P1: SFK/UKS BLBS102-c15 P2: SFK BLBS102-Simpson March 21, 2012 13:21 Trim: 276mm X 219mm Printer Name: Yet to Come 15 Biochemistry of Raw Meat and Poultry Basset O et al 2000 Application of texture image analysis for the classification of bovine meat Food Chem 69: 437–445 Batlle N et al 2000 Early postmortem detection of exudative pork meat based on nucleotide content J Food Sci 65: 413–416 Batlle N et al 2001 ATP metabolites during aging of exudative and non exudative pork meats J Food Sci 66: 68–71 Belury, MA 2002 Dietary conjugated linoleic acid in health: physiological effects and mechanisms of action Ann Rev Nutr 22: 505–531 Brewer S 2010 Technological quality of meat for processing In: F Toldr´a (ed.) Handbook of Meat Processing Wiley-Blackwell, Ames, IA, pp 25–42 Buckely DJ et al 1995 Influence of dietary vitamin E on the oxidative stability and quality of pig meat J Anim Sci 73: 3122–3130 Byrne CE et al 1998 Non-destructive prediction of selected quality attributes of beef by near infrared reflectance spectroscopy between 750 and 1098 nm Meat Sci 49: 399–409 Cassens RG 2000 Historical perspectives and current aspects of pork meat quality in the USA Food Chem 69: 357–363 Castro-Gir´aldez M et al 2009 Physical sensors and techniques In: LML Nollet, F Toldr´a (eds.) Handbook of Processed Meats and Poultry Analysis CRC Press, Boca Raton, FL, pp 7–34 Castro-Gir´aldez M et al 2010a Physical sensors for quality control during processing In: F Toldr´a (ed.) Handbook of Meat Processing Wiley-Blackwell, Ames, IA, pp 443–456 Castro-Gir´aldez M et al 2010b Low-frequency dielectric spectrum to determine pork meat quality Innov Food Sci Emerging Technol 11: 376–386 Chan KM, Decker EA 1994 Endogenous skeletal muscle antioxidants Crit Rev Food Sci Nutr 34: 403–426 Cheah KS et al 1995 Effect of dietary supplementation of vitamin E on pig meat quality Meat Sci 39: 255–264 Dirinck P et al 1996 Studies on vitamin E and meat quality: Effect of feeding high vitamin E levels on time-related pork quality J Agric Food Chem 44: 65–68 Enser M et al 1998 Fatty acid content and composition of UK beef and lamb muscle in relation to production system and implications for human nutrition Meat Sci 49: 329–341 Enser M et al 2000 Feeding linseed to increase the n-3 PUFA of pork: fatty acid composition of muscle, adipose tissue, liver and sausages Meat Sci 55: 201–212 Escudero E et al 2010a Angiotensin I converting enzyme inhibitory peptides generated from in vitro gastrointestinal digestion of pork meat J Agric Food Chem 58: 2895–2901 Escudero E et al 2010b Characterization of Peptides Released by in vitro Digestion of Pork Meat J Agric Food Chem in press Eskin NAM 1990 Biochemical changes in raw foods: meat and fish In: Biochemistry of Foods, 2nd edn Academic Press, San Diego, pp 3–68 Faustman LC 1994 Postmortem changes in muscle foods In: DM Kinsman, AW Kotula, BC Breidenstein (eds.) Muscle Foods Chapman and Hall, New York, pp 63–78 Fern´andez X et al 1995 Effect of muscle type and food deprivation for 24 hours on the composition of the lipid fraction in muscles of large white pigs Meat Sci 41: 335–343 Flores J, Toldra F 1993 Curing: processes and applications In: R MacCrae et al (eds.) Encyclopedia of Food Science, Food Technology and Nutrition Academic Press, London, pp 1277–1282 301 Gianelli MP et al 2000 Effect of carnosine, anserine and other endogenous skeletal peptides on the activity of porcine muscle alanyl and arginyl aminopeptidases J Food Biochem 24: 69–78 Got F et al 1999 Effects of high intensity high frequency ultrasound on aging rate, ultrastructure and some physico-chemical properties of beef Meat Sci 51: 35–42 Greaser ML 1986 Conversion of muscle to meat In: PJ Bechtel (ed.) Muscle as Food Academic Press, Orlando, pp 37–102 Greaser ML 2001 Postmortem muscle chemistry In: YH Hui, et al (eds.) Meat Science and Applications Marcel Dekker, New York, pp 21–37 Grunert KG 1997 What’s in a steak? A cross-cultural study on the quality perception of beef Food Qual Pref 8: 157–174 Hern´andez P et al 1998 Lipid composition and lipolytic enzyme activities in porcine skeletal muscles with different oxidative pattern Meat Sci 49: 1–10 Honikel KO 1997 Reference methods supported by