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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.
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.
Salting is a processing treatment used either to provide a salty flavor or to impart storage stability by decreasing water activity. Optimum salt content can enhance overall flavor acceptability and is a major factor in safe preservation of smoked fish. In this study, rate of salt penetration into fresh or frozen and then thawed fish muscle was studied by dipping mullet fillets in brine. Salt penetration curves resemble a first order change [X = C(1 -emkt)]. Initial salt penetration rate (g salt/g sample/ min) and rate constant (min-') increased respectively from 0.006 and 0.018 for fresh fish to 0.014 and 0.029 after freezing (frozen for 1 wk), then decreased to 0.011 and 0.025 after 3 wk of frozen storage, and leveled off at 0.009 and 0.018 from 5-9 wk of storage. The change in salt penetration rates closely followed changes in extractable actomyosin in muscle, indicating a dependence of the change on the degree of denaturation of fish muscle proteins. The effects of brine concentration and frozen storage on water transfer were also studied. Water migrated from the brine into the flesh if fresh mullet fillets were dipped in brine at concentrations up to 15%. When the brine concentration was 20% or higher, the water migated from fish muscle to the brine. However, after 2 months frozen storage, fish muscle gained water if dipped in brine of 20% or below and lost water when the brine was 25% or higher.
Carotenoids and colour characteristics were studied in raw, steamed and smokecured muscle of rainbow trout fed synthetic astaxanthin (50 or 100 mg kg-' diet) or canthaxanthin (100mg kg-' diet) for 15 months.In raw muscle main carotenoid concentration was strongly correlated with hue, chroma, and lightness. Diet and sex effects were significant on most traits, with little or no interaction. Female muscles contained much more carotenoid and were more coloured than male ones. Astaxanthin fed fish were more red than canthaxanthin fed fish.Significant mean effects of cooking were an apparent increase of carotenoid concentration and an increase of lightness. Hue and chroma were practically unchanged with high correlations between raw and cooked muscle measurements. Lightness increased and differences between sexes and individuals were reduced.Smoke-curing induced an increase of carotenoid concentration and a decrease of hue and lightness. Hue of canthaxanthin fed fish when smoke-cured was similar to that of raw fish fed astaxanthin. Lightness, still dependant on that of the raw muscle, was reduced and differences among diets, sexes and individuals were reduced.
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