Meat proteins

Levels of complexity

C, H, O, N

Amino acids

Actin Myosin

Thin filaments Thick filaments

Myofibrils

Levels of complexity

Muscle fiber

Primary bundle

Secondary bundle

Muscle

MeatChanges happen

Myofibrils

Meat composition

• Water– ~75%

• Protein– ~18%

• Lipid– ~3%

• Non-protein nitrogen– ~1.6%

Meat composition

• Carbohydrate– ~1.2%

• Minerals– ~0.7%

• Traces of vitamins, etc.

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 (10-100 m x several centimeters long)

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

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

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

Muscle fiber divisions

Image courtesy of savell-j.tamu.edu/structure.html.

Endomysium

Image courtesy of www.aps.uoguelph.ca/swatland/html/ch5.0.html.

Endomysium

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

Muscle--electron micrograph

Image courtesy of ortho84-13.ucsd.edu/MusIntro/Fibril.html

Another micrograph

Image courtesy of www.aps.uoguelph.ca/swatland/html/ch5.0.html

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 = 470,000-480,000• There are about 400 myosin molecules

per thick filament

Myosin

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

ATPaseactivity

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

Thin and thick filaments

Image courtesy of www.aps.uoguelph.ca/swatland/html/ch5.1.html#aging.

Actin

• G-actin– Globular– 42,000-48,000 MW

• F-actin– Fibrous– 92% alpha helix

Fibrous actin

Image courtesy of www.pdb.bnl.gov/PPS/course/9_quaternary/aggregs.html

Each sphere represents a globular actin monomer

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

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

Troponin

• Troponin T– 37,000-40,000 MW– Provides a strong association site for binding

troponin to tropomyosin

Thin filaments

Image courtesy of www.pdb.bnl.gov/PPS/course/9_quaternary/aggregs.html

Contraction of skeletal muscle

Image courtesy of www.aps.uoguelph.ca/swatland/html/ch5.1.html#aging.

Image courtesy ofwww.pdb.bnl.gov/PPS/course/10_interactions/a_m_contract.gif

Muscle contraction animation

Role of creatine and creatine kinase in relaxation

ADP + Creatine phosphate

ATP + Creatine

Creatine kinase

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

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.

False colored SEM of connective tissue-individual collagen fibers can be seen

London Research Institute

Adipose tissue

• Adipocytes– Fat cells

• Surface fat– Subcutaneous

• Intramuscular fat– Marbling

• Related to tenderness and flavor in high grades of meat

Marbling•Prime •Choice

•Choice •Select

Heavily marbled Kobe beef

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

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

O2

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

O2 No O2

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

pH decline in meats

Image courtesy of savell-j.tamu.edu/conversion.html

DFD = dark,firm,

and dry

PSE = pale,soft, andexudative

Effect of pH on water holding capacity

Image courtesy of savell-j.tamu.edu/conversion.html

IEP ~ 5.2

Effect of pH on meat color (pH too high)

Image courtesy of savell-j.tamu.edu/conversion.html

An exampleof darkcutting beefor lamb (DFD meat)

Effect of pH on meat color (pH too low)

An exampleof pale, soft and exudative chicken breast(PSE meat)

•OK •pH too low

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

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

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

Effect of rigor on meat tenderness

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

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