OECD and their use in Mediterranean meat products Food Chem 59: 573–582 Houben JH et al 1998 Effect of the dietary supplementation with vitamin E on colour stability and lipid oxidation in packaged, minced pork Meat Sci 48: 265–273 Hovenier R et al 1992 Genetic parameters of pig meat quality traits in a halothane negative population Meat Sci 32: 309–321 Huff-Lonergan E 2010 Chemistry and Biochemistry of meat In: F Toldr´a, (ed.), Handbook of Meat Processing Wiley-Blackwell, Ames, IA, pp 5–24 Ibrahim RM et al 2008 Effect of two dietary concentrate levels on tenderness, calpain and calpastatin activities and carcass merit in Waguli and Brahman sterrs J Anim Sci 86: 1426–1433 Jakobsen K 1999 Dietary modifications of animal fats: status and future perspectives Fett/Lipid 101: 475–483 Jim´enez Colmenero F et al 2006 New approaches for the development of functional meat products In: LML Nollet, F Toldr´a (eds.) Advanced technologies for meat processing CRC Press, Boca Raton, FL, pp 275–308 Johnson MH et al 1990 Differences in cathepsin B+L and calciumdependent protease activities among breed type and their relationship to beef tenderness J Anim Sci 68: 2371–2379 Josell A et al 2003 Sensory and meat quality traits of pork in relation to post-slaughter treatment and RN genotype Meat Sci 66: 113–124 Kauffman RG 2001 Meat composition In: YH Hui, WK Nip, RW Rogers, OA Young, (eds.) Meat Science and Applications Marcel Dekker, New York, pp 1–19 Larick DK et al 1992 Volatile compound contents and fatty acid composition of pork as influenced by linoleic acid content of the diet J Anim Sci 70: 1397–1403 Laurent W et al 2000 Muscle characterization by NMR imaging and spectroscopic techniques Food Chem 69: 419–426 Leszczynski et al 1992 Characterization of lipid in loin and bacon from finishing pigs fed full-fat soybeans or tallow J Anim Sci 70: 2175–2181 Mercier Y et al 1998 Effect of dietary fat and vitamin E on colour stability and on lipid and protein oxidation in turkey meat during storage Meat Sci 48: 301–318 Miller MF et al 1990 Determination of the alteration in fatty acid profiles, sensory characteristics and carcass traits of swine fed P1: SFK/UKS BLBS102-c15 P2: SFK BLBS102-Simpson 302 March 21, 2012 13:21 Trim: 276mm X 219mm Printer Name: Yet to Come Part 3: Meat, Poultry and Seafoods elevated levels of monounsaturated fats in the diet J Anim Sci 68: 1624–1631 Monahan FJ et al 1992 Influence of dietary vitamin E (alpha tocopherol) on the color stability of pork chops Proc 38th Int Congress of Meat Science and Technology, pp 543–544, Clermont-Ferrand, France, August 1992 Monin G, Sellier P 1985 Pork of low technological quality with a normal rate of muscle pH fall in the immediate post-mortem period: the case of the Hampshire breed Meat Sci 13: 49–63 Moody WG, Cassens RG 1968 Histochemical differentiation of red and white muscle fibers J Anim Sci 27: 961–966 Mora L et al 2008 Contents of creatine, creatinine and carnosine in pork muscles of different metabolic type Meat Sci 79: 709–715 Morgan CA et al 1992 Manipulation of the fatty acid composition of pig meat lipids by dietary means J Sci Food Agric 58: 357 368 Niemăoller A, Behmer D 2008 Use of near infrared spectroscopy in the food industry In J Irudayaraj, C Reh (eds.) Nondestructive Testing of Food Quality Blackwell Publishing, Ames, IA, pp 67–118 O’Sullivan MG 1997 The distribution of dietary vitamin E in the muscles of the porcine carcass Meat Sci 45: 297–305 Parolari G et al 1994 Relationship between cathepsin B activity and compositional parameters in dry-cured ham of normal and defective texture Meat Sci 38: 117–122 Pearson AM 1987 Muscle function and postmortem changes In: JF Price, BS Schweigert (eds.) The Science of Meat and Meat Products Food and Nutrition Press, Westport, CT, pp 155– 191 Pearson AM, Young RB 1989 Muscle and Meat Biochemistry Academic Press, San Diego, pp 1–261 Renou JP et al 1994 23Na magnetic resonance imaging: distribution of brine in muscle Magn Reson Imaging 12: 131–137 Resurreccion AVA 2003 Sensory aspects of consumer choices for meat and meat products Meat Sci 66: 11–20 Rhee KS et al 1988 Effect of dietary high-oleic sunflower oil on pork carcass traits and fatty acid profiles