Food Biochemistry and Food Processing (2 edition)

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16 Biochemistry of Processing Meat and Poultry 307

Myofibrillar and sarcoplasmic

proteinsproteins

Calpains

Cathepsins

Tenderness

Protein fragments and polypeptides

Tripeptidylpeptidases

Dipeptidylpeptidases

Small peptides

Aminopeptidases

Free amino acids Taste

Chl

Volatile

compounds Aroma

Chemical reactions

Figure 16.3.General scheme of proteolysis during the processing

of meat and meat products.

the processing conditions, the type of muscle, and the amount

of endogenous proteolytic enzymes, as described later.

Proteolysis in Aged Meat and Cooked Meat

Products

During meat aging, there is proteolysis of important myofibril-

lar proteins such as troponin T by calpains, with the associated

release of a characteristic 30 kDa fragment, which is associ-

ated with meat tenderness, nebulin, desmin, titin, troponin I,

myosin heavy chain, and proteins at the Z-line level (Yates et al.

1983). A 95 kDa fragment is also characteristically generated

(Koohmaraie 1994). Examples of cathepsin and aminopeptidase

activity during beef aging are shown in Figures 16.4 and 16.5,

respectively.

The fastest aging rate is observed in chicken, followed by pork,

lamb, and beef. For instance, it has been reported that chicken

myofibrils are easily damaged by cathepsin L, whereas beef

myofibrils are much more resistant (Mikami et al. 1987). The

reasons for this difference are the differences between species

in enzymatic activity, inhibitor content, and susceptibility to

proteolysis of the myofibrillar structure. These factors are also

strongly linked to the type of muscle metabolism, which has

a strong influence on the aging rate (Toldra 2006a). In fact, ́

proteolysis is faster in fast-twitch white fibers (which contract

rapidly) than in slow-twitch red fibers (which contract slowly),

that is, aging rate increases with increasing speed of contraction

but decreases with the level of heme iron (Ouali 1991). Even

though the effect of muscle type is estimated to be tenfold lower

14

16

18

4

6

8

10

12

14

16

Activity (U/g x 1000)

Cathepsin L

Cathepsin H

Cathepsin B

0

2

4

6

0 5 10 15

Time (d)

Figure 16.4.Evolution of muscle cathepsins during the aging of

beef (Toldra, unpublished data). ́

than the effect of temperature, it is threefold higher than animal

effects (Dransfield 1980–1981). There are also some physical

and chemical conditions in postmortem muscle, listed in Tables

16.1 and 16.2, respectively, that can affect enzyme activity. Of

these conditions, the most significant are pH, which decreases

once the animal is slaughtered, and osmolality, which increases

from 300 to around 550 mOsm within 2 days, due to the release of

ions to the cytosol. The osmolality has been observed to be higher

in fast-twitch muscle, which also experiences a faster aging

rate (Valin and Ouali 1992). Age of the animal also decreases

the aging rate, because collagen content as well as the cross-

links that make the muscle more heat stable and mechanically

resistant, increase with age.

Proteolysis in Fermented Meats

The progressive pH decrease by lactic acid (generated during

the fermentation by lactic acid bacteria), the added salt (2–3%),

5

6

7

8

9

Alanyl

AB

0

1

2

3

4

0 5 10 15

Activity (U/g)

Alanyl

Aap B

Methionyl

0

1

0 5 10 15

Time (d)

Figure 16.5.Evolution of muscle aminopeptidases during the aging

of beef (Adapted from Goransson et al. 2002).