Lipid oxidation in salted‐dried fish: II. The effect of photosensitisers on the rate of oxidation of a fish oil

Abstract

AbstractThe 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 μl O2g−1for the control to 0.335 μl O2g−1min−1, 0.308 μl O2 g−1min−1 and 0.461 μl O2g−1min−1, 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 β‐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 II; the reduction observed in oxygen uptake rate in the presence of β‐carotene shows the Type II mechanism involving singled oxygen to be important. Studying β‐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.