2009_Book_FoodChemistry

586 12 Meat
12.3.8 Purines and Pyrimidines
The total content of purines in fresh beef muscle
tissue is 0.1–0.25% (on a fresh weight basis).
ATP, present predominantly in living tissue,
breaks down to inosine-5 ′ -monophosphate
(IMP) in the post-mortem stages. The breakdown
rate is influenced by the condition of
the animal and by temperature. IMP is then
slowly decomposed through successive steps to
hypoxanthine, with inosine as an intermediary
product:
ATP ⇄ ADP + P in (Myosin-ATPase)
2ADP ⇄ ATP + AMP (Myokinase)
AMP → IMP + NH 3 (Adenylate-deaminase)
IMP → Inosine + P in (5 ′ − Nucleotidase)
Inosine → Hypoxanthine+ Ribose (Nucleosidase)
(12.26)
Post-mortem data on the Psoas major rabbit
muscle are given in Table 12.10. They relate to
nucleotide breakdown and to other important
muscle tissue constituents.
The changes in water holding capacity of meat
resulting from ATP transition to IMP are dealt
with in Section 12.5. Unlike purines, pyrimidine
nucleotide content in muscle is very low (Table
12.9).
Table 12.10. Post-mortem changes in the concentration
of some constituents of rabbit muscle (M. psoas)
Compound
µmol/g Fresh tissue
living
muscle
post-rigor
muscle
Total acid-soluble phosphorus 68 68
Inorganic phosphorus <12 >48
Adenosine triphosphate (ATP) 9 <1
Adenosine diphosphate (ADP) 1 <1
Inosine monophosphate <1 9
Creatine phosphate 20 <1
Creatine 23 42
NAD/NADP 2 1
Glycogen 50 <10
Glucose-1-phosphate <1 <1
Glucose-6-phosphate 5 6
Fructose-1,6-bisphosphate <1 <1
Lactic acid 10 100
12.3.9 Organic Acids
The predominant acid in muscle tissue is the
lactic acid formed by glycolysis (0.2–0.8% on
a fresh meat weight basis), followed by glycolic
(0.1%) and succinic acids (0.05%). Other acids
of the Krebs cycle are present in negligible
amounts.
Table 12.9. Purines and pyrimidines in fresh-beef muscle
Compound Content (%)
Inosine-5 ′ -phosphate
0.02–0.2 a
Inosine
Trace
Hypoxanthine 0.01–0.03
Adenosine-5 ′ -phosphate 0.001–0.01
Adenosine-5 ′ -diphosphate
<0.3 b
Adenosine-5 ′ -triphosphate
Nicotinamide-adenine-dinucleotide 0.1
Guanosine-5 ′ -phosphate 0.002
Cytidine-5 ′ -phosphate 0.001
Uridine-5 ′ -phosphate 0.002
a Until approx. 1 h post-mortem no IMP is found in
muscle.
b There is a fairly rapid decrease in post-mortem concentration
influenced by cooling and other muscle handling
conditions.
12.3.10 Carbohydrates
The glycogen content of muscle varies greatly
(0.02–1.0% on a fresh tissue weight basis) and
is influenced by the age and condition of the
animal prior to slaugher. The rate of the postmortem
decrease in glycogen als varies. Sugars
are only 0.1–0.15% of the weight of fresh muscle,
of which 0.1% is shared by glucose-6-phosphate
and other phosphorylated sugars. The free sugars
present are glucose (0.009–0.09%), fructose and
ribose.
12.3.11 Vitamins
Table 12.11 provides data on water-soluble vitamins
in beef muscle.