Muscle physiology, meat quality and protein functionality

1. 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

2. Muscle cells • “most highly organized cells in mammals” -movement, balance, coordination, energy storage and utilization (aerobic and anaerobic), protein regeneration… • cells → fibrils → sarcomeres → filaments

5. Sarcomere I band A band -estimated 65 proteins as part of the sarcomere (probably more) -thick filament → myosin (isoforms) -thin filament → actin, troponin, tropomyosin

6. 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

7. Actin -globular structure, ~375 amino acids, MW~45 kDa

8. Muscle function- -ATP -ATP-ase (myosin) -calcium -glucose/glycogen -oxygen/myoglobin -lactic acid

9. Postmortem muscle → meat • Cessation of blood flow • Depletion of energy (ATP) • Elevation of temperature • Aerobic to anaerobic shift (consider myoglobin/hemoglobin content for residual oxygen) • Accumulation of lactic acid/declining pH • Loss of membrane function and ion separation • Shift in oxidation/reduction potential = highly significant change in protein properties and functionality

10. Aerobic vs. anaerobic energy and end products

11. Factors affecting “quality” • Genotype a. halothane gene (PSS) b. RendementNapole (Hampshire: high glycogen, low 24 hr. pH) c. polygenic effects • 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)

12. Rate of pH change is critical • Normal---0.005 pH unit/min for beef, sheep; 0.01-0.02 pH unit/min for swine and turkeys • PSE pork---2 X (0.02-0.04 pH/min) or more (0.1 pH/min has been reported)

13. 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…”

14. Protein functionality in processed meats is often termed “technological quality”… …reflecting the importance of practical performance of the protein in meat processing.

15. 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.

16. 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?

17. 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

18. 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

19. 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

20. Meat proteins review Sarcoplasmicproteins -muscle cell cytoplasm -metabolic enzymes, pigments, many peptides -soluble in water/physiological saline -most are globular in structure with MW 20-60 kDa.

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. Meat proteins review (cont’d) Stromal proteins -supporting connective tissue between muscle fibers, bundles and individual muscles -collagen is most prevalent

29. 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

30. Specific functional properties 1. Water holding ability -encompasses inherent and added water -affects yields, appearance, palatability -many extrinsic factors (genetics, nutrition, pre- and post-harvest treatments, chilling, etc.) are important but fundamental factors are pH and salt

31. 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

32. Effect of NaCl on the isoelectric point and water binding ability of meat *maximum at ~1.0 M (5.8%) NaCl

33. 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. 8:245-281.

34. Dependence of Filament Spacing on pH Space between filaments • Diesbourg et al. 1988. J. Animal Sci. 66:1048-1054. pH

35. 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

36. 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

37. 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)

38. 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

39. Myosin’s role in interfacial film formation -likely due to specific hydrophobic region on heavy meromyosin

40. 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

41. Effects of vacuum on protein performance in emulsions Emulsion Capacity Water extractSalt extract Pork Nonvacuum 26.9 43.6 Vacuum 43.4 50.3 % difference 41.3% 13.2% Beef Nonvacuum 30.3 46.7 Vacuum 44.7 51.3 % difference 32.4% 8.8% Tantikarnjathep et.al. 1983. Use of vacuum during formation of meat emulsions. J. Food Sci. 48:1039.

42. Specific functional properties 4. Gelation -heating results in a series of inter- and intra- molecular 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

43. Gelation sequence for myosin HMM LMM S1 S2 Rod 35 to 45oC 45 to 55oC > 55oC Unfolding of S-1 region Hydrophobic head-to head interaction 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.

44. 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

45. Gel strength of normal and PSE pork protein extracts at equal protein concentrations and with different heating rates PSE had 50% or less gel strength than normal at 54 mg/ml 23 34 48 54 Camou and Sebranek, 1991. Gelation characteristics of muscle proteins from pale, soft, exudative (PSE) pork. Meat Sci. 30:207-220.

46. Water losses from normal and PSE pork protein extracts at equal protein concentrations and with different heating rates PSE had 13% greater water loss and 11% more expelled protein than normal 23 34 48 54 Camou and Sebranek, 1991. Gelation characteristics of muscle proteins from pale, soft, exudative (PSE) pork. Meat Sci. 30:207-220.

47. 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.

48. 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