of raw tissues Meat Sci 24: 249–260 Robson RM et al 1997 Postmortem changes in the myofibrillar and other cytoskeletal proteins in muscle Proceedings of the 50th Annual Reciprocal Conference, Ames, IA 50: 43–52 Rodbotten R et al 2000 Prediction of beef quality attributes from early post mortem near infrared reflectance spectra Food Chem 69: 427–436 Rosenvold A, Andersen HJ 2003 Factors of significance for pork quality: A review Meat Sci 64: 219–237 Stoeva S et al 2000 Isolation and identification of proteolytic fragments from TCA soluble extracts of bovine M Longissimus dorsi Food Chem 69: 365–370 Swatland HJ 1994 Structure and Development of Meat Animals and Poultry Technomic Publishing, Lancaster, pp 143–199 Toldr´a F 1992 The enzymology of dry-curing of meat products In: FJM Smulders, et al (eds.) New Technologies for Meat and Meat Products Audet, Nijmegen, pp 209–231 Toldr´a F 2002 Dry-Cured Meat Products Food and Nutrition Press, Trumbull, CT, pp 1–238 Toldr´a F 2006 Meat: chemistry and biochemistry In: YH Hui et al (eds.) Handbook of Food Science, Technology and Engineering, Vol CRC Press, Boca Raton, FL, pp 28–1 a 28–18 Toldr´a F 2007 Biochemistry of muscle and fat In: F Toldr´a et al (eds.) Handbook of Fermented Meat and Poultry Blackwell Publishing, Ames, IA, pp 51–58 Toldr´a F, Flores, M 2000 The use of muscle enzymes as predictors of pork meat quality Food Chem 69: 387–395 Toldr´a F, Flores M 2004 Analysis of meat quality In: LML Nollet (ed.) Handbook of Food Analysis Marcel Dekker, New York, pp 1961–1977 Toldr´a F et al 1996a Pattern of muscle proteolytic and lipolytic enzymes from light and heavy pigs J Sci Food Agric 71: 124–128 Toldr´a F et al 1996b Lipids from pork meat as related to a healthy diet Recent Res Devel Nutr 1: 79–86 Urich K 1994 Comparative Animal Biochemistry Springer, Berlin, pp 526–623 Warner RD et al 1993 Quality attributes of major porcine muscles: a comparison with the Longissimus lumborum Meat Sci 33: 359–372 Warner RD et al 1997 Muscle protein changes post mortem in relation to pork quality traits Meat Sci 45: 339–372 Wheeler TL et al 1990 Mechanisms associated with the variation in tenderness of meat from Brahman and Hereford cattle J Anim Sci 68: 4206–4220 Wood JD et al 2003 Effects of fatty acids on meat quality Meat Sci 66: 21–23 P1: SFK/UKS BLBS102-c16 P2: SFK BLBS102-Simpson March 21, 2012 10:54 Trim: 276mm X 219mm Printer Name: Yet to Come 16 Biochemistry of Processing Meat and Poultry Fidel Toldr´a Background Information Description of the Muscle Enzymes Muscle Proteases Neutral Proteinases: Calpains Lysosomal Proteinases: Cathepsins Proteasome Complex Exoproteases: Peptidases Exoproteases: Aminopeptidases and Carboxypeptidases Lipolytic Enzymes Muscle Lipases Adipose Tissue Lipases Muscle Oxidative and Antioxidative Enzymes Oxidative Enzymes Antioxidative Enzymes Proteolysis Proteolysis in Aged Meat and Cooked Meat Products Proteolysis in Fermented Meats Proteolysis in Dry-Cured Ham Nucleotide Breakdown Glycolysis Lipolysis Lipolysis in Aged Meat and Cooked Meat Products Lipolysis in Fermented Meats Lipolysis in Dry-Cured Ham Oxidative Reactions Oxidation to Volatile Compounds Antioxidants References Abstract: The biochemical changes happening during meat conditioning (aging) were abundantly reported during the 1970s and 1980s It has been in recent decades that more information has been available for the biochemical changes in other products such as cooked, dry-fermented, and dry-cured meats The processing and quality of these meat products have been improved based on a better knowledge of the biochemical mechanisms involved in the genera- tion of color, flavor, and texture The endogenous enzyme systems that play important roles in these processes mainly through proteolysis and lipolysis reactions are described in this chapter Other biochemical reactions like oxidation, glycolysis, and nucleotides breakdown are also described BACKGROUND INFORMATION There are a wide variety of meat products that are attractive to consumers because of their characteristic color, flavor, and texture This perception varies depending on local traditions and heritage Most of these products have been produced for many years or even centuries