Food Biochemistry and Food Processing

13 Biochemistry of Raw Meat and Poultry 297

proteins, present in a significant proportion, that are
located in the void space between the filaments and
the Z-line and contribute to the integrity of the mus-
cle cells (Robson et al. 1997). Desmin is located on
the external area of the Z-line and connects adjacent
myofibrils at the level of the Z-line.

Sarcoplasmic Proteins

These are water-soluble proteins, comprising about
30–35% of the total protein in muscle. Sarcoplasmic
proteins contain a high diversity of proteins (summa-
rized in Table 13.2.), mainly metabolic enzymes
(mitochondrial, lysosomal, microsomal, nuclear, or
free in the cytosol) and myoglobin. Some of these
enzymes play a very important role in postmortem
meat and during further processing, as described in
Chapter 14. Minor amounts of hemoglobin may be
found in the muscle if blood has not been drained
properly. Myoglobin is the main sarcoplasmic pro-
tein, responsible for the red meat color of meat
as well as the typical pink color of drippings. The
amount of myoglobin depends on many factors. Red
fibers contain higher amounts of myoglobin than
white fibers. The species is very important, and thus
beef and lamb contain more myoglobin than pork and
poultry. For a given species, the myoglobin content in
the muscle increases with the age of the animal.

Connective Tissue Proteins

Collagen, reticulin, and elastin constitute the main
stromal proteins in connective tissue. There are sev-

eral types (I to V) of collagen containing different

polypeptide chains (up to 10 
chains). Type I colla-

gen is the major component of the epimysium and

perimysium that surround the muscles. Types III, IV,

and V collagen are found in the endomysium, which

provides support to the muscle fiber (Eskin 1990).

There are a high number of cross-linkages in the

collagen fibers that increase with age, and this is

why meat is tougher in older animals. Elastin is

found in lower amounts, usually in capillaries, ten-

dons, nerves, and ligaments.

NONPROTEINCOMPOUNDS

Dipeptides

Muscle contains three natural dipeptides: carno-

sine (-alanyl-L-histidine), anserine (—alanyl-L-

1-methylhistidine), and balenine (—alanyl-L-3-

methylhistidine). These dipeptides perform some-

physiological functions in muscle, for example, as

buffers, antioxidants, neurotransmitters, and modu-

lators of enzyme action (Chan and Decker 1994,

Gianelli et al. 2000). Dipeptide content is especially

higher in muscles with glycolytic metabolism (see

Table 13.3), but it varies with the animal species,

age, and diet (Aristoy and Toldrá 1998, Aristoy et al.

2003). Beef and pork have a higher content of carno-

sine and are lower in anserine, lamb has similar

amounts of carnosine and anserine, and poultry is

very rich in anserine (see Table 13.3). Balenine is

present in minor amounts in pork muscle but at very

low concentrations in other animal muscle, except in

Table 13.3.Example of the Composition in Dipeptides of the Porcine Glycolytic

Muscle Longissimus dorsiand Oxidative Muscle Trapezius

Carnosine Anserine

(mg/100 g muscle) (mg/100 g muscle)

Effect of muscle metabolism

Glycolytic (M. Longissimus dorsi) 313 14.6

Oxidative (M. Trapezius) 181.0 10.7

Animal species

Pork (loin) 313.0 14.5

Beef (top loin) 372.5 59.7

Lamb (neck) 94.2 119.5

Chicken (pectoral) 180.0 772.2

Sources:From Aristoy and Toldrá 1998, Aristoy et al. 2003.