Protein changes in frozen fish.

Storage of frozen fish brings about a decrease of extractability of myofibrillar proteins. There is also deterioration of the texture and functional properties of the flesh. In model systems, aggregation of myosin, actin, tropomyosin, and whole myofibrils have been described. These changes are caused by concurrent action of partial dehydration due to the freezing out of water, exposure of the proteins to inorganic salts which are concentrated in the remaining nonfrozen fluid, interactions with free fatty acids liberated from phospholipids and with lipid oxidation products, and cross-linking by formaldehyde produced in some species of fish as a result of enzymic decomposition of trimethylamine oxide. The extent of protein alterations increases with time and temperature of storage as well as with advanced disintegration of the tissues and intermixing of their components. The role played by the individual factors and the significance of different types of bonds, i.e., hydrophobic adherences, ionic bonds, and covalent cross-links in particular cases are not yet fully disclosed. Retardation of the deteriorative changes of proteins in frozen fish is possible by avoiding high storage temperatures and oxidation of lipids, removing hematin compounds and other constituents promoting cross-linking reactions, and by adding cryoprotectors like sugars, several organic acids, amino acids, or peptides.

[Protein denaturation in frozen fish]. / Zur Frage der Proteindenaturierung im gefrorenen Fischfleisch

To inhibit the rapid deterioration of the production-technological properties of fish meat minced and frozen on board, it is necessary to become acquainted with the causes of protein denaturation under these conditions. It was found that the protein solubility in codfish is impaired by formaldehyde which develops from trimethyl-amine oxide during storage and also by the salt content. After 7 days at -20 degrees C, the solubility of the sarcoplasmic and myofibrillary proteins in minced fish meat added with 80 mg of formaldehyde per 100 g of total protein amounted to almost 70% and 35%, respectively, of the value determined in controls, i.e., samples without formaldehyde. After this period of storage, only 30% of the added formaldehyde were in the free state. After 1 month, the solubility of the myofibrillary proteins in waterleached fish meat was by 30% higher than in unleached controls, 80 mg of formaldehyde per 100 mg of total protein having been added in both cases. After 1 month at -5 degrees C and -20 degrees C, only 90 and 15 p.p.m. of formaldehyde, respectively, were found in minced fish meat, whereas leached samples contained no formaldehyde. The solubility of the myofibrillary proteins in leached fish meat (the initial salt content of which had been restituted) was by 30% lower than in the unleached controls.