based on traditional practices For instance, cured meat products reached America with settlers Pork was cured in New England for consumption in the summer Curers expanded these products by trying different recipes based on the use of additives such salt, sugar, pepper, spices, and so forth, and smoking (Toldr´a 2002) Although scientific literature on biochemical changes during meat conditioning (aging) and in some meat products were abundantly reported during the 1970s and 1980s, little information was available on the origin of the biochemical changes in other products such as cooked, dry-fermented, and dry-cured meats The need to improve the processing and quality of these meat products prompted research in the last decades on endogenous enzyme systems that play important roles in these processes, which has been later demonstrated (Toldr´a 2007) It is important to remember that the potential role of a certain enzyme in a specific observed or reported biochemical change can only be established if all the following requirements are met (Toldr´a 1992): (1) the enzyme is present in the skeletal muscle or adipose tissue, (2) the enzyme is able to degrade in vitro the natural substance (i.e., a protein in the case of a protease, a triacylglycerol in the case of a lipase, etc.), (3) the enzyme and substrate Food Biochemistry and Food Processing, Second Edition Edited by Benjamin K Simpson, Leo M.L Nollet, Fidel Toldr´a, Soottawat Benjakul, Gopinadhan Paliyath and Y.H Hui C 2012 John Wiley & Sons, Inc Published 2012 by John Wiley & Sons, Inc 303 P1: SFK/UKS BLBS102-c16 P2: SFK BLBS102-Simpson March 21, 2012 10:54 Trim: 276mm X 219mm 304 Printer Name: Yet to Come Part 3: Meat, Poultry and Seafoods are located close enough in the real meat product for an effective interaction, and (4) the enzyme exhibits enough stability during processing for the changes to be developed DESCRIPTION OF THE MUSCLE ENZYMES There are a wide variety of enzymes in the muscle Most of them have an important role in the in vivo muscle functions, but they also serve an important role in biochemical changes such as the proteolysis and lipolysis that occur in postmortem meat, and during further processing of meat Some enzymes are located in the lysosomes, while others are free in the cytosol or linked to membranes The muscle enzymes with most important roles during meat processing are grouped by families and are described in the succeeding sections Muscle Proteases Proteases are characterized by their ability to degrade proteins, and they receive different names depending on respective mode of action (see Fig 16.1) They are endoproteases or proteinases, when they are able to hydrolyze internal peptide bonds, but they are exopeptidases, when they hydrolyze external peptide bonds, either at the amino termini or the carboxy termini Neutral Proteinases: Calpains Calpains are cysteine endopeptidases consisting of heterodimers of 110 kDa, composed of an 80 kDa catalytic subunit and a 30 kDa subunit of unknown function They are located in the cytosol, around the Z-line area Calpains have received different names in the scientific literature, such as calcium-activated neutral proteinase, calcium-dependent protease, and calcium- O H3N O C C C HN HN C O C C C HN HN C O C OH C O n Endopeptidase Dipeptidylpeptidase O H3N C C O O HN C HN C O C C C HN HN C C C O n Aminopeptidase Carboxypeptidase Figure 16.1 Mode of action of the different types of muscle proteases OH activated factor Calpain I is also called µ-calpain because it needs micromolar amounts (50–70 µM) of Ca2+ for activation Similarly, calpain II is called m-calpain because it requires millimolar amounts (1–5 mM) of Ca2+ Both calpains show maximal activity around pH 7.5 Calpain activity decreases very quickly when pH decreases to 6.0, or even reaches ineffective activity at pH 5.5 (Etherington 1984) Calpains have shown good ability to degrade important myofibrillar proteins, such as titin, nebulin, troponins T and I, tropomyosin, C-protein, filamin, desmin, and vinculin, which are responsible for the fiber structure On the other hand, they are not active against myosin, actin, α-actinin and troponin C (Goll et al 1983, Koohmaraie 1994) The