Contributions of blood and blood components to lipid oxidation in fish muscle
There was wide variation in hemoglobin levels extracted from muscle tissue of bled and unbled fish. Averaged values indicated that bleeding removed 23–44% of the hemoglobin. This suggested that the residual blood level in muscle of bled fish was high. There also was a wide variation in sensory scores and TBARS values from bled and unbled fish stored at 2°C. Bleeding significantly reduced rancidity in minced trout whole muscle, minced mackerel light muscle, intact mackerel dark muscle, but not minced mackerel dark muscle stored at 2°C. The reduction was in the number of fish that had longer shelf-life; the shelf-life of some bled fish was not improved. Hemoglobin levels in tail-bled and gill-bled trout were not significantly different. Evidence of hemoglobin subunits in extracts was observed.
Hemoglobin could account for all of the lipid oxidation capacity of whole blood added to washed cod muscle. The evidence suggested that the active form of hemoglobin had the heme iron intact. When a critical level of hemoglobin was present, breakdown of lipid hydroperoxides occurred that was accompanied by oxidation odor. At a low level of preformed lipid hydroperoxides (3.6 μM), reduced hemoglobins stimulated lipid hydroperoxide formation, whereas methemoglobin did not. This suggests that reduced hemoglobins played an important role in the early stages of lipid oxidation. Lowering pH in the range of 7.6 to 6.0 greatly accelerated lipid oxidation induced by reduced hemoglobins. The sharp decrease in hemoglobin oxygenation in this pH range suggested a possible role for deoxyhemoglobin. Hemoglobin autoxidation also occurred more rapidly at pH 6.0 compared to 7.2. Methemoglobin promoted oxidation of washed cod lipids equally at pH 6.0 and 7.6. Increasing the level of preformed lipid hydroperoxides activated methemoglobin as a catalyst when linoleic acid was the lipid substrate.
In addition to increased pH, blood plasma, ascorbate, and prevention of erythrocyte lysis inhibited hemolysate-induced lipid oxidation. The water-extractable fraction of cod muscle was highly inhibitory at pH 6.7. Finally, adding in-situ levels of trout blood to reduced-lipid washed cod caused strong rancid odor to develop during 2°C storage in samples containing no more than 0.1% lipid.