2009_Book_FoodChemistry

12.3 Muscle Tissue: Composition and Function 577
Fig. 12.16. Myoglobin reactions
(Mb: myoglobin,
MMb + : metmyoglobin,
MbO 2 : oxymyoglobin,
MbNO: nitrosylmyoglobin,
MMb + NO: nitrosylmetmyoglobin)
both axial binding sites:
(12.8)
MbNO antioxidatively protects the meat against
lipid peroxidation. As shown in Formula 12.9, it
traps fatty acid peroxyl radicals ROO • with the
formation of myoglobin and nitrite. MbNO is reformed
in the presence of the above mentioned
reducing agents.
ROO • + MbNO + H 2 O → ROOH + Mb + HNO 2
ROOH + 2MbNO + H 2 O → ROH + 2Mb + 2HNO 2
(12.9)
A color change to brown is observed when noncured
meat is heated. A Fe 3+ complex is present
which has its fifth and sixth coordination sites occupied
by histidine residues of denatured meat
proteins.
The myoglobin reactions relevant to meat color
are presented schematically in Fig. 12.16.
12.3.2.3 Insoluble Proteins
The main fraction of proteins insoluble in water
or salt solutions are the proteins of connective tissue.
Membranes and the insoluble portion of the
contractile apparatus are included in this group
(cf. 12.3.2.1.4).
Connective tissue contains various types of cells.
These cells synthesize many intercellular amorphous
substances (carbohydrates, lipids, proteins)
in which the collagen fibers are embedded.
Lipoproteins are present mostly in membranes.
The lipids make up 3–4% of muscle tissue and are
located in membranes. They consist of phospholipids,
triacylglycerols and cholesterol. The phospholipid
portion varies greatly: it makes up 50%
of the plasma membrane and 90% of the mitochondrial
membrane.
12.3.2.3.1 Collagen
Collagen constitutes 20–25% of the total protein
in mammals. Table 12.6 shows data on its amino
acid composition. The high contents of glycine
and proline and the occurrence of 4-hydroxyproline
and 5-hydroxylysine are characteristic.
Since the occurrence of hydroxyproline is
confined to connective tissue, its determination
may provide quantitative data on the extent
of connective tissue incorporation into a meat
product.
Collagen also contains carbohydrates (glucose
and galactose). These are attached to hydroxylysine
residues of the peptide chain by O-glycosidic
bonds. The presence of 2-O-α-D-glucosyl-O-β-
D-galactosyl-hydroxylysine and of O-β-D-galactosyl-hydroxylysine
has been confirmed.
Various types of collagen are known. They are
characteristic of different organs and also of dif-