Muscle physiology, meat quality and
protein functionality
- “Technological quality”
= functionality in processing applications
-starts with muscle physiology as modified by
post mortem biochemical changes, structural
changes and enzymatic activity
Muscle cells
- “most highly organized cells in mammals”
-movement, balance, coordination, energy
storage and utilization (aerobic and
anaerobic), protein regeneration…
- cells → fibrils → sarcomeres → filaments
Sarcomere
I band
A band
-estimated 65 proteins as part of the
sarcomere (probably more)
-thick filament → myosin (isoforms)
-thin filament → actin, troponin,
tropomyosin
Myosin
-long filamentous molecule, L:D ratio 100:1(1 in.: 8 ft.)
-~4,500 amino acids, 6 polypeptide chains
(4 light, 2 heavy) , MW ~ 500 kDa
Actin
-globular structure, ~375 amino acids,
MW~45 kDa
Muscle function-ATP
-ATP-ase (myosin)
-calcium
-glucose/glycogen
-oxygen/myoglobin
-lactic acid
Postmortem muscle → meat
1.
2.
3.
4.
Cessation of blood flow
Depletion of energy (ATP)
Elevation of temperature
Aerobic to anaerobic shift (consider
myoglobin/hemoglobin content for residual oxygen)
5. Accumulation of lactic acid/declining pH
6. Loss of membrane function and ion separation
7. Shift in oxidation/reduction potential
= highly significant change in protein properties and
functionality
Aerobic vs. anaerobic energy and end
products
Factors affecting “quality”
1. Genotype
a. halothane gene (PSS)
b. Rendement Napole (Hampshire: high
glycogen, low 24 hr. pH)
c. polygenic effects
2. Diet
a. muscle glycogen manipulation (fasting)
b. dietary fats
c. vitamin E/antioxidants
3. Pre-slaughter handling
4. Stunning (electrical, carbon dioxide, captive bolt)
5. Post-slaughter handling (chilling)
Rate of pH change is critical
1. Normal---0.005 pH unit/min for beef, sheep;
0.01-0.02 pH unit/min for swine
and turkeys
2. PSE pork---2 X (0.02-0.04 pH/min) or more
(0.1 pH/min has been reported)
What is “protein functionality”?
According to Merriam-Webster’s Dictionary…
“functionality” is…
“…performing or being able to perform a
function…concerned with actual use rather
than theoretical possibilities…”
Protein functionality in processed meats is often
termed “technological quality”…
…reflecting the importance of practical
performance of the protein in meat
processing.
Thus, protein functionality in processed meats
may be considered to be the combination of
properties that affects the utilization and
performance of the protein in a finished
product.
Most often, this means the ability…
…to bind and retain fat and water through the
manufacturing process and during storage,
and …to create the desired texture in the
finished product at the point of consumption.
Protein properties of interest for
functionality in processed meats
1. Water binding/moisture retention/product yield
2. Protein solubility and extractability
3. Emulsion/batter forming ability (encapsulation
and binding of fat)
4. Gelation properties (trapping fat and water,
creating texture, inter-surface bonding of meat
pieces)
5. Color?
Proteins are “extremely complex” and
properties are very diverse
Hydrophilic/hydrophobic balance
-dictates interaction with water to determine
solubility and water binding
Interphasic properties
-ability to form films
-critical to forming the emulsion phase of
meat batters
Protein complexity (cont’d)
Intramolecular interactions
-determines tertiary structure
Intermolecular interaction
-protein-protein attraction/binding, etc.
-determines viscosity, elasticity of films,
foaming
Thermal effects
-unfolding, denaturation, gelation
Factors that determine protein
functionality
Intrinsic factors-dependent on protein source
-amino acid composition
-amino acid sequence
-molecular size, shape, conformation, flexibility
Extrinsic-environmental factors
-pH, salts, temperature, redox potential, etc.
-very strong effects
-role of processing technology
Meat proteins review
Sarcoplasmic proteins
-muscle cell cytoplasm
-metabolic enzymes, pigments, many
peptides
-soluble in water/physiological saline
-most are globular in structure with MW
20-60 kDa.
