- A study was completed to determine the extent of the
protein changes occurring in ground beef stored at
2°C for 10 days.
Sections of semitendinosus muscle were obtained
immediately after the slaughter of three beef animals.
Each section was divided into two equal portions, one of
which was ground and the other remained intact (control).
All samples were handled and stored under aseptic
Grinding markedly accelerated glycolysis as
manifested by the rapid pH decline in the ground samples
during the initial 24 hours of postmortem storage. After
this storage interval, however, there was little
difference in pH values between the ground and intact
Sodium dodecyl sulfate-polyacrylamide gel
electrophoresis (SDS-PAGE) was employed to monitor
changes in myofibrillar and sarcoplasmic proteins
extracted from at-death muscles and samples stored for 1,
3, 6, and 10 days at 2°C. The gels were examined
visually and scanned densitometrically to detect protein
The principal electrophoretic changes in
myofibrillar proteins of the ground samples were the
gradual disappearance of nebulin and desmin components
and the gradual appearance of 110,000-, 95,000-, and
30,000-dalton polypeptides. In addition, there was a
progressive increase in the content of a protein around
55,000 daltons and myosin light chain-3. Intact muscles
showed similar changes to those of the ground samples
except that the latter had a faster initial rate in some
of the changes, notably the disappearance of nebulin and
the appearance of the 30,000-dalton polypeptide. It seems
probable that grinding caused an early release of
Ca⁺⁺ from the sarcoplasmic reticulum, which
activated the Ca⁺⁺-activated proteinase (CAF).
Electrophoretic changes in sarcoplasmic proteins of
the ground samples closely resembled those of the intact
muscles. The major alterations in both muscle treatments
included the gradual appearance of a 100,000-dalton
polypeptide and three proteins having molecular weights (M.W.) between 500,000 and 1,000,000 daltons, and the
progressive disappearance of 300,000- and 24,000-dalton
proteins. The appearance of a 100,000-dalton polypeptide
and the three large M.W. proteins presumably originated
from myofibrils since they appeared to be related to the
changes in myofibrillar proteins.
Results of microbial testing indicated very little,
if any, sample contamination by psychrotrophic
microorganisms. Thus, microbial proteolysis was not a
factor in this study.
It was concluded that grinding had no pronounced
effect on the protein changes of beef muscle other than
changes in pH.