Freezing Technology

1.2.1.2. Proteins

2.1.2. Proteins

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

1. Structural proteins (actin, myosin, tropomyosin and actomyosin), which constitute 70-80% of the total protein content (compared with 40% in mammals). These proteins are soluble in neutral salt solutions of fairly high ionic strength (> 0.5M).
2. Sarcoplasmic proteins (myoalbumin, globulin and enzymes) which are soluble in neutral salt solutions of low ionic strength (<0.15M). This fraction constitutes 25-30% of the protein.
3. Connective tissue proteins (collagen), which constitute approximately 3% of the protein in teleost and about 10% in elasmobranch (compared with 7% in mammals).

 The majority of the sarcoplasmic proteins are enzymes participate in the cell metabolism, such as the anaerobic energy conversion from glycogen to ATP. If the organelles with in the muscle cells are broken, this protein fraction may also contain the metabolic enzymes localized inside the endoplasmic 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. The proteins in the sarcoplasmic fraction are excellently suited to distinguish between different fish species, as all the different species have their characteristic band pattern, when separated by the isoelectric focusing method. Fish protein is an excellent source of all essential amino-acids. In diets based mainly on cereals, a supplement of fish can raise the biological value significantly.