Sarcoplasmic proteins (cont’d)
-relatively high surface charge and polarity
resulting in significant functionality in meat
systems
-role in functionality becomes more
important at low salt concentrations (<1%)
-some, i.e. calpain, may impact functionality by
affecting availability of other proteins like myosin
and actin
Meat proteins review (cont’d)
Myofibrillar proteins
-structural, contractile proteins
-soluble in 2% salt (2.5%-3.0% brine) or more
-myosin (40%-50%); actin (20%-25%) play
overwhelming role in processed meats and
vast majority of functionality research
Myofibrillar proteins (cont’d)
Myosin
-long filamentous molecule, L:D ratio 100:1
(1 in.: 8 ft.)
-~4,500 amino acids, 6 polypeptide chains
(4 light, 2 heavy) , MW ~ 500 kDa
Myosin (cont’d)
-highly “functional” blend of hydrophobic
and hydrophilic amino acids and a tertiary
structure that facilitates interactions with
polar and nonpolar media
-at least 10 isoforms of heavy chains and
several of light chains which in
combination give several distinct isomyosins
Actin
-signficantly less functional than myosin
-probably reduces myosin functionality
somewhat as actomyosin in postmortem
muscle
-at least 4 isoforms in muscle
-vary with species, fiber type, age
-differ in 10 amino acids
Meat proteins review (cont’d)
Titin (a.k.a. Connectin)
-3rd most abundant myofibrillar protein
(~10%)
-largest protein with over 34,000 amino
acids, MW ~ 3.5 MDa
-connects ends of myosin filaments to
Z-lines
Meat proteins review (cont’d)
Other myofibrillar proteins
-5% or less of total protein
-functionality has not be well-studied
-limited even if functionality is high
Meat proteins review (cont’d)
Stromal proteins
-supporting connective tissue between
muscle fibers, bundles and individual muscles
-collagen is most prevalent
Collagen (cont’d)
-tropocollagen-3 polypeptide chains in a
long filamentous molecule, (L:D ratio =
200:1), highly crosslinked, nonpolar amino
acids, nonfunctional
-pretreated collagen can be effective as
gelatin
Specific functional properties
1. Water holding ability
-encompasses inherent and added water
-affects yields, appearance, palatability
-many extrinsic factors (genetics, nutrition, preand post-harvest treatments, chilling, etc.) are
important but fundamental factors are pH and
salt
Water retention in meat
-monomolecular layer of water on protein
charged/polar sites
-2-3 additional layers held by the mono-layer
-~80% of the water is retained in capillary
spaces within and between myofibrillar
proteins, very sensitive to structural change,
manipulated by pH and salt
Effect of NaCl on the isoelectric point and
water binding ability of meat
140
Bound
water
as %
of meat
weight*
120
100
Salted
80
Unsalted
60
40
20
0
3
4
5
6
pH
*maximum at ~1.0 M (5.8%) NaCl
7
8
Cross-sectional diagram of myofibrils
illustrating capillary space effects of
swelling and shrinking
Swelling
Shrinking
Offer and Trinick, 1983. On the mechanism of water holding in
meat: the swelling and shrinking of myofibrils. Meat Sci. 8:245281.
Dependence of Filament Spacing on pH
425
400
Space
between
filaments
375
350
325
5.5
5.75
6
6.25
pH
6.5
6.75
7
Diesbourg et al. 1988. J.
Animal Sci. 66:1048-1054.