stability of calpain I in postmortem muscle is very poor because it is readily autolyzed, especially at high temperatures, in the presence of the released Ca2+ (Koohmaraie 1994) Calpain II appears as more stable, just 2–3 weeks before losing its activity (Koohmaraie et al 1987) In view of this rather poor stability, the importance of calpains should be restricted to short-term processes A minor contribution, just at the beginning, has been observed in long processes such as dry curing of hams (Rosell and Toldr´a 1996) or in fermented meats where the acid pH values makes calpain activity rather unlikely (Toldr´a et al 2001) Calpastatin is a polypeptide (between 50 and 172 kDa) acting as an endogenous reversible and competitive inhibitor of calpain in the living muscle In postmortem muscle, calpastatin regulates the activity of calpains, through a calcium-dependent interaction, although only for a few days, because it is destroyed by autolysis (Koohmaraie et al 1987) The levels of calpastatin vary with animal species, and pork muscle has the lowest level (Valin and Ouali 1992) Lysosomal Proteinases: Cathepsins There are several acid proteinases in the lysosomes that degrade proteins in a nonselective way The most important are cathepsins B, H, and L, which are cysteine proteinases, and cathepsin D, which is an aspartate proteinase The optimal pH for activity is slightly acid (pH around 6.0) for cathepsins B and L, acid (pH around 4.5) for cathepsin D, and neutral (pH 6.8) for cathepsin H (Toldr´a et al 1992) Cathepsins require a reducing environment such as that found in postmortem muscle to express their optimal activity (Etherington 1987) All of them are of small size, within the range 20–40 kDa, and are thus able to penetrate into the myofibrillar structure Cathepsins have shown a good ability to degrade different myofibrillar proteins Cathepsins D and L are very active against myosin heavy chain, titin, M and C proteins, tropomyosin, and troponins T and I (Matsukura et al 1981, Zeece and Katoh 1989) Cathepsin L is extremely active in degrading both titin and nebulin Cathepsin B is able to degrade myosin heavy chain and actin (Schwartz and Bird 1977) Cathepsin H exhibits both endo- and aminopeptidase activity, and this is the reason for its classification as an aminoendopeptidase (Okitani et al 1981) In the muscle, there are endogenous inhibitors against cysteine peptidases These inhibitory compounds, known as cystatins, are able to inhibit cathepsins B, H, and L Cystatin C and chicken cystatin are the most well-known cystatins P1: SFK/UKS BLBS102-c16 P2: SFK BLBS102-Simpson March 21, 2012 10:54 Trim: 276mm X 219mm Printer Name: Yet to Come 305 16 Biochemistry of Processing Meat and Poultry Proteasome Complex Exoproteases: Aminopeptidases and Carboxypeptidases The proteasome is a multicatalytic complex with different functions in living muscle, even though its role in postmortem muscle is still not well understood The 20S proteasome has a large molecular mass, 700 kDa, and a cylinder structure with several subunits Its activity is optimal at pH above 7.0, but it rapidly decreases when pH decreases, especially below 5.5 It exhibits three major activities: (1) chymotrypsin-like activity, (2) trypsin-like activity, and (3) peptidyl-glutamyl hydrolyzing activity (Coux et al 1996) This multiple activity behavior is the reason why there was originally some confusion among laboratories over its identification The 20S proteasome concentration is higher in oxidative muscles than in glycolytic ones (Dahlmann et al 2001) This enzyme has shown degradation of some myofibrillar proteins such as troponin C and myosin light chain and could be involved in postmortem changes in slow twitch oxidative muscles or in high-pH meat, where an enlargement of the Z-line with more or less density loss is observed (Sentandreu et al 2002) A new family of peptidases, named as caspases or apoptosisgenerating peptidases, are cysteine peptidases that have been recently proposed to be involved in cell death and thus immediate postmortem changes in proteins having some impact on the phases of rigor and meat aging Three main pathways of cellular death development have been proposed (Herrera-M´endez