87Chapter 4
Aging/Tenderization Mechanisms
Brian C. Bowker , Janet S. Eastridge , Ernie W. Paroczay , Janice A. Callahan , and
Morse B. Solomon
Introduction
Humans (both men and women alike) con-
tinue to look for ways to slow down, control,
or virtually eliminate the aging process,
often looking for the infamous “ fountain of
youth. ” Scientists (Strehler 1986 ) from the
Molecular Biology Division at the University
of California, Los Angeles, have demon-
strated that gene loss is the primary cause of
the human aging process. On the contrary,
the “ aging ” of meat is viewed positively, and
the practice of storing meat for extended
periods of time after the death of an animal,
in order to improve its texture, has been a
practice followed by many for a very long
time.
Most consumers do not understand the
basic notion that all meat — red, white, or
pink — is muscle. Its origin is muscle, which
converts over time into meat, and this con-
version is by no means instantaneous. The
widely held view that postmortem storage
of meat at refrigerated temperatures (referred
to as aging) results in a signifi cant improve-
ment in meat tenderness has been adhered to
for a very long time, and aging still remains
an important method used for producing
tender meat. The fi rst scientifi c reports
looking at postmortem tenderization of meat
were those of Bouley (1874) and Lehman
(1907) , who reported that there was an
increase in meat tenderness during extended
postmortem storage. Hoagland et al. (1917)
provided indication of protein breakdown by
showing that there was an increase in nonco-
agulable nitrogen during the storage of meat.
Postmortem “ aging ” has been called “ condi-
tioning ” or “ ripening ” and is a natural process
when meat is subjected to controlled refriger-
ated storage conditions. While meat from any
species could be aged, postmortem aging is
generally limited to beef, due to the relative
youth of pork, lamb, and veal at the time of
slaughter. Muscle from poultry carcasses
also undergoes aging, but the time course for
aging in poultry muscle is hours and minutes
as compared with days for beef.
Tenderness development in postmortem
skeletal muscle is a very complex phenom-
enon. Immediately post - exsanguination,
muscle is soft and tender until the onset of
rigor mortis causes irreversible linkages
between the actin and myosin proteins, which
cause the meat to toughen. With aging,
however, protein degradation and ultrastruc-
tural changes occur that cause tenderization
of the meat. During this tenderization phase,
degradation of myofi brillar and cytoskeletal
proteins such as troponin, desmin, vinculin,
dystrophin, nebulin, and titin occurs within
the muscle (Taylor et al. 1995 ; Robson et al.
1997 ). As these proteins degrade, muscle
ultrastructure changes as: (1) breaks at the
junction of the I - band and Z - disk occur, (2)
Z - to Z - line attachments are disrupted by the
degradation of intermediate fi laments, and
(3) Z - and M - line attachments to the sarco-
lemma are disrupted. Ultimately, the ultra-
structural changes that occur within muscle
during aging are thought to be the result of
enzymatic degradation of myofi brillar and