The relationship between the malonaldehyde formed as oxidation product of lipids, and changes of proteins in frozen Baltic herring (Clupea harengus var. membranus) was studied. Changes occurred in the muscle proteins due to freezing, so that water-binding capacity, solubility, and the amount of free epsilon-amino groups in these proteins decreased. A close relationship was found between the amount of malonaldehyde and changes of proteins in the different phases of storage. The reaction of malonaldehyde with the free amino groups of the proteins may be one of the factors on which the changes of proteins during frozen storage are based.
Changes in the quality of freshly caught fish are chiefly caused by autolysis and bacterial decomposition. The rate of autolysis and spoilage depends very much on the temperature. Freezing prevents or at least delays these processes and thus frozen fish can be kept in good condition for long periods a t low temperatures. However, freezing and subsequent storage may effect changes of another kind which are undesirable and may make the fish less palatable. These changes vary depending on the species, the freshness and pretreatment of the fish, the freezing methods, and storage conditions. One of the deteriorations in question involves primarily an irreversible change in the protein fraction of frozen fish. This change results in a drier muscle with a coarser textwe. The cause of the change seems to be a denaturation of the proteins, which lose their ability to swell and retain the muscle juice and to return to their original condition after defrosting. No complete explanation of fish protein denaturation caused by freezing is forthcoming at present in spite of the numerous investigations conducted in these fields, Reay (9, lo), Reay and Kuchel ( l l ) , Notevarp and Heen (6, 7), Dyer, French and Snow ( 3 ) , Snow (12), and Dyer (2).The conversion of native protein to denaturated protein can be brought about by various means, for example, by a high or low temperature. A protein denatured so that it is insoluble in water or in dilute salt solution undergoes a change in a number of its other properties. I n this investigation the term denaturation has been used to denote loss in brine extractibility of the myosin fraction. By electrophoresis, by ultracentrifugation and by other methods this fraction has been subdivided further. Our present view of the fundamental nature and behavior of these substances is primarily based on the work of the Szent-Gyorgyi and Dubuisson schools. Actually, however, the results and conclusions of these workers are not uniform in all details. Furthermore, the nomenclature of muscle proteins is confusing. For convenience, the fraction extracted by a salt solution and precipitated on dilution with water is called myosin in this paper. METHODSThe soluble proteins from fillets of Baltic herring (Clupea harengw var. menbranus) were extracted at 0"G. with a 5% sodium chloride solution according to the method described by Dyer, French and Snow (3). The myosin present in the brine extract was precipitated by diluting 10-fold with distilled water at 0-5°C. After standing overnight in the refrigerator, the myosin was centrifuged off and the myosin nitrogen was determined by the micro-Kjeldahl modification described in the A.O.A.C. (1). I n agreement with the results of Hamoir ( 4 ) , no myosin was observed in the electrophoretic pattern of extracts dialyzed against a buffer of ionic strength, p = 0.11 and pH 7.2, and therefore the precipitation of myosin was considered complete. Total nitrogen and soluble 200
Problems of freezing and frozen storage of fish are very much like those for other frozen foods such as meat and ponltry. During cold storage these products undergo changes of two general types, chemical and physical. Chemical changes include those brought about by the action of bacteria, by naturally occurring enzymes, by oxidative and hydrolytic processes in the fats and oils, and by denaturation of proteins. The principal physical changes are ice crystal formation and desiccation or drying out of the flesh.The main effect of freezing and frozen storage on the quality of fish is a change in texture which is connected with the accompanying rearrangement and coagulation of the proteins in the muscle. The tissues become tough because of the denaturation of the proteins. The present investigators (6) have shown that thawing and refreezing also alter the state of the muscle proteins (myosin), making them more liable t o undergo denaturation during frozen storage, defrosting, and subsequent storage.The theory of quick freezing is based on the observation that there is a critical zone at about -4 to -10°C. in which rapid denaturation occurs ( 3 ) ; this temperature zone should be passed as rapidly as possible during the freezing process. However, the rate of freezing is not as important aa it wm considered t o be formerly (2, 9,10,11). Reay (10) concludes that the freshness of fish and the temperature of storage are more important, the rate of freezing being a minor factor. On the other hand, the present investigators (6) have found that freezing of fish during rigor mortis makes myosin more prone to undergo denaturation during cold storage and defrosting than when the fish are frozen after the resolution of PigoT mortis. This conclusion is confirmed by Marsh ( 5 ) , who found that freezing of meat before t*igor mortis is completed can lead to abnormal behavior on thawing. Under all circumstances there is a definite change in some characteristics of the fresh fish after freezing and subsequent thawing, even if we must admit that the changes occurring during storage are more serious.This paper deals with problems concerning the quality of products that arise in the cutting, filleting, packaging, freezing, and frozen storage of fish. METHODSBaltic herring (Clupen harengus var. membranus) was used in this study. Fish or fillets packed in waxed cartons were frozen in a freezer with forced air circulation or in a metal contact freezer at -35°C. The frozen fish were stored at -2O"C., defrosted after suitable intervals in pliofilm bags during 7 hours at room temperature, and then examined for quality. The quality of the fish was estimated by determining the myosin denaturation using the procedure described previously by the authors (6,7). The details of preparing the fish, packaging, etc. are reported in connection with the results. 42
SUMMARY Solubility measurements have shown that the “myosin” of Baltic herring is denatured rapidly in situ even in a 2% sodium chloride solution at 0°C. This denaturation is inhibited by various alkali phosphates and citrate. A higher concentration of sodium chloride requires a higher concentration of phosphate to prevent denaturation by the former. This action of alkali phosphates varies with the pH of the solution, probably as a consequence of changes in the net charge of the proteins and in the dissociation of the phosphates. Possible mechanisms of the inhibiting effect of the phosphates and citrates on the denaturation by sodium chloride are discussed.
1. Serratamic acid, m.p. 1380 (decomp.), is produced by several strains of organisms of the Serratia group. 2. Serratamic acid is readily hydrolysed by both mineral acids and alkalis. Acid hydrolysis yields L-serine and an acid, ClOH203, tentatively identified as a hydroxydecanoic acid, which is oxidized to octanoic acid or one of its isomers. 3. The linkage between the amino acid and hydroxy acid appears to be of the peptide type. 4. Serratamic acid does not account for the antibiotic activity of strains of S. mar"escens producing this acid. ,5. The growth of organisms of the Serratia group to produce serratamic acid affords a convenient source of L-serine.
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