Strategies to prevent hemoglobin-mediated lipid oxidation in fish

During the last 20 years, numerous studies have revealed how hemoglobin (Hb) strongly controls development of lipid oxidation in fish muscle; more so than the total lipid content or the relative amount of polyunsaturated fatty acids. In Chalmers, three main routes have been explored to combat Hb-mediated lipid oxidation in fish muscle; (i) preventing lysis of the red blood cells (RBC), (ii) removing Hb during processing, or (iii), introducing natural antioxidants. Key results from these strategies will be revealed. In (i), which is mainly applicable to very early steps of the fish process chain, we have found for example that avoiding osmotic or mechanic pressure as well as temperature raises keeps the RBC intact. Important RBC-stabilizing factors were also identified in blood plasma of herring and trout. In terms of Hb-removal, washing or pH-shift processing of different types of fish raw materials have been studied. Among findings are that repeated washing of fish mince (3×3 volumes), e.g. in surimi production, can remove nearly 85% heme and yield relatively stable minces. On the other hand, limited washing (1×1 volumes) destabilizes fish mince despite >60% heme removal, pointing at wash-out of antioxidants counteracting the effect of Hb removal. Work with antioxidants has been done both in simpler models such as washed fish mince and with more complex systems as filleting rest raw materials. Particularly interesting results have been found when using bladderwrack (Fucus vesiculosus) or rosemary extracts as well as grape or lingonberry press cakes. Extracts/solutions provides the possibility to apply antioxidants onto the surface of intact fish tissue via dipping or incubation treatments which is easy to implement in industry. The solid materials can on the other hand be co-processed with fish (rest)raw materials proving a sustainable concept to valorize different food side streams while at the same time preventing oxidation.