P1: SFK/UKS BLBS102-c15 P2: SFK BLBS102-Simpson March 21, 2012 13:21 Trim: 276mm X 219mm Printer Name: Yet to Come 291 15 Biochemistry of Raw Meat and Poultry Table 15.4 Example of the Composition in Dipeptides (Expressed as mg/100g of Muscle) of the Porcine Glycolytic Muscle Longissimus Dorsi and Oxidative Muscle Trapezius data in Table 15.5) When triacylglycerols are rich in polyunsaturated fatty acids (PUFA) such as linoleic and linolenic acids, fats tend to be softer and prone to oxidation These fats may even have an oily appearance when kept at room temperature Carnosine Anserine Phospholipids Effect of muscle metabolism Glycolytic (muscle Longissimus dorsi) Oxidative (muscle Trapezius) 313 181.0 14.6 10.7 Animal species Pork (loin) Beef (top loin) Lamb (neck) Chicken (pectoral) 313.0 372.5 94.2 180.0 14.5 59.7 119.5 772.2 Source: Aristoy and Toldr´a 1998, Aristoy and Toldr´a 2004 These compounds are present in cell membranes, and although present in minor amounts (see Table 15.1), they have a strong relevance to flavor development due to their relatively high proportion of PUFA (see polar fraction in Table 15.5) Major constituents are phosphatidylcholine (lecithin) and phosphatidylethanolamine The phospholipid content may vary depending on the genetic type of the animal and the anatomical location of the muscle (Hern´andez et al 1998, Armero et al 2002) For instance, red oxidative muscles have a higher amount of phospholipids than white glycolytic muscles Triacylglycerols Triacylglycerols are the major constituents of fat, as shown in Table 15.1 The fatty acid content mainly depends on age, production system, type of feed, and environment (Toldr´a et al 1996b) Monogastric animals such as swine and poultry tend to reflect the fatty acid composition of the feed in their fat In the case of ruminants, the nutrients and fatty acid composition are somehow standardized due to biohydrogenation by the microbial population of the rumen (Jakobsen 1999) The properties of the fat will depend on its fatty acid composition A great percentage of the triacylglycerols are esterified to saturated and monounsaturated fatty acids (see neutral muscle fraction and adipose tissue CONVERSION OF MUSCLE TO MEAT A great number of chemical and biochemical reactions take place in living muscle Some of these reactions continue, while others are altered due to changes in pH, the presence of inhibitory compounds, the release of ions into the sarcoplasm, and so on during the early postmortem time In a few hours, these reactions are responsible for the conversion of muscle to meat; this process is basically schematized in Figure 15.2 and consists of the following steps: Once the animal is slaughtered, the blood circulation is stopped, and the importation of nutrients and the removal of metabolites to the muscle cease This fact has very important Table 15.5 Example of Fatty Acid Composition (Expressed as Percentage of Total Fatty Acids) of Muscle Longissimus Dorsi and Adipose Tissue in Pigs Feeded with a Highly Unsaturated Feed Neutral and Polar Fractions of Muscle Lipids are also Included Fatty Acid Total Muscle Neutral Polar Adipose Tissue Myristic acid (C 14:0) Palmitic acid (C 16:0) Stearic acid (C 18:0) Palmitoleic acid (C 16:1) Oleic acid (C 18:1) C 20:1 Linoleic acid (C 18:2) C 20:2 Linolenic acid (C 18:3) C 20:3 Arachidonic acid (C 20:4) C 22:4 Total SFA Total MUFA Total PUFA Ratio MUFA/SFA Ratio PUFA/SFA 1.55 25.10 12.62 2.79 36.47 0.47 16.49 0.49 1.14 0.30 2.18 0.25 39.42 39.74 20.84 1.01 0.53 1.97 26.19 11.91 3.49 42.35 0.52 11.38 0.43 1.17 0.10 0.25 0.08 40.23 46.36 13.41 1.15 0.33 0.32 22.10 14.49 0.69 11.45 0.15 37.37 0.66 0.97 1.04 9.83 0.84 37.03 12.26 50.70 0.33 1.37 1.40 23.78 11.67 1.71 31.64 0.45 25.39 0.78 2.64 0.10 0.19 0.07 37.02 33.81 29.17 0.91 0.79 SFA, saturated fatty acids; MUFA, monosaturated fatty acids; PUFA, polysaturated fatty acid P1: SFK/UKS BLBS102-c15 P2: SFK BLBS102-Simpson March 21, 2012 13:21 Trim: 276mm X 219mm 292 Printer Name: Yet to Come Part 3: Meat, Poultry and Seafoods Muscle Slaughter: Blood circulation is stopped g pp Very fast decrease of oxygen concentration in the muscle Lack of available oxygen Redox potential decreases down to –50 mV Cell respiration stops Cease of