370 Part III: Muscle Foods
causes dehydration and solute concentration, and
thus disturbs the conformation of protein, leading to
protein denaturation. Lipid degradation and enzy-
matic degradation of TMAO during freezing affect
the textural and sensory properties of frozen sea-
food. Therefore, it is important to understand the
mechanisms of various biochemical and physico-
chemical reactions occurring during freezing and
frozen storage so that the quality of frozen food can
be maintained.
BIOCHEMISTRY OF DRIED,
FERMENTED, PICKLED, AND
SMOKED SEAFOOD
Salting, fermenting, marinating (pickling), drying,
and smoking of fish and marine invertebrates in-
crease the shelf life and develop in the products’ de-
sirable sensory properties. Extension of storage life
is achieved mainly through the combined effects of
- A reduction in water activity from the addition of
salt, - A decrease in microbial load by the application
of heat, - The presence of inherent preservatives such as
acetic acid - The use of chemical preservatives such as
ascorbic acid, BHA, and BHT, and - The antibacterial and antioxidant activities of
various smoke components.
The enzymatic and spoilage processes are con-
trolled not only by these chemical components, but
also by the temperature, pH, and availability of oxy-
gen. Desirable changes in sensory properties are thus
developed as a result of these carefully controlled
chemical and enzymatic processes (Doe and Olley
1990, Fuke 1994, Haard 1994, Miler and Sikorski
1990, Shenderyuk and Bykowski 1990, Sikorski and
Ruiter 1994, Shewan 1944, Perez-Villarreal 1992).
There are some changes in proteins. In the salting
of fish, salt will penetrate slowly into the tissues, af-
fecting the stability of the native proteins and re-
ducing their extractability. Heavily salted fish, when
compared with less salted fish, has disadvantages:
- More water loss due to osmosis,
- Tougher texture, and
- Less developed flavor.
It should be remembered that enzymes are also
proteins, and their activities are affected by salt con-
centration.
Another effect of salting is the changes in texture
of the final product. It is believed that the calcium
and magnesium ions present as impurities in salt
may penetrate the fish, giving rise to a soft, “mushy”
texture in the fillet. This may be undesirable for
most salted fish, but this effect is considered highly
desirable in some Chinese salted products (tender
salted threadfin, tender salted mackerel, and others),
and Scandinavian products (such as kryddersild,
tidbit, and gaffelbiter) (Shewan 1944). Salting of
whole fish should be controlled precisely, as over-
ripening will result in excessively soft products with
sensory properties that are undesirable to most con-
sumers. However, changes in salted fish fillets de-
pend mostly on endogenous muscle proteases.
Fermented fish paste and sauce are popular prod-
ucts prepared and consumed in southern China and
Southeast Asian countries as a source of nutrients
and as condiments. Generally, whole fish or shrimp
are used as the raw materials for the preparation of
these products. It is believed that extensive proteoly-
sis occurs under carefully controlled conditions. It is
difficult to differentiate the endogenous and bacteri-
al actions of proteolysis because of the use of whole
fish or shrimp and the way these products are
produced. Sikorski and Ruiter (1994) summarized
some of the work on proteolysis in fermented fish
products. Cathepsins A and C as well as trypsin-like
enzyme are endogenous proteases that appear to
contribute to the fish sauce production both in yield
and quality (Orejana and Liston 1982, Raksakul-
kthai et al. 1986, Rosario and Maldo 1984).
Chiou et al. (1989) reported that cathepsin D–like
and aminopeptidase activities release a large amount
of free amino acids during roe processing and con-
tribute to the flavor. In the drying of squid, endoge-
nous cathepsin C appears to contribute to desirable
qualities (texture and flavor) of traditional products
(Haard 1983). Simpson and Haard (1984) reported
that added trypsin appears to be a key enzyme con-
tributing to the texture and flavor of matjes herring.
Marinating fish (mainly herring) by means of salt
and acetic acid is one of the oldest ways of preserv-
ing food in European countries. The acid condition
of the marinades, with pH 4–4.5, makes the tissue
cathepsins much more active. This results in the
degradation of muscle proteins into peptides and