Food Biochemistry and Food Processing

14 Biochemistry of Processing Meat and Poultry 325

tively, due to their good activity, stability, and an
optimal pH near that in ham. The other peptidases
would play a minor role (Sentandreu and Toldrá
2002). The generation of free amino acids during the
processing of dry-cured ham is very high (Toldrá
2004b). Alanine, leucine, valine, arginine, lysine, and
glutamic and aspartic acids are some of the amino
acids generated in higher amounts. An example of
generation is shown in Figure 14.7. The final con-
centrations depend on the length of the process and
the type of ham (Toldrá et al. 2000). Based on the
specific enzyme characteristics and the process con-
ditions, alanyl and methionyl aminopeptidases ap-
pear to be the most important enzymes involved
in the generation of free amino acids, while arginyl
aminopeptidase would mostly generate arginine and
lysine (Toldrá 2002).

NUCLEOTIDE BREAKDOWN

The disappearance of ATP is very fast; in fact, it
only takes a few hours to reach negligible levels.
Many enzymes are involved in the degradation of
nucleotides and nucleosides, as described in Chapter

13. The main changes in the nucleotide breakdown
    products occur during a few days postmortem, as
    shown in Figure 14.8. So, adenosine triphosphate
    (ATP) and adenosine monophosphate (AMP), which
    are intermediate degradation compounds, also dis-

appear within 24 h postmortem. Inosine monophos-

phate (IMP) reaches a maximum by 1 day post-

mortem, but some substantial amount is still recov-

ered after 7 days postmortem. On the other hand,

inosine and hypoxanthine, as final products of these

reactions, increase up to 7 days postmortem (Batlle

et al. 2001).

GLYCOLYSIS

Glycolysis consists in the hydrolysis of carbohy-

drates, mainly glucose, either that remaining in the

muscle or that formed from glycogen, to give lactic

acid as the end product. As lactic acid 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 be-

cause pH affects numerous chemical and biochemi-

cal (all the enzymes) reactions. There are many en-

zymes involved in the glycolytic chain; some of the

most important are phosphorylase, phosphofructoki-

nase, pyruvic kinase, and lactate dehydrogenase

(Demeyer and Toldrá 2004). Lactate dehydrogenase

Figure 14.7.Example of the generation of some free amino acids during the processing of dry-cured ham. (Adapted
from Toldrá et al. 2000.)