Food Biochemistry and Food Processing (2 edition)

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16 Biochemistry of Processing Meat and Poultry 309

inhibited or activated by salt), and processing conditions (time,

temperature, and water activity (aw) that affect the enzyme

activity) (Toldr ́a 1998). All these factors affect the contribution

of the different aminopeptidases, as described in Tables 16.1

and 16.2; thus, the pH reached at the fermentation stage (pH<

5.0) is decisive because it reduces substantially both muscle and

microbial aminopeptidase activity (Sanz et al. 2002). So, the

processing environmental conditions at the beginning of drying

are very important for the accumulation of free amino acids in

the final products (Roseiro et al. 2010).

Finally, it must be taken into account that some microorgan-

isms, grown during fermentation, might have decarboxylase (an

enzyme able to generate amines from amino acids) activity.

Proteolysis in Dry-Cured Ham

The analysis of muscle sarcoplasmic proteins and myofibrillar

proteins by sodium dodecyl sulfate–polyacrylamide gel elec-

trophoresis reveals an intense proteolysis during the process.

This proteolysis appears to be more intense in myofibrillar than

in sarcoplasmic proteins (Toldra and Aristoy 2010). The pat- ́

terns for myosin heavy chain, myosin light chains 1 and 2, and

troponins C and I show a progressive disappearance during the

processing (Toldr ́a et al. 1993). Several fragments of 150, 95,

and 16 kDa and in the ranges of 50–100 kDa and 20–45 kDa are

formed (Toldr ́a 2002). The analysis of ultrastructural changes

by both scanning and transmission electron microscopy shows

weakening of the Z-line as well as important damage to the

fibers, especially at the end of salting (Monin et al. 1997). An

excess of proteolysis may create unpleasant textures because of

intense structural damage. The result is a poor firmness that is

poorly rated by sensory panelists and consumers. This excess of

proteolysis is frequently due to the breed type and/or age, which

have a marked influence on some enzymes, or just a higher level

of cathepsin B activity (Toldra 2004a). A high residual cathepsin ́

B activity and/or low salt content, a strong inhibitor of cathepsin

activity, are correlated with the increased softness. The action of

calpains is restricted to the initial days of processing due to their

poor stability. Cathepsin D would contribute during the initial 6

months, and cathepsins B, L, and H, which are very stable and

have an optimal pH closer to that in ham, would act during the

full process (Toldr ́a 1992). An example of the evolution of these

enzymes is shown in Figure 16.6.

Numerous peptides are generated during processing: mainly

in the range 2700–4500 Da during postsalting and early ripen-

ing, and below 2700 Da during ripening and drying (Aristoy and

To l d r ́a 1995). Some of these peptides have been generated from

specific myofibrillar and sarcoplasmic proteins degradation. Pro-

teomic tools have been used for the identification of long-chain

peptides resulting from proteolysis of actin (Sentandreu et al.

2007), titin, and light-chain myosin I (Mora et al. 2009a) and

creatine kinase (Mora et al. 2009b). These peptides are object of

further research for its potential bioactivity and contribution to

the nutritional properties of dry-cured ham (Jim ́enez-Colmenero

et al. 2010). Some of the smaller tri- and dipeptides recently have

been sequenced. DPP I and TPP I appear to be the major enzymes

involved in the release of di- and tripeptides, respectively, due

12

14

Cat B

4

6

8

10

12

14

Activity (U/g

× 1000)

Cat B

Cat B+L

Cat H

Cat D

0

2

4

0 5 10 15

Time (m)

Figure 16.6.Evolution of cathepsins during the processing of

dry-cured ham (Toldra, unpublished data). ́

to their good activity, stability, and an optimal pH near to that in

ham. The other peptidases would play a minor role (Sentandreu

and Toldra 2002). The generation of free amino acids during the ́

processing of dry-cured ham is very high (Toldra 2004b). Ala- ́

nine, 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 16.7. The final

concentrations depend on the length of the process and the type

of ham (Toldr ́a et al. 2000). On the basis of the specific enzyme

characteristics and the process conditions, alanyl and methionyl

aminopeptidases appear to be the most important enzymes in-

volved in the generation of free amino acids, while arginyl

aminopeptidase would mostly generate arginine and lysine

(Toldr ́a 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 15. The main changes in the nucleotide breakdown

products occur during a few days postmortem, as shown in Fig-

ure 16.8. So, adenosine triphosphate (ATP), adenosine diphos-

phate, and adenosine monophosphate, which are intermediate

degradation compounds, also disappear within 24 hours post-

mortem. Inosine monophosphate reaches a maximum by 1 day

postmortem, but some substantial amount is still recovered after

7 days postmortem. On the other hand, inosine and hypoxan-

thine, as final products of these reactions, increase up to 7 days

postmortem (Batlle et al. 2001).

GLYCOLYSIS

Glycolysis consists in the hydrolysis of carbohydrates, mainly

glucose, either that remaining in the muscle or that formed from

glycogen, to give lactic acid as the end product. As lactic acid