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

15 Biochemistry of Raw Meat and Poultry 293

NADH → NAD+

2ADP → 2ATP

+ H 2 O

2ADP → 2ATP

2NAD+ → 2NADH

ATP → ADP

ATP → ADP

Other

Only under

anaerobic

conditions

Lactate dehydrogenase Pyruvic acid → lactic acid

If O 2 available,

produces CO 2 via

TCA cycle

Requires K+, Mg2+

Pyruvate kinase Phosphoenolpyruvate → pyruvic acid

Enolase 2-Phosphoglycerol → phosphoenolpyruvate

Phosphoglyceromutase 3-Phosphoglycerol → 2-phosphoglycerol Requires Mg2+

Phosphoglycerokinase 1,3-Diphosphoglycerol → 3-phosphoglycerol Requires Mg2+

Triose phosphate Glyceraldehyde-3-P → 1,3diphosphoglycerol

dehydrogenase

Fructose-1,6-biP → dihydroxyacetone-3-P

↓↑

Fructose-1,6-biP → glyceraldehyde-3-P

Aldolase

Inhibited by

excess of ATP

Requires Mg2+

Phosphofructokinase Fructose-6-P → fructose-1,6-biP

Phosphoglucoisomerase Glucose-6-P → fructose-6-P Requires Mg2+

Hexokinase Glucose → glucose-6-P Requires Mg2+

Enzymes Reactions Comments

Figure 15.4.Main steps in glycolysis during early postmortem. (Adapted from Greaser 1986.)

Creatine phosphate Glucose

AT P 2ATP Glycogen

Lactic acid

(^2) Glycogen
Energy-requiring
processes
AT P
pH drop
(contraction-relaxation,
Na/K and Ca pumps, ...)
Protein
denaturation
Activation of
acid hydrolases
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 beefSternomandibulariskept
at 10–15◦C is around 5μmol/g at 1.5 hours postmortem and
decreases to 3.5μmol/g at 8–9 hours postmortem. However,
when that muscle is kept at 38◦C, ATP content is below
0.5μmol/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, form-
ing 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 en-
vironmental conditions such as the temperature of the chilling