the activity of the mitochondrial system Glycolysis Lactic acid is generated and accumulated The enzymatic generation of ATP is reduced Enzyme inhibition ATP consumption Actomyosin is formed pH drops down to around 5.6 Contraction Decrease in water-binding capacity Reduction in red color Proteins are denaturated Rigor mortis Release of water and soluble nutrients Figure 15.2 Summary of main changes during conversion of muscle to meat and drastic consequences The first consequence is the reduction of the oxygen concentration within the muscle cell because the oxygen supply has stopped An immediate consequence is a reduction in mitochondrial activity and cell respiration (Pearson 1987) Under normal aerobic values (see an example of resting muscle in Fig 15.3), the muscle is able to produce 12 moles of adenosine triphosphate (ATP) per mole of glucose, and thus the ATP content is kept around 5–8 µmol/g of muscle (Greser 1986) ATP constitutes the main source of energy for the contraction and relaxation of the muscle structures as well as other biochem- Resting muscle Stressed muscle Glycogen y g Glucose Blood Glucose O2 12ATP (TCA cycle) 2ATP (anaerobic) Glycogen Energy-requiring processes as creatin phosphatein mitochondria Energy-requiring processes Lactic acid CO2 Blood Figure 15.3 Comparison of energy generation between resting and stressed muscles ical reactions in postmortem muscle As the redox potential is reduced toward anaerobic values, ATP generation is more costly So, only moles of ATP are produced per mole of glucose under anaerobic conditions (an example of a stressed muscle is shown in Fig 15.3) The extent of anaerobic glycolysis depends on the reserves of glycogen in the muscle (Greaser 1986) Glycogen is converted to dextrins, maltose, and finally, glucose through a phosphorolytic pathway; glucose is then converted into lactic acid with the synthesis of moles of ATP (Eskin 1990) In addition, the enzyme creatine kinase may generate some additional ATP from adenosine diphosphate (ADP) and creatine phosphate at very early postmortem times, but only while creatine phosphate remains The contents of creatine have been reported to vary depending on the type of muscle (Mora et al 2008) The main steps in glycolysis are schematized in Figure 15.4 The generation of ATP is strictly necessary in the muscle to supply the required energy for muscle contraction and relaxation and to drive the sodium-potassium pump of the membranes and the calcium pump in the sarcoplasmic reticulum The initial situation in postmortem muscle is rather similar to that in the stressed muscle, but with an important change: the absence of blood circulation Thus, there is a lack of nutrient supply and waste removal (see Fig 15.5) Initially, the ATP content in postmortem muscle does not drop substantially because some ATP may be formed from ceratin phosphate through the action of the enzyme creatine kinase and through anaerobic glycolysis As mentioned earlier, once creatine phosphate and glycogen are exhausted, ATP drops within a few hours to negligible P1: SFK/UKS BLBS102-c15 P2: SFK BLBS102-Simpson March 21, 2012 13:21 Trim: 276mm X 219mm Printer Name: Yet to Come 293 15 Biochemistry of Raw Meat and Poultry Enzymes Reactions Other Comments ATP → ADP Requires Mg2+ Hexokinase Glucose → glucose-6-P Phosphoglucoisomerase Glucose-6-P → fructose-6-P Phosphofructokinase Fructose-6-P → fructose-1,6-biP Aldolase Fructose-1,6-biP → dihydroxyacetone-3-P ↓↑ Fructose-1,6-biP → glyceraldehyde-3-P Triose phosphate dehydrogenase Glyceraldehyde-3-P → 1,3diphosphoglycerol 2NAD+ → 2NADH Phosphoglycerokinase 1,3-Diphosphoglycerol → 3-phosphoglycerol 2ADP → 2ATP Phosphoglyceromutase 3-Phosphoglycerol → 2-phosphoglycerol Enolase 2-Phosphoglycerol → phosphoenolpyruvate Pyruvate kinase Phosphoenolpyruvate → pyruvic acid 2ADP → 2ATP If O2 available, produces CO2 via TCA cycle Requires K+, Mg2+ Lactate dehydrogenase Pyruvic acid → lactic acid NADH → NAD+ Only under anaerobic conditions Requires Mg2+ ATP → ADP Inhibited by excess of ATP Requires Mg2+ Requires Mg2+ Requires Mg2+ + H2O Figure 15.4 Main steps in glycolysis during early postmortem (Adapted from Greaser 1986.) Creatine phosphate ATP Glucose 22ATP Glycogen Energy-requiring Lactic acid