Food Biochemistry and Food Processing (2 edition)

BLBS102-c16 BLBS102-Simpson March 21, 2012 10:54 Trim: 276mm X 219mm Printer Name: Yet to Come

310 Part 3: Meat, Poultry and Seafoods

2000

Glutamic acid

Tyrosine

800

1000

1200

1400

1600

1800

Concentration

(mg/100 g dry weight)

Lysine

Valine

Leucine

200

0

400

600

051015

Time (m)

Figure 16.7.Example of the generation of some free amino acids during the processing of dry-cured ham (Adapted from Toldra et al. 2000). ́

accumulates in the muscle, pH falls from neutral values to acid

values around 5.3–5.8. The glycolytic rate, or speed of pH fall,

depends on the animal species and the metabolic status. On

the other hand, the glycolytic potential, which depends on the

amount of stored carbohydrates, gives an indication of ultimate

pH. The pH drop due to the lactic acid accumulation is perhaps

the main consequence of glycolysis, and it has very important

effects on meat processing because pH affects numerous chemi-

cal and biochemical (all the enzymes) reactions (Toldra 2006a). ́

There are many enzymes involved in the glycolytic chain;

some of the most important are phosphorylase, phosphofruc-

tokinase, pyruvic kinase, and lactate dehydrogenase (Demeyer

and Toldr ́a 2004). Lactate dehydrogenase is involved in the last

step, which consists in the conversion of pyruvic acid into lactic

acid. The contribution of glycolysis is restricted to a few hours

postmortem, although it is also important in fermented meats

where sugar is added for microorganism growth (see Chapter

18). The evolution of some glycolytic enzymes is shown in

Figure 16.9.

LIPOLYSIS

Lipolysis makes an important contribution to the quality of meat

products by the generation of free fatty acids, some of which

have a direct influence on flavor, and others that, with polyun-

saturations, may be oxidized to volatile aromatic compounds,

acting as flavor precursors. The general scheme for lipolysis in

muscle and adipose tissue is shown in Figure 16.10. In addition,

the breakdown of triacylglycerols affects the texture of the adi-

pose tissue, and an excess of lipolysis/oxidation may contribute

to the development of rancid aromas or yellowish colors in fat

(Toldr ́a 1998).

Lipolysis in Aged Meat and Cooked

Meat Products

The relative amounts of released free fatty acids depends not only

on the enzyme preference, but also on many other factors such

as raw materials (especially affected by feed composition), type

8

3

4

5

6

7

Concentration (umol/g)

AT P

ADP

AMP

IMP

Inosine

0

1

2

0 5 10 15

Time (d)

IMP

Inosine

Hypoxanthine

Figure 16.8.Evolution of nucleotides and nucleosides during the aging of pork meat. (Adapted from Batlle et al. 2001.)