Graduate Thesis Or Dissertation
 

Gel electrophoretic analysis of the protein changes in ground beef stored at 2⁰C

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  • 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 conditions. 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 samples. 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 changes. 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.
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