Handbook of Meat Processing

Aging/Tenderization Mechanisms 95

uted to differences in the extent of troponin - T

proteolysis in excised beef semitendinosus

muscle from 2 to 10 days postmortem

(Weaver et al. 2008 ). Using an in vitro model,

in which myofi brils with varying sarcomere

lengths were digested with exogenous μ -

calpain, it was observed that sarcomere

length infl uenced the rate and extent of tro-

ponin - T degradation (Weaver et al. 2009 ). In

both of these studies, less proteolysis was

observed in samples with shorter sarcomeres.

The limited protein degradation with shorter

sarcomeres was hypothesized to be a func-

tion of limited substrate availability (Weaver

et al. 2008, 2009 ), but more data is required

to elucidate this mechanism. It is also unclear

from these studies if the range in sarcomere

lengths that resulted in proteolytic differ-

ences is indicative of the natural variation in

sarcomere length that exists in muscles from

commercially processed beef. Nevertheless,

these studies indicate that besides having a

direct effect on tenderness, large differences

in sarcomere length may also impact the

postmortem proteolysis responsible for the

aging tenderization of meat.

Nonenzymatic Mechanisms of

Aging Tenderization

Since there is nearly universal agreement that

aging reduces meat toughness in all but sar-

comere - shortened muscle, and an over-

whelming number of studies have shown that

a decrease in toughness of meat is accompa-

nied by a corresponding increase in protein

degradation and protein solubility as meat

ages, most research efforts have focused

solely on the endogenous proteolytic enzymes

as the primary mechanism regulating tender-

ization. The accumulated evidence on pro-

teolytic systems, however, has numerous

contradictions and more importantly, does

not fully explain the large variation in meat

tenderness or great differences in tenderiza-

tion rates among species. It has been demon-

for activity, it is not surprising that oxidizing
conditions inhibit proteolysis by μ - calpain
(Guttmann et al. 1997 ). It has also been dem-
onstrated that oxidation alters calpain activity
and the inhibition of calpains by calpastatin
differently, depending on the pH and ionic
strength conditions of the muscle (Maddock
et al. 2006 ). Overall, these studies indicate
that oxidative conditions in postmortem
muscle may infl uence the postmortem prote-
olysis associated with the aging tenderization
of meat.

Effect of Muscle Shortening on

Calpain - Mediated Proteolysis

The direct impact of muscle shortening and
sarcomere length on meat tenderness has
been well - established and it is thought that
sarcomere shortening dictates tenderness
early postmortem ( < 24 h) while variations in
proteolysis control differences in tenderness
during aging (Wheeler and Koohmaraie
1994 ). Numerous studies have demonstrated
that muscles with longer sarcomeres have
lower resistance to shearing than those
with shorter sarcomeres (Herring et al. 1965 ;
Herring et al. 1967 ; Hostetler et al. 1972 ;
Marsh and Carse 1974 ; Smulders et al.
1990 ; Wang et al. 1994 ). Past data has
reported that sarcomere length does not
impact postmortem proteolysis and that the
negative impact of short sarcomeres on ten-
derness is solely due to the increased overlap
of the thick and thin fi laments (Young et al.
1980 ; Locker and Wild 1982 ; Jaime et al.
1992 ; Wheeler and Koohmaraie 1999 ). More
recently, however, several studies have dem-
onstrated that there may be some interaction
between sarcomere length and the degrada-
tion of myofi brillar proteins (Weaver et al.
2008, 2009 ). Using a muscle stretching
model to generate muscle samples with a
wider range in sarcomere lengths than have
been used in previous investigations, one
study found that sarcomere length contrib-