Food Chemistry

12.3 Muscle Tissue: Composition and Function 577

Fig. 12.16.Myoglobin re-
actions (Mb: myoglobin,
MMb+: metmyoglobin,
MbO 2 : oxymyoglobin,
MbNO: nitrosylmyoglobin,
MMb+NO: nitrosylmetmyo-
globin)

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 re-
formed 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 non-
cured meat is heated. A Fe^3 +complex is present
which has its fifth and sixth coordination sites oc-
cupied 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 tis-
sue. 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 amor-

phous 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 phospho-

lipids, triacylglycerols and cholesterol. The phos-

pholipid portion varies greatly: it makes up 50%

of the plasma membrane and 90% of the mito-

chondrial 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-hydro-

xyproline and 5-hydroxylysine are characteris-

tic. 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 hydroxyly-

sine 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-ga-

lactosyl-hydroxylysine has been confirmed.

Various types of collagen are known. They are

characteristic of different organs and also of dif-