Levels of complexity C, H, O, N. Amino acids. Actin Myosin. Thin filaments Thick filaments. Myofibrils

Transcription

1 Meat proteins

2 Levels of complexity C, H, O, N Amino acids Actin Myosin Thin filaments Thick filaments Myofibrils

3 Levels of complexity Myofibrils Muscle fiber Primary bundle Secondary bundle Muscle Changes happen Meat

4 Meat composition Water ~75% Protein ~18% Lipid ~3% Non-protein nitrogen ~1.6%

5 Meat composition Carbohydrate ~1.2% Minerals ~0.7% Traces of vitamins, etc.

6 Muscle cells Muscle cells are unique in that they allow the conversion of chemical energy in the form of ATP (high energy phosphate bonds) into mechanical energy and, hence, the ability to do work The basic unit of muscle tissue is the muscle cell ( m x several centimeters long)

7 Muscle structure Image courtesy of M. W. King, Ind. State (web.indstate.edu:80/ thcme/mwking/muscle.html #sarcomere)

8 Sarcolemma Each muscle fiber is surrounded by a thin membrane called the sarcolemma Motor nerve endings terminate on the sarcolemma This is how the signal gets to the muscle cell that it is supposed to contract

9 Sarcoplasm Inside the muscle fiber are the myofibrils These are suspended in a fluid called the sarcoplasm There are about 2000 myofibrils per muscle fiber

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11 Muscle fiber divisions Image courtesy of savell-j.tamu.edu/structure.html.

12 Endomysium Endomysium Image courtesy of

13 Myofibrils Some protein in the myofibril forms the thick filaments Other proteins comprise the thin filaments The particular arrangement of thick and thin filaments in the myofilament produces a distinct light/dark repeating pattern in electron microscopy

14 Muscle--electron micrograph Image courtesy of ortho84-13.ucsd.edu/musintro/fibril.html

15 Another micrograph Image courtesy of

16 Thick filaments The A band is principally thick filament protein This is almost entirely myosin Myosin comprises about 45% of muscle protein MW of myosin = 470, ,000 There are about 400 myosin molecules per thick filament

17 Myosin ATPase activity Image courtesy of M. W. King, Ind. State (web.indstate.edu:80/ thcme/mwking/muscle.html#sarcomere)

18 Thin filaments The I band is almost completely thin filament protein This protein is called actin, and it constitutes about 20-25% of muscle protein The actin comes in two forms, G-actin and F-actin

19 Thin and thick filaments Image courtesy of #aging.

20 Actin G-actin Globular 42,000-48,000 MW F-actin Fibrous 92% alpha helix

21 Fibrous actin Each sphere represents a globular actin monomer Image courtesy of aggregs.html

22 Tropomyosin Tropomyosin 2 stranded alpha helix About 5% of muscle protein 33,000-37,000 MW Length 400 Å Lies in actin double helix groove Each tropomyosin interacts with seven G- actin monomers

23 Troponin Troponin C 17,000-18,000 MW Contains many acidic amino acids, responsible for calcium ion binding Troponin I 20,000-24,000 MW Strongly inhibits ATPase activity of actomyosin

24 Troponin Troponin T 37,000-40,000 MW Provides a strong association site for binding troponin to tropomyosin

25 Thin filaments Image courtesy of aggregs.html

26 Contraction of skeletal muscle Image courtesy of #aging.

27 Muscle contraction animation Image courtesy of PPS/course/ 10_interactions/ a_m_contract.gif

28 Role of creatine and creatine kinase in relaxation ADP + Creatine phosphate Creatine kinase ATP + Creatine

29 Connective tissue Binds muscle fibers in bundles to form muscle Amount and kind of connective tissue determines tenderness in meat Reticulin Small structural fibers around cells, blood vessels, and neural structures

30 Connective tissue Elastin Gristle Collagen Principal connective tissue protein. Built up from tropocollagen units. The more collagen in a muscle tissue, the tougher it is. Is converted into (tender) gelatin by treatment with moist heat.

31 False colored SEM of connective tissue-individual collagen fibers can be seen London Research Institute

32 Adipose tissue Adipocytes Fat cells Surface fat Subcutaneous Intramuscular fat Marbling Related to tenderness and flavor in high grades of meat

33 Marbling Prime Choice Choice Select

34 Heavily marbled Kobe beef

35 Post-mortem changes in muscle On the death of the animal, a series of complex biochemical reactions occur that bring about the conversion of muscle into meat As the animal is bled at slaughter, the aerobic pathway for energy metabolism can no longer function

36 Post-mortem changes in muscle For a time, an anaerobic pathway takes over This results in the conversion of D-glucose into lactic acid, which builds up in the muscle and causes a drop in tissue ph This ph drop affects meat quality O 2

37 Post-mortem changes in muscle For a time, an anaerobic pathway takes over This results in the conversion of D-glucose into lactic acid, which builds up in the muscle and causes a drop in tissue ph This ph drop affects meat quality O 2 No O 2

38 Effect of ph on meat quality If ph decreases too rapidly Muscle proteins will denature and cause a loss of water holding capacity A pale myoglobin color If ph remains too high Meat will be too dark in color Meat surface will be dry

39 ph decline in meats DFD = dark, firm, and dry PSE = pale, soft, and exudative Image courtesy of savell-j.tamu.edu/conversion.html

40 Effect of ph on water holding capacity IEP ~ 5.2 Image courtesy of savell-j.tamu.edu/conversion.html

41 Effect of ph on meat color (ph too high) An example of dark cutting beef or lamb (DFD meat) Image courtesy of savell-j.tamu.edu/conversion.html

42 Effect of ph on meat color (ph too low) An example of pale, soft and exudative chicken breast (PSE meat) OK ph too low

43 Generation of meat flavor potentiators during aging As meat ages ATP is converted into ADP, then AMP, and finally in IMP (inosine monophosphate) and ammonia These molecules provide some of the characteristic flavors of cooked meat

44 Rigor mortis and aging Rigor is a stiffening of the carcass a short time post mortem As muscle ATP is depleted and cannot be replaced, actin and myosin react to form the contracted muscle protein actomyosin

45 Rigor mortis and aging Accompanying rigor are Loss of muscle elasticity and extensibility Increase in muscle tension Sarcomere shortening Note that meat cooked in rigor tends to be quite tough

46 Effect of rigor on meat tenderness

47 Rigor mortis and aging Aging is the process of simply holding the meat so as to allow for resolution of rigor (muscle relaxation) Alterations in the structure of the myofibrils is noted during this process

48 Rigor mortis and aging The microscopic appearance of the sarcomere changes Some actin filaments become dissociated from the Z line The Z line itself becomes less distinct Sarcomere length increases during aging Muscle protein increases its water holding capacity