processes (contraction-relaxation, Na/K and Ca pumps, ) pH drop Activation of acid hydrolases Protein denaturation Water release Loss of nutrients Figure 15.5 Scheme of energy generation in postmortem muscle values by conversion into ADP, adenosine monophosphate, and other derived compounds such as -inosine monophosphate, -guanosine monophosphate, and inosine (see Fig 15.6) An example of the typical content of ATP breakdown products in pork at hours and 24 hours postmortem is shown in Table 15.6 The reaction rates depend on the metabolic status of the animal prior to slaughter For instance, reactions proceed very quickly in pale, soft, exudative (PSE) muscle, where ATP can be almost fully depleted within few minutes The rate is also affected by the pH and temperature of the meat (Batlle et al 2000, 2001) For instance, the ATP content in beef Sternomandibularis kept at 10–15◦ C is around µmol/g at 1.5 hours postmortem and decreases to 3.5 µmol/g at 8–9 hours postmortem However, when that muscle is kept at 38◦ C, ATP content is below 0.5 µmol/g at 6–7 hours postmortem Once the ATP concentration is exhausted, the muscle remains contracted, as no more energy is available for relaxation The muscle develops a rigid condition known as rigor mortis, in which the crossbridge of myosin and actin remains locked, forming actomyosin (Greaser 1986) The postmortem time necessary for the development of rigor mortis is variable, depending on the animal species, size of carcass, amount of fat cover, and environmental conditions such as the temperature of the chilling P1: SFK/UKS BLBS102-c15 P2: SFK BLBS102-Simpson March 21, 2012 13:21 Trim: 276mm X 219mm 294 Printer Name: Yet to Come Part 3: Meat, Poultry and Seafoods Creatine phosphate Creatin kinase Adenosine triphosphate (ATP) ATPase Adenosine diphosphate (ADP) Myokinase Adenosine monophosphate (AMP) AMP deaminase Inosine monophosphate (IMP) Guanosine monophosphate (GMP) Inosine guanosine Nucleoside phosphorylase Hypoxanthine Figure 15.6 Main adenosine triphosphate (ATP) breakdown reactions in early postmortem muscle tunnel and the air velocity (see pork pieces after cutting in a slaughterhouse in Fig 15.7) The rates of enzymatic reactions are strongly affected by temperature In this sense, the carcasscooling rate will affect glycolysis rate, pH drop rate, and the time course of rigor onset (Faustman 1994) The animal species and size of carcass have a great influence on the cooling rate Table 15.6 Example of Nucleotides and Nucleosides Content (Expressed as µmol/g Muscle) in Pork Postmortem Muscle at hours and 24 hours Compound ATP ADP AMP ITP + GTP IMP Inosine Hypoxanthine h Postmortem Time 24 h Postmortem Time 4.39 1.08 0.14 0.18 0.62 0.15 0.05 − 0.25 0.20 − 6.80 1.30 0.32 Source: Batlle et al 2001 ATP, adenosine triphosphate; ADP, adenosine diphosphate; AMP, adenosine monophosphate; ITP, inosine triphosphate; GMP, guanosine monophosphate; IMP, inosine monophosphate of the carcass Furthermore, the location in the carcass is also important because surface muscles cool more rapidly than deep muscles (Greaser 2001) So, when carcasses are kept at 15◦ C, the time required for rigor mortis development may be about 2–4 hours in poultry, 4–18 hours in pork, and 10–24 hours in beef (Toldr´a 2006) Muscle glycolytic enzymes hydrolyze the glucose to lactic acid, which is accumulated in the muscle because muscle waste substances cannot be eliminated due to the absence of blood circulation This lactic acid accumulation produces a relatively rapid (in a few hours) pH drop to values of about 5.6–5.8 The pH drop rate depends on the glucose concentration, the temperature of the muscle, and the metabolic status of the animal previous to slaughter Water binding decreases with pH drop because of the change in the protein’s charge Then, some water is released out of the muscle as a drip loss (DL) The amount of released water depends on the extent and rate of pH drop Soluble compounds such as sarcoplasmic proteins, peptides, free amino acids, nucleotides, nucleosides, B vitamins, and minerals may be partly lost in the drippings, affecting nutritional quality (Toldr´a and Flores 2004) The pH drop during early postmortem has a great influence on the quality of pork and poultry