Food Biochemistry and Food Processing

322 Part III: Muscle Foods

as well as the cross-links that make the muscle more
heat stable and mechanically resistant, increase with
age.

PROTEOLYSIS INFERMENTEDMEATS

The progressive pH decrease by lactic acid (generat-
ed during the fermentation by lactic acid bacteria),
the added salt (2–3%), and the heating/drying condi-
tions during the fermentation and ripening/drying
affect protein solubility. The reduction may reach
50–60% in the case of myofibrillar proteins and
20–47% for sarcoplasmic proteins (Klement et al.
1974). There is a variable degree of contribution to
proteolysis by both endogenous proteases and those
of microbial origin. This contribution mainly
depends on the raw materials, the type of product,
and the processing conditions. The pH drop during
the fermentation stage is very important. So, when
pH drops below 5.0, the proteolytic activity of endo-
genous cathepsin D becomes very intense (Toldrá et
al. 2001). Several myofibrillar proteins, such as my-
osin and actin, are degraded, and some fragments of
135, 38, 29, and 13 kDa are formed. The major role
of cathepsin D has been confirmed in model systems
using antibiotics and specific protease inhibitors in
order to inhibit bacterial proteinases or other endo-
genous muscle proteinases (Molly et al. 1997). A

minor role is played by other muscle cathepsins

(B, H, and L) and bacterial proteinases. Peptides and

small protein fragments are produced during fermen-

tation, heating (smoking), and ripening. The genera-

tion of free amino acids, as final products of proteol-

ysis, depends on the pH reached in the product

(as aminopeptidases are affected by low pH values),

concentration of salt (these enzymes are inhibited or

activated by salt), and processing conditions (time,

temperature, and water activity that affect the en-

zyme activity) (Toldrá 1998). All these factors affect

the contribution of the different aminopeptidases,

as described in Tables 14.1 and 14.2; thus, the pH

reached at the fermentation stage (pH 5.0) is deci-

sive because it reduces substantially both muscle

and microbial aminopeptidase activity (Sanz et al.

2002). Finally, it must be taken into account that

some microorganisms, grown during fermentation,

might have decarboxylase (an enzyme able to gener-

ate amines from amino acids) activity.

PROTEOLYSIS INDRY-CUREDHAM

The analysis of muscle sarcoplasmic proteins and

myofibrillar proteins by sodium dodecyl sulfate

(SDS)-polyacrylamide electrophoresis reveals an

intense proteolysis during the process. This proteol-

ysis appears to be more intense in myofibrillar than

Figure 14.5.Evolution of muscle aminopeptidases during the ageing of beef. (Adapted from Goransson et al. 2002.)