Water retention in meat
-mechanical inputs are important, i.e.,
tumbling
-equilibration of salts within tissue
-disruption of some structural constraints
to swelling
-thermal treatments
-temperature, heating rate affect
trapping and retention of water in the
heat-set protein gel
Specific functional properties
2. Solubility and extractability
-not so critical by itself but a “pre-condition”
for other functions particularly emulsion
films, adhesion of meat pieces and gelation
-hydrophilic/hydrophobic balance and protein
conformation allows surface exposure of
charged/polar amino acids
2. Solubility and extractability (cont’d)
-affected by
-intrinsic pH (i.e., PSE pork, pre-rigor
meat), ionic strength (NaCl and
phosphates) and added water
-fiber type and species
-mechanical input (grinding, chopping,
mixing, tumbling)
Specific functional properties
3. Emulsion/batter stability
-requires
-interfacial protein film for raw stability
-protein gelation for cooked stability
-extraction/solubility of “emulsifying”
proteins is critical thus role of pH, salts and
mechanical input
Myosin’s role in interfacial film formation
-likely due to specific hydrophobic region on
heavy meromyosin
Myosin role in interfacial film formation (cont’d)
-fat globules appear to have 3 layers or more
of protein forming the film
-represents some loss of water binding due to
protein “consumption” for this role
-reason for pre-emulsified fat concept
-vacuum is important to protein capacity to
form films
Effects of vacuum on protein performance
in emulsions
Emulsion Capacity
Water extract
Salt extract
Pork
Nonvacuum
Vacuum
% difference
Beef
Nonvacuum
Vacuum
% difference
26.9
43.4
41.3%
30.3
44.7
32.4%
43.6
50.3
13.2%
46.7
51.3
8.8%
Tantikarnjathep et.al. 1983. Use of vacuum during
formation of meat emulsions. J. Food Sci. 48:1039.
Specific functional properties
4. Gelation
-heating results in a series of inter- and intramolecular changes that create a continuous,
interlinked 3-dimensional gel
-critical to finished product properties by
providing trapping/retention of water and
fat, adhesion and mouthfeel of cooked
products
Gelation sequence for myosin
HMM
LMM
S2
S1
Rod
35 to 45oC
Unfolding of
S-1 region
Hydrophobic
head-to head
interaction
45 to 55oC
> 55oC
Unfolding of LMM;
tail to tail
interaction
Final matrix
formation
From Xiong. 2007. Meat Binding: Emulsions and
Batters. Meat Processing Technology Series. AMSA,
Champaign, IL.
4. Gelation (cont’d)
-actin, troponin, tropomyosin and myosin
light chains are at least partially expelled
with water during gel formation
-myosin “availability” is critical thus pH,
salts, etc. that increase myosin are
important
-protein properties determine gel properties,
i.e., PSE pork
Gel strength of normal and PSE pork protein extracts
at equal protein concentrations and with different
heating rates
Gel strength as force, N/cm2
8
HR 17 C/h
7
PSE had
50% or
less gel
strength
than
normal
at 54
mg/ml
HR 39 C/h
6
HR 93 C/h
5
4
3
2
1
0
PSE
Normal PSE Normal PSE Normal PSE
23
34
48
Salt-Soluble Protein Concentration, mg/ml
Normal
54
Camou and Sebranek, 1991. Gelation characteristics of muscle
proteins from pale, soft, exudative (PSE) pork. Meat Sci. 30:207-220.
Water losses from normal and PSE pork protein
extracts at equal protein concentrations and with
different heating rates
HR 17 C/h
70
HR 39 C/h
Percent water loss
60
HR 93 C/h
50
PSE had 13%
greater water
loss and 11%
more expelled
protein than
normal
40
30
20
10
0
Normal
PSE
23
Normal
PSE
34
Normal
PSE
48
Normal
PSE
54
Salt-soluble protein concentration mg/ml
Camou and Sebranek, 1991. Gelation characteristics of muscle
proteins from pale, soft, exudative (PSE) pork. Meat Sci. 30:207220.
4. Gelation (cont’d)
-pH has been reported to affect gel
strength and water binding during
heating above 60°C such that this might
be used to modify or control product
texture and improve consistency
Westphalen et.al. 2005. Influence of pH on rheological
properties of porcine myofibrillar protein during heat
induced gelation. Meat Sci. 70:293-299.
The bottom line…
Much is known, more is needed particularly as
new processing technologies develop, i.e.,
-high pressure processing
-interactions of temperature, pressure,
duration of pressure on protein
gelation
-novel ingredients, i.e., transglutaminase
-application of nontraditional uses, i.e.,
surimi, etc. to red meat/poultry proteins