Abstract:In relation to studies of lipid oxidation during the processing and storage of salted sun-dried fish, the measurement of the initial rate of oxygen uptake has been studied on a model system consisting of a highly polyunsaturated fish oil. This parameter has been used for assessing the effects of temperature and light conditions on the model system and has been compared to changes in the peroxide value and the polyene index. Assessment of oxygen uptake using a Gilson respirometer was found to give a clearer indication than other methods of assessment of the effect of different conditions on fish oil oxidation. At 30°C the initial rate of oxygen uptake increased from 0.103 p10, g-' min-' to 0.1 58 p 1 0 , g-' min-l with a change in light conditions from dark to photosynthesis light. At 40"C, the rate of oxygen uptake was faster than at 30 "C, and the effect of light was more pronounced, giving values of 0.188p10, g-lmin-' and 0.483 p l 0 , g-' min-', for dark and photosynthesis light, respectively. The observed results for the measurement of the initial rate of oxygen uptake suggest its use as a sensitive and reliable method for the investigation of the contribution of the many components in salted sun-dried fish towards the rate of oxidation and subsequent rancidity.
The contribution of photosensitised oxidation to the extent of lipid oxidation of a model fish oil system has been considered using a number of methods of measurement, namely peroxide value, polyene index and the initial rate of oxygen uptake. This represents the initial stages of the development of a model system which will eventually simulate the processing and storage of salted, sun-dried fish. The effects of the photosensitisers, protoporphyrin IX, riboflavin and myoglobin on the rate of oxidation of a highly unsaturated fish oil in a model system, have been studied under dark or photosynthetic light conditions at 30°C. As has previously been established, the initial rate of oxygen uptake proved to be a more reliable method than the peroxide value or the polyene index in determining the extent of lipid oxidation of the oil in the model system. Protoporphyrin IX, riboflavin and myoglobin all increased the rate of oxygen uptake significantly in photosynthetic light at 30°C, from a rate of 0.158 p1 02.g-l' min-' for the control to 0.335 p1 0, g-' min-', 0.308 pI 0, g-' min-and 0.461 pl 0, g-' min-', respectively. However, in the dark at 30°C their addition had no effect on the initial rate of oxygen uptake, showing that these components increased the rate via a photosensitised oxidation mechanism rather than by autoxidation. The addition of B-carotene, a well-established singlet oxygen quencher, to the systems containing the photosensitisers at 30°C in the light significantly reduced the rate of oxygen uptake, but not to the same rate as in the control. Photosensitised oxidation has been classified as Type I or Type 11; the reduction observed in oxygen uptake rate in the presence of Bcarotene shows the Type I1 mechanism involving singlet oxygen to be important.Studying /3-carotene on its own in the model system at 30°C showed an unexpected increase in the rate of oxygen uptake both in the light and in the dark.
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