8 food biochemistry and food phần 61

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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.


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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 2 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 2 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


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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 5 -inosine monophosphate,
5 -guanosine monophosphate, and inosine (see Fig. 15.6). An
example of the typical content of ATP breakdown products in
pork at 2 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 5 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


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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 2 hours and 24 hours
Compound
ATP
ADP
AMP
ITP + GTP
IMP
Inosine
Hypoxanthine

2 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 2 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


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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 2 hours (Batlle et al. 2001).
Red, firm, normal meat experiences a progressive pH drop down
to values around 5.8–6.0 at 2 hours postmortem. In this meat,
full ATP breakdown may take up to 8 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