et al 2006) These peptidases are active at neutral pH, and one of the limitations to its activity in postmortem meat is the acid pH in the postmortem muscle There are five aminopeptidases, known as leucyl, arginyl, alanyl, pyroglutamyl, and methionyl aminopeptidases, based on their respective preference or requirement for a specific N-terminal amino acid They are able, however, to hydrolyze other amino acids, although at a lower rate (Toldr´a 1998) Aminopeptidases are metalloproteases with a very high molecular mass and complex structures All of them are active at neutral or basic pH Alanyl aminopeptidase, also known as the major aminopeptidase because it exhibits very high activity, is characterized by its preferential hydrolysis of alanine, but it is also able to act against a wide spectrum of amino acids such as aromatic, aliphatic, and basic aminoacyl bonds Methionyl aminopeptidase has preference for methionine, alanine, lysine, and leucine, but also has a wide spectrum of activity This enzyme is activated by calcium ions Arginyl aminopeptidase, also known as aminopeptidase B, hydrolyzes basic amino acids such as arginine and lysine (Toldr´a and Flores 1998) Carboxypeptidases are located in the lysosomes and have optimal acid pH They are able to release free amino acids from the carboxy termini of peptides and proteins Carboxypeptidase A has preference for hydrophobic amino acids, whereas carboxypeptidase B has a wide spectrum of activity (McDonald and Barrett 1986) Exoproteases: Peptidases There are several peptidases in the muscle with the ability to release small peptides of importance for taste Tripeptidylpeptidases (TPPs) are enzymes capable of hydrolyzing different tripeptides from the amino termini of peptides, while dipeptidylpeptidases (DPPs) are able to hydrolyze different dipeptide sequences There are two TPPs and four DPPs, and their molecular masses are relatively high, between 100 and 200 kDa, or even as high as 1000 kDa for TPP II, and have different substrate specificities TPP I is located in the lysosomes, has an optimal acid pH (4.0), and is able to hydrolyze tripeptides Gly-Pro-X, where X is an amino acid, preferentially of hydrophobic nature TPP II has optimal neutral pH (6.5–7.5) and wide substrate specificity, except when Pro is present on one of both sides of the hydrolyzed bond DPPs I and II are located in the lysosomes and have optimal acid pH (5.5) DPP I has a special preference for hydrolyzing the dipeptides Ala-Arg and Gly-Arg, while DPP II prefers a terminal Gly-Pro sequence DPP III is located in the cytosol and has special preference for terminal Arg-Arg and Ala-Arg sequences DPP IV is linked to the plasma membrane and prefers a terminal Gly-Pro sequence Both DPP III and IV have an optimal pH around 7.5–8.0 All these peptidases have been purified and fully characterized in porcine skeletal muscle (Toldr´a 2002) Lipolytic Enzymes Lipolytic enzymes are characterized by their ability to degrade lipids, and they receive different names depending on their mode of action (see Fig 16.2) They are known as lipases when they are able to release long-chain fatty acids from triacylglycerols, while they are know as esterases when they act on short-chain fatty acids Lipases and esterases are located either in the skeletal muscle or in the adipose tissue Phospholipases, mainly found in the skeletal muscle, hydrolyze fatty acids at positions or in phospholipids Lipase O C H2CO R2 C R1 Phospholipase A1 O C OCH H2CO R2 O H2CO C R3 C R1 OCH O H2CO O P Phospholipase A2 O OCH2CH2(CH)3 O- Figure 16.2 Mode of action of muscle lipases and phospholipases ... (eds.) Handbook of Food Science, Technology and Engineering, Vol CRC Press, Boca Raton, FL, pp 28? ??1 a 28? ?? 18 Toldr´a F 2007 Biochemistry of muscle and fat In: F Toldr´a et al (eds.) Handbook of... normal and defective texture Meat Sci 38: 117–122 Pearson AM 1 987 Muscle function and postmortem changes In: JF Price, BS Schweigert (eds.) The Science of Meat and Meat Products Food and Nutrition... heavy chain, titin, M and C proteins, tropomyosin, and troponins T and I (Matsukura et al 1 981 , Zeece and Katoh 1 989 ) Cathepsin L is extremely active in degrading both titin and nebulin Cathepsin