An improved method for determining the thiobarbituric acid-reactive substances (TBARS) in fish tissue has been described. The biological sample was digested in acidic media and then separated by using specific reflux distillation. The compounds in distillate were estimated by spectrophotometric measurement at 538 nm after the reaction with 2-thiobarbutanic acid. The operational errors, interferences and the recovery of malonaldehyde standard and TBARS from tissue sample using the described procedure have been studied. The recommended technique is simple and reproducible with a detection limit of 0.2 nmol TBARS per 10 g of tissue sample and an overall deviation of less than 7%. The recommended method has been applied satisfactorily for evaluation of rancidity studies in various fish samples.
Kinetic variations in lipid oxidation in various parts of mackerel stored at -15, -30 and -40°C were studied by measuring the changes of POV and of TBA molar values as a function of time. Oxidation developed in the skin sample (subcutaneous fat) was found to be eight times faster in TBA change than in the white and dark muscles from mackerel held at -15°C for two months. This rapid development of rancidity in the skin was effectively inhibited by lowering the frozen storage temperature to -4OoC, where relatively the degree of lipid oxidation in the dark muscle was found to be of the same order of magnitude. The in vitro rate of autoxidation at 60°C for the lipids extracted from skin and the fillet muscle also showed an unusual difference which indicated that unknown pro-oxidative substances in mackerel skin were fat solvent extractable. The activity of the unknown compounds in the skin was also temperature dependent even in the frozen state, but could be effectively mitigated by lowering temperature to -40°C. Lipid hydrolysis in mackerel was also retarded completely when the fish were stored at -40°C. The comparative compositions of fatty acids and volatile carbonyls were determined in head, skin, white and dark muscle, and suggested an active non-specific oxidation system in the skin fat.
Kinetic effects of added copper, zinc, and iron compounds have been investigated in the oxidation of lipids in mackerel skin and meat at 60 C using a simple weight gain method. Inorganic Fe(II) and Cu(II) were found to be strong catalysts in mackerel lipid oxidation. The meat lipids were particularly sensitive to oxidation in the presence of Fe(II) and Cu(II) below 5 ppm, and increments above this level did not result in a further increase in catalytic influence. The oxidation of skin lipids increased very rapidly with increased Fe(II) and Cu(II) concentrations of < 10 ppm, and slight increases in prooxidative effects were still recorded when additional metals were added. The oxidations catalyzed by hemoglobin and zinc for skin lipids, and hematin for meat lipids, were proportional on a semilogarithmic basis with the increases in the metal catalysts. The slight variations in fatty acids between mackerel skin and meat samples did not seem adequate to explain the rapid oxidation in skin tipids. Thus, we believe that in the skin lipids one or more fat-solvent-extractable prooxidants, alone or associated with trace metals, were present and were responsible for the high susceptibility to oxidation found for this lipid.aThe contents of Cd and Hg (< 0.05 ppm) and Co, Ni, and Mn (< 0.4 ppm) showed no significant differences between skin and meats.bFor the dark meat only. CFor the white meat only.
The formation of potentially “fishy” off flavor components, especially 2,4,7‐decatrienals, in various rancid mackerel oils has been semiquantitatively investigated using preparative thin layer chromatography (TLC) and gas liquid chromatography (GLC) methods. A combination of 2 GLC analyses can be directly employed for free aldehyde analysis. This GLC method is faster and gives a better recovery than the alternative TLC proceeding through the dinitrophenylhydrazone derivatives of the carbonyl compounds. Kinetic relations between decatrienal formation and the degree of autoxidation of polyenoic fatty compounds present in mackerel oil are discussed. The decreases in major polyenoic fatty acids rancid oils, measured by the ratiosurn:x-wiley:0003021x:equation:aocs0349-math-0001or
urn:x-wiley:0003021x:equation:aocs0349-math-0002
can be related to the formation of 2,4,7‐decatrienals and other unsaturated aldehydes.
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