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Summary Frozen fillets from cod which had been starved in an aquarium developed less cold store flavour or odour at −10°C than was developed by fed controls. Cis-4-heptenal, the major compound responsible for the off-flavour, was found to be present at a much lower level in the starved fish. At the same time the relative proportions of phospholipids and of the most important polyunsaturated fatty acids were found to be reduced by starvation. It was concluded that selective mobilization of precursors reduced the formation of the flavour compound during subsequent frozen storage. A seasonal variation in cold store flavour (and odour) therefore seems almost inevitable.

The post-mortem pH of cod muscle, which strongly influences its attributes as a food, is subject to seasonal variation. For most of the year the average of a batch lies above about 6.7, but for a short time in the summer it suddenly drops to low values, rising again just as suddenly. It is now thought that heavy feeding immediately after starvation causes this sharp fall. Fillets from fish frozen at such a time gape and have a tough texture after cooking. The low-pH period is fairly short, so cod are in fact suitable for freezing for all of the year outside certain summer months. The phenomenon has been observed in cod from a wide range of fishing grounds. Haddock show no such seasonal variation, but their post-mortem pH is lower on average than that of cod.

The post-mortem pH value of the muscle of cod, Gadus morhua L., is shown to be a valid indicator of the muscle glycogen level after rapid capture, stunning and rapid sampling (a level thought to be little different from that current before capture). Good correlations between initial glycogen values and post-mortem pH were obtained in fish of a wide range of nutritional states comprising fed, starved for different periods, and refed on different diets after starvation. Inconsistencies in results published in the literature are attributed to variations in the initial glycogen values as measured, which in turn stem from the fish struggling on capture, differing degrees of which reduce muscle glycogen by different amounts but probably do not affect the post-mortem pH. Variations in the time taken to sample the glycogen would also affect the results.

Revue bibliographique concernant les etudes de texture et d'ultrastructure et le comportement a l'homogeneisation a la congelation, a la decongelation, de chair de poisson (cabillaud). Analyses au niveau tissulaire et moleculaire (proprietes des proteines)

Fish to be used as food are judged on the nature of the texture, flavor, odor, surface appearance, and color of the fillet. Investigations on Atlantic cod (Gadus morhua) have shown that all of these attributes can vary according to the season and the place where the fish were caught.Factors that influence one or more of the attributes are: firstly, the postmortem pH of the muscle, itself influenced by the supply of food and in some way by the body length; secondly, the lipid content, also influenced by feeding; and thirdly, the swimming activity of the fish.

1. In the early stages of starvation in cod (Gadus morhua L.), liver lipids and the glycogens of liver and white muscle were mobilised simultaneously, but liver lipids were exhausted first. At about that point, proteins began to be mobilised in red and white muscle, and red muscle glycogen also began to decrease. After about 20 weeks at 9°C, the latter was virtually exhausted, leaving muscle proteins as the only source of energy. 2. Starvation caused a significant fall in the glucose, lipids and nonesterified fatty acids (NEFA) of the blood. Ketone bodies seemed to be unimportant as sources of energy in starving cod, in contrast to elasmobranchs and mammals. Little change was observed in blood proteins or ninhydrin-positive substances. 3. When the cod were refed after starvation, overcompensation occurred in several metabolites, levels rising temporarily to values much higher than in continuously-fed controls. These comprised the glycogen contents of liver, red muscle and white muscle, NEFA in plasma and the RNA/DNA ratios of liver, red muscle and white muscle. Other constituents simply increased until their levels approached those of the controls. Liver lipids during refeeding did not rise above the low levels of starvation until the water content of the white muscle had dropped below about 82%. 4. When fish were refed after a shorter starvation period with squid muscle or herring muscle, only those refed with squid showed a significant rise in the muscle glycogen concentrations. Those fed on herring increased their liver lipids the more. 5. The differing periods of refeeding which gave rise to maximum values of red muscle glycogen and white muscle glycogen coincided respectively with maximum RNA/DNA ratios, suggesting that the purpose of unusually high glycogen values was to supply the energy for muscle regeneration.

The viscosity of cod muscle, homogenised in 65% sucrose solution, was measured after the fish had been cold stored at-15° or -30° for various periods. The results differed according to the locality where the fish were caught, probably because of variations in the post-rigor pH of the muscle arising from differences in nutritional state. It was found that the values decreased during cold storage, and it was concluded that the method showed promise for assessing the texture of the fish after thawing and cooking.

SUMMARY Earlier work has shown that when the concentration of tissue salts in liquid solution in frozen cod is increased by freezing at a certain rate, the rate of denaturation of the proteins of the flesh is enhanced during subsequent cold storage. The present work is concerned with conditions under which the proportion of ice and concentration of tissue salts decreases, and with the subsequent behavior of the muscle in cold storage. Cod were frozen before the onset of rigor mortis, vacuum-dried at –29°, embedded in paraffin and histologically sectioned. The ice crystals showed up as holes. Measurements of the areas of muscle and ice in photographic enlargements of the sections showed that the pre-rigor frozen fish formed relatively less ice than post-rigor frozen fish under the same conditions. It was further shown that the pre-rigor fish then underwent less denaturation after cold storage than did the controls. Storage time/denaturation curves at narrowly-spaced temperatures near the melting point showed that the rate of denaturation did not continue to increase as the melting point was approached. Above a certain temperature the rate began to decrease, and this again appeared to be due to the decrease in the proportion of ice, increase in free water, and consequent dilution of tissue salts. The effect of decreasing the amount of ice in the tissue by the use of glycerol, and of the abolition of ice altogether by ultra-rapid freezing, is described.

Collagen in the muscles of fish constitutes the main component of the connective tissue membranes joining individual myotomes and is responsible for the integrity of the fillets. The content of collagen in fish muscles is from about 0.2 to 1.4% and in squid mantel about 2.6%. Fish and invertebrata collagens contain slightly more essential amino acids than intramuscular bovine connective tissue collagen. The invertebrata collagens are exceptionally rich in sugars linked mainly O-glycosidically to hydroxylysine residues. During maturation of fish the proportion of collagen to total protein in the muscles increases while the extent of crosslinking does not change significantly. The thermal properties of fish collagens depend significantly on the content of hydroxyproline and proline residues which in turn is correlated to the temperature of the habitat. Generally the shrinkage temperature of fish skin collagens is about 20 degrees C lower than that of mammalian hide collagens. In several species of fish the weakening of the connective tissues post mortem may lead to serious quality deterioration that manifests itself by disintegration of the fillets, especially under the strain of rough handling and of rigor mortis at ambient temperature. Thermal changes in collagen are the necessary result of the cooking of fish, squid, and minced fish products and contribute to the desirable texture of the meat. However, they may lead to serious losses during hot smoking due to a reduction in the breaking strength of the tissues when heating is conducted at high relative humidity. Because of the high viscosity of gelatinized collagen, it is not possible to concentrate the fish stickwaters, a proteinaceous byproduct of the fish meal industry, to more than 50% dry matter. Better knowledge of the contents and properties of fish collagens could be helpful in rationalizing many aspects of fish processing.

The solubility of Lemon Sole muscle in 5% sodium chloride after storage at — 14 to — 15°C was shown to fall to the same minimum level as that of cod, but the reaction was very much slower. After storage for 5 years the protein had still been fairly soluble, and appeared to have reached the asymptote, but in the present experiment the fish were stored for over 17 years and the reaction was now evidently complete. It is complete in cod at this temperature in about 15 weeks.

Furan-containing fatty acids, which occur widely at a low level in fish lipids, become dominant in the liver lipids of starving cod.