Food Biochemistry and Food Processing (2 edition)

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292 Part 3: Meat, Poultry and Seafoods

Muscle

Slaughter: Blood circulation is stopped

Very fast decrease of oxygen concentration in the muscle

g pp

Lack of available oxygen

Redox potential decreases

down to –50 mV Cell respiration stops

Cease of the activity

of the mitochondrial system

Lactic acid is generated The enzymatic generation of ATP is reduced

Glycolysis

and accumulated

pH drops down to around 5.6 Actomyosin is formed

ATP consumption

Enzyme inhibition

Proteins are denaturated Decrease in water-binding capacity

Contraction

Reduction in red color

Release of water and soluble nutrients

Rigor mortis

Figure 15.2.Summary of main changes during conversion of muscle to meat.

and drastic consequences. The first consequence is the reduc-

tion 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 5–8μmol/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

Blood Glucose

12ATP

(TCA cycle)

Glucose

2ATP

(anaerobic)

O 2

y g

Glycogen

Energy-requiring processes

as creatin phosphatein

mitochondria

Energy-requiring

processes

CO 2 Lactic acid

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 addi-

tion, 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 phos-

phate 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