13 Biochemistry of Raw Meat and Poultry 311tidylpeptidase IV at just 2 hours postmortem, has
shown good ability to predict the water-holding
capacity of the meat (Toldrá and Flores 2000).
Modified assay procedures, based on the use of syn-
thetic fluorescent substrates, have been developed to
allow relatively fast and simple measurements of
enzyme activity with enough sensitivity.
Some peptides have been proposed as biochemi-
cal markers for meat tenderness, which is particularly
important in beef. These are peptides with molecular
masses ranging from 1282 to 5712 kDa, generated
from sarcoplasmic and myofibrillar proteins (Stoeva
et al. 2000). The isolation and identification of these
peptides is tedious and time consuming, but once the
full sequence is known, ELISA test kits can be de-
veloped, and this would allow rapid assay and on-
line detection at the slaughterhouse.
In summary, there are many biochemical and
chemical reactions of interest in raw meats that con-
tribute to important changes in meat and affect
its quality. Most of these changes, as have been de-
scribed in this chapter, have an important role in
defining the aptitude of the meat as raw material for
further processing.
GLOSSARY
Ageing—Holding meat at refrigeration temperatures
(0–5°C) for brief period of time (several days or few
weeks) to improve meat tenderness and palatability.
ATP—Adenosine triphosphate, a high energy com-
pound in metabolism.
DFD—Pork meat with dark, firm, and dry characteris-
tics due to a lack of carbohydrates in muscle and
thus poor glycolysis and reduced latic acid genera-
tion. These meats have pH values above 6.0 after 24
hours postmortem and are typical of exhausted
stressed pigs before slaughtering. It is known as
dark-cutting in the case of beef.
Drip loss—This indicates the amount of water lost as
drippings during postmortem treatments. It can be
easily measured by hanging a weighed muscle por-
tion within a sealed plastic bag for 72 hours under
refrigeration. The difference in weights between 0
and 72 hours, expressed as percent, gives the drip loss.
Glycolysis—Enzymatic breakdown of carbohydrates
with the formation of pyruvic acid and lactic acid
and the release of energy in the form of ATP.
Krebs cycle—Also known as citric acid cycle. It con-
sists in a sequence of enzymatic reactions to provide
energy for storage in the form of ATP.
Lactate dehydrogenase—Enzyme that catalyzes the
oxidation of pyruvic acid to lactic acid.
Lipolysis—Enzymatic breakdown of lipids with the
formation of free fatty acids
Lysosome—Cell organelle surrounded by a single
membrane that contains different types of digestion
enzymes with hydrolytic activity.
Mitochondria—Cytoplasmic organelles, surrounded by
a membrane system. Its main function is to recover
energy through the Krebs cycle and the respiratory
chain and convert it by phosphorylation into ATP.
Peroxide value—Term used to measure rancidity and
expressed as millimoles of peroxide taken up by
1000 g of fat.
Proteolysis—Enzymatic breakdown of proteins with
the formation of peptides and free amino acids.
PSE—Meat with pale, soft, and exudative characteris-
tics resulting from an accelerated glycolysis and thus
rapid lactic acid generation. The pH drop is very fast,
reaching values as low as 5.6 in just 1 hour post-
mortem.
Rigor mortis—Stiffening and rigidity of the muscle
after death. It takes a few hours to develop, depend-
ing on the species and temperature.
Skeletal muscle fiber—Elongate, thick-walled with a
characteristic striated or banded pattern; multinu-
cleate.
Water holding capacity—This expresses the capacity
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