Food Biochemistry and Food Processing

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14 Biochemistry of Processing Meat and Poultry 321

polypeptides are further hydrolzyed to small pep-
tides by di- and tripeptidylpeptidases; and (3) dipep-
tidases, aminopeptidases, and carboxypeptidases are
the last proteolytic enzymes that act on previous poly-
peptides and peptides to generate free amino acids.
The progress of proteolysis varies depending on the
processing conditions, the type of muscle, and the
amount of endogenous proteolytic enzymes, as des-
cribed below.


PROTEOLYSIS INAGEDMEAT ANDCOOKED
MEATPRODUCTS


During meat ageing, there is proteolysis of impor-
tant myofibrillar proteins such as troponin T by cal-
pains, with the associated release of a characteristic
30 kDa fragment, which is associated with meat ten-
derness; nebulin; desmin; titin; troponin I; myosin
heavy chain; and proteins at the Z-line level (Yates
et al. 1983). A 95 kDa fragment is also characteristi-
cally generated (Koohmaraie 1994). Examples of
cathepsin and aminopeptidase activity during beef
ageing are shown in Figures 14.4 and 14.5, respec-
tively.
The fastest ageing rate is observed in chicken, fol-
lowed by pork, lamb, and beef. For instance, it has
been reported that chicken myofibrils are easily


damaged by cathepsin L, while beef myofibrils are
much more resistant (Mikami et al. 1987). The rea-
sons for this difference are the differences between
species in enzymatic activity, inhibitor content, and
susceptibility to proteolysis of the myofibrillar struc-
ture. These factors are also strongly linked to the
type of muscle metabolism, which has a strong
influence on the ageing rate. In fact, proteolysis is
faster in fast-twitch white fibers (which contract rap-
idly) than in slow-twitch red fibers (which contract
slowly), that is, ageing rate increases with increasing
speed of contraction but decreases with the increas-
ing level of heme iron (Ouali 1991). Even though
the effect of muscle type is estimated to be 10-fold
lower than the effect of temperature, it is three-fold
higher than animal effects (Dransfield 1980–81).
There are also some physical and chemical condi-
tions in postmortem muscle, listed in Tables 14.1 and
14.2, respectively, that can affect enzyme activity. Of
these conditions, the most significant are pH, which
decreases once the animal is slaughtered, and os-
molality, which increases from 300 to around 550
mOsm within 2 days, due to the release of ions to
the cytosol. The osmolality has been observed to be
higher in fast-twitch muscle, which also experiences
a faster ageing rate (Valin and Ouali 1992). Age also
decreases the ageing rate, because collagen content

Figure 14.4.Evolution of muscle cathepsins during the ageing of beef. (Toldrá, unpublished data.)

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