|dc.description.abstract||Salt (NaCl) is the most established food additive in the world and still widely used for its good preservative effects, sensorial properties and low cost. A large part of the food consumed today contain higher levels of salt than necessary. The industry has been challenged to find strategies to formulate products with reduced sodium content. However, this will lead to challenges regarding taste, safety, texture and physiochemical properties of the product. The focus in this thesis has been on marinated herring (Clupea harengus), which contains high amount of salt. Marinating is one of the oldest preservative methods for fish, and work by lowering of pH and action of salt. Ripening of fish occur in brine, often containing more than 10% salt, for up to 12 months at chilling temperature. During the long ripening period, chemical and biochemical changes take place, leading to formation of the characteristic taste and texture of marinated herring. Both the presence of salt and storage history are relevant for the stability and quality of the product. The herring fillets contain high amounts of polyunsaturated fatty acids, which are susceptible to oxidation following successive degradation.
The main objective of this thesis was to evaluate to which extent lipid oxidation and lipid hydrolysation are affected by different salt concentrations and salt types (NaCl and KCl) in marinated herring during storage of 35 days and 10 months at -4oC. Two different storage experiments with marinated herring were analysed. The experiments differed in concentration of salt in the marinades, and in time of catch (January and October). In this thesis, the stability of lipids during storage was evaluated by following the development of lipid oxidation by assessing peroxide value (PV) and conjugated dienes (CD) to measure primary oxidation products, and TBARS to measure secondary oxidation products. A comparison of the methods used for measuring of primary oxidation products was determined to be included in the thesis. Lipid hydrolysation was measured by determination of free fatty acids. In addition, an evaluation of PV in sensory samples from the pre-marinated herring in experiment 1 was included.
In both experiments the primary and secondary oxidation increased with the pre-marinating time, regardless of the salt concentration. It is suggested that salt concentration has no effect on CD formation in experiment 2, while larger variations in the development of CD between the different marinades were observed in experiment 1. High PV values was found for both experiments, exceeding 10 meq O2/kg oil for some of the marinades. After 10 months, the level of PV for three of the marinades were found to decrease. Despite high PV, the results from determination of secondary lipid oxidation products displays a low and relatively stable TBARS development. The concentration of salt in the marinades is found to have no clear influence on the oxidative stability of lipids. Based on the results, a partial replacement of Na+- ions with K+- ions could be possible without affecting the lipid oxidative stability. Rancidity descriptors was not found to contribute to the total sensory profile of the marinated herring in experiment 1, indicating that primary and secondary oxidation products have not increased enough to affect the sensorial properties. The results presents an increase in FFA for both experiments, indicating that the muscle enzymes were active during storage. Salt does not inhibit the lipases responsible for the liberation of FFA. After 10 months (experiment 1) the FFA increased to 6-8 %, from around 2 % after 35 days. Comparison of experiment 1 ( winter herring) and experiment 2 ( autumn herring) shows significantly higher numbers for experiment 2, which could be an evidence of seasonal differences. In both experiments, a barrel-to-barrel variation was observed for some of the marinades. For the relationship between PV and CD measurements, a highly significant ([P] < 1%) correlation was found for all marinades in both experiments, except marinade B, D and R (experiment 1). CD for measuring of primary oxidation might be used as an index of stability of lipids in addition to or instead of PV, however it has less specificity than the PV method.||en