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Proteins from fish and fish products


The proteins in fish muscle tissue can be divided into three groups:

  • structural proteins (actin, myosin, tropormyosin and actomyosin) which constitute 70-80 percent of the total protein content (compared with 40 percent in mammals). These proteins are soluble in neutral salt solutions of fairly high ionic strength (0.5 M);
  • sarcoplasmic proteins (myoalbumin, globulin and enzymes) which are soluble in neutral salt solutions of low ionic strength (<0.15 M). This fraction constitutes 25-30 percent of the protein; and
  • connective tissue proteins (collagen), which constitute approximately 3 percent of the protein in teleostei and about 10 percent in elasmobranchii (compared with 17 percent in mammals).


Structural proteins make up the contractile apparatus responsible for muscle movement. The amino-acid composition is roughly similar to corresponding proteins in mammalian muscle, although the physical properties can differ slightly. When the proteins are denatured under controlled conditions, their properties may be utilized for technological purposes. A good example is the production of surimi-based products in which the gel-forming ability of the myofibrillar proteins is used. After salt and stabilizers are added to a washed, minced preparation of muscle proteins and after a controlled heating and cooling procedure, the proteins form a very strong gel.

The majority of sarcoplasmic proteins are enzymes participating in cell metabolism, such as the anaerobic energy conversion from glycogen to ATP. If the organelles within the muscle cells are broken, this protein fraction may also contain the metabolic enzymes localized inside the endoplasmatic reticulum, mitochondria and lysosomes.

The fact that the composition of the sarcoplasmic protein fraction changes when the organelles are broken was suggested as a method for differentiating fresh from frozen fish, under the assumption that the organelles were intact until freezing. However, it was later stated that these methods should be used with great caution as some of the enzymes are liberated from the organelles during iced storage of fish as well.

The proteins in the sarcoplasmic fraction are excellently suited to distinguishing fish species, as each species has a characteristic band pattern when separated by the isoelectric focusing method.

The chemical and physical properties of collagen proteins are different in tissues such as skin, swim bladder and the myocommata in muscle. In general, collagen fibrils form a delicate network structure with varying complexity in the different connective tissues in a pattern similar to that found in mammals. However, the collagen in fish is much more thermolabile and contains fewer, but more labile, cross-links than collagen from warm-blooded vertebrates.
Different fish species contain varying amounts of collagen in body tissues. This has led to a theory that the distribution of collagen may reflect the swimming behaviour of the species. Furthermore, the varying amounts and varying types of collagen in different fishes may also have an influence on the textural properties of fish muscle.

Fish proteins contain all the essential amino-acids and, like milk, eggs and mammalian meat proteins, have a very high biological value.

Essential amino-acids (percentage) in various proteins

Amino-acid Fish Milk Beef Eggs
Lysine 8.8 8.1 9.3 6.8
Tryptophan 1.0 1.6 1.1 1.9
Histidine 2.0 2.6 3.8 2.2
Phenylalanine 3.9 5.3 4.5 5.4
Leucine 8.4 10.2 8.2 8.4
Isoleucine 6.0 7.2 5.2 7.1
Threonine 4.6 4.4 4.2 5.5
Methionine-cystine 4.0 4.3 2.9 3.3
Valine 6.0 7.6 5.0 8.1

 

 

Cereal grains are usually low in lysine and/or sulphur-containing amino-acids (methionine and cysteine), whereas fish protein is an excellent source of these amino-acids. A supplement of fish can therefore significantly raise the biological value in cereal-based diets.In addition to the fish proteins already mentioned there is a renewed interest in specific protein fractions that can be recovered from by-products, particularly in the viscera. One such example is the basic protein or protamines found in the milt of the male fish, which can contain as much as 65 percent arginine. The best sources are salmonids and herring, whereas ground fish such as cod are not found to contain protamines.

The extreme basic character of protamines makes them interesting for several reasons. As they adhere to most other proteins less basic, they can enhance the functional properties of other food proteins (if all the lipids present in the milt are removed from the protein preparation to avoid off-flavour development in the finished products). But the most promising feature of basic proteins is their ability to prevent growth of microorganisms.

 
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