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

290 Part 3: Meat, Poultry and Seafoods

metabolic enzymes (mitochondrial, lysosomal, microsomal, nu-

clear, or free in the cytosol) and myoglobin. Some of these en-

zymes play a very important role in postmortem meat and during

further processing, as described in Chapter 16. Minor amounts

of hemoglobin may be found in the muscle if blood has not been

drained properly. Myoglobin is the main sarcoplasmic protein,

responsible for the red 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 pro-

teins in connective tissue. There are several types (I–V) of colla-

gen containing different polypeptide chains (up to tenα-chains).

Type I collagen is the major component of the epimysium and

perimysium that surround the muscles. Types III, IV, and V col-

lagen 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 tougher in older animals. Elastin is found in lower

amounts, usually in capillaries, tendons, nerves, and ligaments.

Enzymes

Muscle contains a large number of enzymes, mainly peptidases

like calpains, cathepsins, proteasome, tri- and dipeptidylpep-

tidases, aminopeptidases, carboxypeptidases, and dipeptidases,

but also lipases, glycohydrolase, nucleotidases, and so on (Toldra ́

2007). All these enzymes have a special relevance for meat pro-

cessing and they are covered in-depth in Chapter 16.

Non-protein Compounds

Free Amino Acids

The action of muscle aminopeptidases contributes to the gener-

ation of free amino acids in living muscle. An example of the

typical content of free amino acids, at less than 45 minutes post-

mortem, in glycolytic and oxidative porcine muscles is shown

in Table 15.3. It can be observed that most of the amino acids

are present in significantly higher amounts in the oxidative mus-

cle (Aristoy and Toldra 1998). The free amino acid content is ́

relatively low just postmortem, but it is substantially increased

during postmortem storage due to the action of the proteolytic

chain, which is very active and stable during meat aging.

Dipeptides

Muscle contains three natural dipeptides: carnosine (β-alanyl-l-

histidine), anserine (β-alanyl-l-1-methylhistidine), and balenine

(β-alanyl-l-3-methylhistidine). These dipeptides perform some

physiological functions in muscle, for example, as buffers, an-

Table 15.3.Example of the Composition in Free Amino

Acids (Expressed as mg/100g of Muscle) of the

Glycolytic MuscleLongissimus Dorsiand Oxidative

MuscleTrapezius.

Amino Acids

Muscle

Longissimus

dorsi

Muscle

Trapezius

Essential

Histidine 2.90 4.12

Threonine 2.86 4.30

Valine 2.78 2.09

Methionine 0.90 1.01

Isoleucine 1.52 1.11

Leucine 2.43 1.82

Phenylalanine 1.51 1.25

Lysine 2.57 0.22

Nonessential

Aspartic acid 0.39 0.74

Glutamic acid 2.03 5.97

Serine 2.02 4.43

Asparagine 0.91 1.63

Glycine 6.01 12.48

Glutamine 38.88 161.81

Alanine 11.29 26.17

Arginine 5.19 5.51

Proline 2.83 4.45

Tyrosine 2.11 1.63

Ornithine 0.83 0.83

Source: Aristoy and Toldra 1998. ́

tioxidants, neurotransmitters, and modulators of enzyme action

(Chan and Decker 1994, Gianelli et al. 2000). Dipeptide content

is especially higher in muscles with glycolytic metabolism (see

Table 15.4), but it varies with the animal species, age, and diet

(Aristoy and Toldr ́a 1998, Aristoy et al. 2004). Beef and pork

have a higher content of carnosine and are lower in anserine,

lamb has similar amounts of carnosine and anserine, and poultry

is very rich in anserine (see Table 15.3). Balenine is present in

minor amounts in pork muscle but at very low concentrations

in other animal muscle, except in marine mammalians such as

dolphins and whales (Aristoy and Toldr ́a 2004).

Muscle and Adipose Tissue Lipids

Skeletal muscle contains a variable amount of lipids, between

1% and 13%. Lipid content mainly depends on the degree of

fattening and the amount of adipose tissue. Lipids can be found

within the muscle (intramuscular), between muscles (intermus-

cular), and in adipose tissue. Intramuscular lipids are mainly

composed of triacylglycerols, which are stored in fat cells, and

phospholipids, which are located in cell membranes. The amount

of cholesterol in lean meat is around 50–70 mg/100 g. Intermus-

cular and adipose tissue lipids are mainly composed of tria-

cylglycerols and small amounts of cholesterol, around 40–60

mg/100g (Toldr ́a and Flores 2004).