Food Biochemistry and Food Processing (2 edition)

BLBS102-c15 BLBS102-Simpson March 21, 2012 13:21 Trim: 276mm X 219mm Printer Name: Yet to Come

294 Part 3: Meat, Poultry and Seafoods

Creatine phosphate Adenosine triphosphate (ATP)

Creatin kinase

Adenosine diphosphate (ADP)

ATPase

Adenosine monophosphate (AMP)

Myokinase

Inosine monophosphate (IMP)

Guanosine monophosphate (GMP)

AMP deaminase

Inosine

guanosine

Hypoxanthine

Nucleoside phosphorylase

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 carcass-

cooling 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 asμmol/g Muscle) in Pork

Postmortem Muscle at 2 hours and 24 hours

Compound

2 h Postmortem

Time

24 h Postmortem

Time

ATP 4.39 −

ADP 1.08 0.25

AMP 0.14 0.20

ITP+GTP 0.18 −

IMP 0.62 6.80

Inosine 0.15 1.30

Hypoxanthine 0.05 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 (Toldra 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, nu-

cleotides, nucleosides, B vitamins, and minerals may be partly

lost in the drippings, affecting nutritional quality (Toldraand ́

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