Food Biochemistry and Food Processing (2 edition)

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306 Part 3: Meat, Poultry and Seafoods

Muscle Lipases

Lysosomal acid lipase and acid phospholipase are located in

the lysosomes. Both have an optimal acid pH (4.5–5.5) and are

responsible for the generation of long-chain free fatty acids.

Lysosomal acid lipase has preference for the hydrolysis of tri-

acylglycerols at positions 1 or 3 (Fowler and Brown 1984). This

enzyme also hydrolyzes di- and monoacylglycerols but at a lower

rate (Imanaka et al. 1985, Negre et al. 1985). Acid phospholi-

pase hydrolyzes phospholipids at position 1 at the water-lipid

interface.

Phospholipase A and lysophospholipase have optimal pH in

the basic region and regulate the hydrolysis of phospholipids, at

positions 1 and 2, respectively. The activities of these enzymes

are higher in oxidative muscles than in glycolytic muscles, and

this fact would explain the high content of free fatty acids in

oxidative muscles. The increase in activity is about 10- to 25-

fold for phospholipase A and 4- to 5-fold for lysophospholipase

(Alasnier and Gandemer 2000).

Acid and neutral esterases are located in the lysosomes and

cytosol, respectively, and are quite stable (Motilva et al. 1992).

Esterases are able to hydrolyze short-chain fatty acids from tri-,

di-, and monoacylglycerols, but they exert poor action due to the

lack of adequate substrate.

Adipose Tissue Lipases

Hormone-sensitive lipase is the most important enzyme present

in adipose tissue. This enzyme is responsible for the hydrolysis

of stored adipocyte lipids. It has a high specificity and prefer-

ence for the hydrolysis of long-chain tri- and diacylglycerols

(Belfrage et al. 1984). This enzyme has positional specificity

since it hydrolyzes fatty acids at positions 1 or 3 in triacylglyc-

erols four times faster than it hydrolyzes fatty acids in position

2 (Belfrage et al. 1984). The hormone-sensitive lipase has a

molecular mass of 84 kDa and neutral optimal pH, around 7.0.

The monoacylglycerol lipase is mainly present in the adipocytes,

and very little is present in stromal and vascular cells. It has a

molecular mass of 32.9 kDa and hydrolyzes medium- and long-

chain monoacylglycerols resulting from previous hydrolysis by

the hormone-sensitive lipase (Tornquist et al. 1978). Lipopro-

tein lipase is located in the capillary endothelium and is able to

hydrolyze the acylglycerol components at the luminal surface

of the endothelium (Smith and Pownall 1984), with preference

for fatty acids at position 1 over those at position 3 (Fielding

and Fielding 1980). Lipoprotein lipase is an acylglycerol lipase

responsible for the degradation of lipoprotein triacylglycerol. Its

molecular mass is around 60 kDa and it has an optimal basic pH.

Unsaturated monoacylglycerols are more quickly hydrolyzed

than saturated compounds (Miller et al. 1981).

The lipolysis phenomenon in adipose tissue is not so complex

as in muscle. The hormone-sensitive lipase hydrolyzes tri- and

diacylglycerols, as a rate-limiting step (Xiao et al. 2010). The

resulting monoacylglycerols from this reaction or from lipopro-

tein lipase (Belfrage et al. 1984) are then further hydrolyzed by

the monoacylglycerol lipase. The end products are glycerol and

free fatty acids.

Acid and neutral esterases are also present in adipose tissue

(Motilva et al. 1992). During mobilization of depot lipids, es-

terases can participate by mobilizing stored cholesteryl esters.

Esterases can also degrade lipoprotein cholesteryl esters taken

up from the plasma (Belfrage et al. 1984).

Muscle Oxidative and Antioxidative Enzymes

Oxidative Enzymes

Lipoxygenase contains iron and catalyzes the incorporation of

molecular oxygen into polyunsaturated fatty acids, especially

arachidonic acid, and esters containing a Z,Z-1,4-pentadien

(Marczy et al. 1995). They receive different names, 5-, 12-,

or 15-lipoxygenase, depending on the position where oxygen

is introduced. The final product is a conjugated hydroperoxide.

They usually require millimolar concentrations of Ca+^2 , and

their activity is stimulated by ATP (Yamamoto 1992). Lipoxy-

genase has been found to be stable during frozen storage and is

responsible for rancidity development in chicken, especially in

the muscleGastrocnemius(Grossman et al. 1988).

Antioxidative Enzymes

Antioxidative enzymes and their regulation in the muscle consti-

tute a defense system against oxidative susceptibility (i.e., an in-

creased concentration of polyunsaturated fatty acids) and physi-

cal stress (Young et al. 2003). Glutathione peroxidase contains a

covalently bound selenium atom that is essential for its activity.

This enzyme catalyzes the dismutation of alkyl hydroperoxides

by reducing agents like phenols. Its activity has been reported to

be lower in oxidative muscles than in glycolytic muscles (Daun

et al. 2001). Superoxide dismutase is a copper metalloenzyme,

and catalase an iron metalloenyzme. Both enzymes catalyze the

dismutation of hydrogen peroxide to less harmful hydroxides.

These enzymes influence the shelf life of the meat and pro-

tect against the pro-oxidative effects of chloride during further

processing.

PROTEOLYSIS

Proteolysis constitutes an important group of reactions during

the processing of meat and meat products. In fact, proteolysis has

a high impact on texture, and thus meat tenderness, because it

contributes to the breakdown of the myofibrillar proteins respon-

sible for muscle network, but proteolysis also generates peptides

and free amino acids that have a direct influence on taste and

also act as substrates for further reactions contributing to aroma

(Toldr ́a 1998, 2002). In general, proteolysis has several consec-

utive stages (see Fig. 16.3) as follows: (1) action of calpains and

cathepsins on major myofibrillar proteins, generating protein

fragments and intermediate size polypeptides; (2) these gener-

ated fragments and polypeptides are further hydrolzyed to small

peptides by DPPs and TPPs; and (3) dipeptidases, aminopepti-

dases, and carboxypeptidases are the last proteolytic enzymes

that act on previous polypeptides and peptides to generate free

amino acids. The progress of proteolysis varies depending on