meats The pH decrease is very fast, below 5.8 after hours postmortem, in muscles from animals with accelerated metabolism This is the case of the PSE pork meats and red, soft, exudative pork meats ATP breakdown 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 295 Figure 15.7 Pork hams after cutting in a slaughterhouse, ready for submission to a processing plant (Courtesy of Industrias Carnicas ´ Vaquero SA, Madrid, Spain.) also proceeds very quickly in these types of meats, with almost full ATP disappearance in less than hours (Batlle et al 2001) Red, firm, normal meat experiences a progressive pH drop down to values around 5.8–6.0 at hours postmortem In this meat, full ATP breakdown may take up to hours Finally, the dark, firm, dry pork meat (DFD) and dark cutting beef meat are produced when the carbohydrates in the animal are exhausted from before slaughter, and thus almost no lactic acid can be generated during early postmortem due to the lack of a substrate Very low or almost negligible glycolysis is produced, and the pH remains high in these meats, which constitutes a risk from the microbiological point of view These meats constitute a risk because they are prone to contamination by foodborne pathogens and must be carefully processed, with extreme attention to good hygienic practices Protein oxidation is another relevant change during postmortem aging Some amino acid residues may be converted into carbonyl derivatives and cause the formation of inter- and intraprotein disulfide links that can reduce the functionality of proteins (Huff-Lonergan 2010) FACTORS AFFECTING BIOCHEMICAL CHARACTERISTICS Effect of Genetics Genetic Type The genetic type has an important relevance for quality, not only due to differences among breeds, but also to differences among animals within the same breed Breeding strategies have been focused toward increased growth rate and lean meat content and decreased backfat thickness Although grading traits are really improved, poorer meat quality is sometimes obtained Usually, large ranges are found for genetic correlations between production and meat quality traits, probably due to the reduced number of samples when analyzing the full quality of meat, or to a large number of samples but with few determinations of quality parameters This variability makes it necessary to combine the results from different research groups to obtain a full scope (Hovenier et al 1992) Current pig breeding schemes are usually based on a backcross or on a three- or four-way cross For instance, a common cross in the European Union is a three-way cross, where the sow is a Landrace × Large White (LR × LW) crossbreed The terminal sire is chosen depending on the desired profitability per animal, and there is a wide range of possibilities For instance, the Duroc terminal sire grows faster and shows a better food conversion ratio but accumulates an excess of fat; Belgian Landrace and Pietrain are heavily muscled but have high susceptibility to stress and thus usually present a high percentage of exudative meats; or a combination of Belgian Landrace × Landrace gives good conformation and meat quality (Toldr´a 2002) Differences in tenderness between cattle breeds have also been observed Brahman cattle is used extensively in the southwest of the United States since it is tolerant to adverse environmental conditions but may give some tenderness issues (Brewer 2010) Even toughness was associated to an increased amount of calpastatin, the endogenous muscle calpain inhibitor (Ibrahim 2008) Studies have been performed for cattle breeding For instance, after 10 days of aging, the steaks from an Angus breed were more ... grows faster and shows a better food conversion ratio but accumulates an excess of fat; Belgian Landrace and Pietrain are heavily muscled but have high susceptibility to stress and thus usually... Muscle at hours and 24 hours Compound ATP ADP AMP ITP + GTP IMP Inosine Hypoxanthine h Postmortem Time 24 h Postmortem Time 4.39 1. 08 0.14 0. 18 0.62 0.15 0.05 − 0.25 0.20 − 6 .80 1.30 0.32 Source:... (ATP) per mole of glucose, and thus the ATP content is kept around 5? ?8 µmol/g of muscle (Greser 1 986 ) ATP constitutes the main source of energy for the contraction and relaxation of the muscle