0.97) during
the time course of rigor mortis (Ouali 1990 ),
yet it has received comparatively little atten-
tion in meat research studies. It was sug-
gested that the pH drop was likely the major
cause for the large increase in osmotic pres-
sure through alteration of proteins to which
ions (mainly Na + , K + , Ca 2+ , and Mg 2+ ) are
normally bound (Ouali et al. 1991 ). In
general, salt concentrations above physiolog-
ical values ( ∼ 0.15 M) raise myofi brillar
protein solubility; consequently, it was pos-
tulated the ionic strength attained at the com-strated that a minimal amount of proteolysis
occurs during the fi rst 3 days of aging, yet
the largest changes ( ∼ 65%) in postmortem
tenderization occur during the fi rst 3 days
postmortem (Wheeler and Koohmaraie 1994 ;
Taylor et al. 1995 ). Although much less
investigated, some mechanisms not related to
proteolysis appear to contribute to postmor-
tem tenderization.
Calcium Theory of Tenderization
The rise in free sarcoplasmic Ca 2+ from
10 −^4 mM in living skeletal muscle to 0.2 mM
in postmortem muscle has been hypothesized
to be responsible for postmortem tenderiza-
tion, regardless of proteolysis (Takahashi
1992, 1996, 1999 ). The calcium theory of
meat tenderization is based on evidence that
all structural weakening of myofi brils and
rigor linkages, which contain molecular con-
stituents with an affi nity for binding with
Ca 2+ , are fully induced when the concentra-
tion of free Ca 2+ increases to more than
0.1 mM (Takahashi 1992, 1996, 1999 ). This
concept, however, has not received wide-
spread acceptance. Based on reports over the
last four decades, the mechanism underlying
the weakening of myofi brils (Takahashi et al.
1967 ; Hattori and Takahashi 1979 ) has been
related to the liberation of phospholipids
from Z - disks (Ahn et al. 2003 ), and the frag-
mentation of cytoskeletal structure proteins
titin (Tatsumi et al. 1999 ), nebulin (Tatsumi
and Takahashi 1992, 2003 ), and desmin
(Takahashi 1996 ) through direct binding
reactions with free Ca 2+. The second key
element, weakening of rigor linkages, was
attributed to the translocation of paratropo-
myosin from the A - I junction region onto
thin fi lament actin (Hattori and Takahashi
1988 ; Takahashi et al. 1995 ; Fei et al. 1999 ).
All these ultrastructural changes were dem-
onstrated specifi cally by 0.1 mM Ca 2+ ion
treatments in vitro, in muscles from beef,
pork, chicken, and rabbits. With regard to the
different speeds of meat tenderization among