Thermal Processing 179cluded that the change in volume was mostly
due to moisture loss. Expulsion of water from
the myofi ber was slow and incomplete from
40 ° C to 52.5 ° C but accelerated markedly to
maximal rate between 57.5 ° C and 60 ° C. The
acceleration of moisture lost was attributed
to collagen shrinkage.
Changes in connective tissue are also seen
when meat is cooked. Welke et al. (1986)
observed increased weights of epimysial con-
nective tissue after cooking, indicating
hydration and hydrolysis of the collagen.
Martens et al (1982) reported collagen dena-
tured between 53 – 63 °. The denaturation of
collagen involved the breaking up of the
fi brous structure, probably fi rst by the break-
age of hydrogen bonds. If collagen is not
stabilized by intermolecular bonds, it will
dissolve and form gelatin on further heating
(Tornberg 2005 ), especially when meat is
cooked with moisture.Surface Drying
Reduction of moisture at the surface of meat
and meat products serves several purposes.
Lowering surface moisture reduces the water
activity on the surface and thus reduces
microbial growth. The reduced surface mois-
ture plays a key role in preventing not only
the growth of surviving bacteria, but also the
growth of any bacteria that may recontami-
nate the surface of the product.
Surface drying during cooking is also
responsible for skin formation in produc -
tion of hams and other similar products.
Coagulation of the surface proteins results in
the formation of an outer layer that serves as
a “ skin ” when the casings are removed. The
skin formed during cooking is a function of
the temperature the product reached during
cooking and the time it was held at that tem-
perature. The nature of the skin is most
important for peelability or removal of casing
or netting. Drying of the surface also aids in
giving the skin a dense texture and imparts
the characteristic appearance of skinlessest between 40 ° C and 60 ° C, with the proteins
being essentially insoluble above 60 ° C
(Hamm and Deatherage 1960 ; Lyon et al.
1986 ). Hamm and Deatherage (1960)
reported denaturation occurred in different
steps. The fi rst reaction was the unfolding of
the tertiary structure of the protein. The
second was the aggregation of protein chains,
resulting in the coagulation of proteins. The
initial changes are confi ned to the surface,
but as time and temperature increase, the
action penetrates further into the interior of
the meat.
Changes in the muscle structure are seen
during cooking. Leander et al. (1980) reported
slight disfi gurement of the myofi brils after
cooking to an internal temperature of 63 ° C,
with some evidence of induced swelling of
the perimysial connective tissue. Increased
temperatures to 68 ° C resulted in more swell-
ing in the A - band due to thermally induced
contraction of the sarcomeres. Muscle fi bers
remained intact, while connective tissue
sheaths underwent coagulation and appeared
granular. These researchers reported the
greatest effects were observed in samples
heated to 73 ° C. Sarcomeres exhibited ther-
mally induced contraction and breakage at
the Z - line, while some transverse lines
remained intact. Coagulation of the sarco-
lemma and exposure of myofi brils were also
observed. Increased loss of sarcomeric struc-
ture was observed, with increased fi nal
internal temperature. Hearne et al. (1978)
also observed greater fi ber disintegration,
with increased fi nal internal temperature.
Furthermore, these researchers found faster
cooking rates to result in greater fi ber disin-
tegration compared with slow rates of
cooking to an endpoint of 60 ° C.
Bendall and Restall (1983) reported no
change in sarcomere length when fi bers were
heated, but diameter of fi bers changed mark-
edly. Myofi bers heated in aqueous medium
to fi nal temperatures of 40 ° C to 90 ° C resulted
in a decrease in diameter of myofi bers but
no change in length. These researchers con-