Chemistry Reference
In-Depth Information
combinations post mortem. These factors seem to be mainly responsible for the gradual loos-
ening of the myofibrillar network soon after the formation of
rigor mortis
.
Cathepsins, another group of muscle proteases, are located in lyzosomes. Cathepsins
degrade various myofibrillar components including myosin F-actin and troponin.
8
In connective tissue, located within or between muscles, collagen molecules array to
fibrils of 10-500 nm diameter and of unknown length. The collagen molecules of the fib-
rils are bound together with cross-bridges that are stabile in physiological conditions and
make the fibrils inextensible. The degradation of collagen requires collagenase and another
neutral protease activity. Once cleaved the collagen, fibril fractions can be degraded by a
wide range of proteases even to the peptide and amino acid level. Denatured collagen is
more easily degraded by proteases. However, degradation of native collagen is very slow.
5
Endogenic proteolytic system of meat itself is responsible for tenderization during natural
aging. However, when tenderization is desired to be enhanced or tough pieces of meat need to
be tenderized, plant or microbial enzymes can be added to boost tenderization. Tenderization
with non-meat proteases is discussed in Section 12.4.
12.2.4
Factors in meat processing
12.2.4.1
Heating
Heating causes dramatic changes in meat protein systems. The myofibrillar proteins denature
and thus open up and aggregate, thereby loosing a large proportion of their water binding.
Additionally, there will be an increase of cross-bridges between the filaments, thereby re-
ducing the space between the filaments. Upon heating, the collagen in connective tissue
also denaturizes and shrinks, creating a contractive force within the meat. Collagen also is
partly dissolved by heating, depending on the type of collagen, the age of the animal and
temperature-time-pH of the heating process.
9
The endogenic proteolytic enzymes will also
be denatured, but the muscle proteins become more susceptible to exogenous proteolytic
enzymes. Heating and acid pH renders collagen, resistant to proteolysis in its native state,
more susceptible to proteolysis.
5
12.2.4.2
Water binding
Water binding is the most important technological property in meat and meat products. Good
water binding is generally obtained when the myofibrillar proteins carry high (in most cases
negative) net charge, when there are as little as possible cross-bridges in and between the
structural elements and when the connective tissue membranes are thin and the collagen
fibrils are non-mature.
10
The great cause for water binding and variation therein are interactions between and in
the myofilaments (actin and myosin). The amount of the water bound is determined by the
net charge of the proteins, causing an increase in binding and by the number and strength of
cross-bridges that limit the binding.
11
The swelling depends on pH. Without salt there is a relative maximum of swelling at pH
3.0, a minimum at pH 5.0 (the average isoelectric point of meat proteins) and from there is a
constant increase within the physiological pH range of 5.0-7.5. The water binding increases
also with increasing salt content up to 5% NaCl.
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