Docosahexaenoic acid (DHA; 22:6-n3, 4Z,7Z,10Z,13Z,16Z,19Z) is a long-chain polyunsaturated omega-3 fatty acid with several health benefits. It promotes cardiovascular health and is considered essential for the normal neural and visual development of infants. It also has beneficial effects on anti-inflammatory and immune functions. Despite the well-documented health benefits, the intake of DHA is lower than recommended in most Western countries. The primary sources of DHA in the diet are fatty fish and food supplements. Due to its structure with six double bonds, DHA is highly susceptible to oxidation. Autoxidation is initiated by a radical-induced interaction between atmospheric oxygen and fatty acid, resulting in a complex and multiphase series of chemical reactions. The reactions result in fatty acid decomposition and generation of a diverse array of lipid oxidation products. Oxidation introduces off-flavors and lowers the nutritional quality and safety of food. It is a challenge to the food industry's ability to fortify foods with DHA, which would otherwise be a convenient way to increase the insufficient dietary intake. Oxidation also results in losses within the omega-3 industry supply chains. In the supplements, the fatty acids are found as esterified to triacylglycerol, ethyl ester, or phospholipid structure. These lipid structures, as well as the fatty acid position in the glycerol backbone (sn-1, sn-2, or sn-3), can influence the oxidative stability and antioxidant response of the oil.

This research aimed to investigate novel means to improve the oxidative stability of DHA by the choice of lipid structure and examination of a promising sphingoid base antioxidant, dihydrosphingosine. Additionally, the study aimed to elucidate the oxidation pattern of DHA because only a few previous studies are available on the topic. A new omics-type analytical approach was applied with liquid- and gas chromatographic separation coupled to mass spectrometric detection for the non-volatile and volatile oxidation products, nuclear magnetic resonance spectroscopy, and substrate and antioxidant concentration monitoring during the oxidation trials.

The results showed that the oxidative stability of DHA is influenced by the lipid structure into which it is integrated, and lipid structures were observed to interact differently with tocopherols. Tridocosahexaenoin was more stable than DHA ethyl ester without added α-tocopherol, while in the presence of 0.2% α- tocopherol, the opposite was observed. In the triacylglycerols with two palmitic acids and one DHA either at sn-1, sn-2, or sn-3 position, DHA at the sn-2 position was the most stable structure, both with and without 0.14% RRR-α-tocopherol. Without antioxidant, there was no significant difference between the sn-1 and sn-3 positions, while with RRR-α-tocopherol, the sn-1 position was slightly more stable than the sn-3 position. Increased stability might indicate favorable diastereomeric interactions between the enantiopure RRR-α-tocopherol and DHA at the sn-1 position. This was the first study conducted on the oxidative stability of enantiopure triacylglycerols. Dihydrosphingosine (d18:0) showed an improved antioxidative effect after the initial stages of oxidation, indicating antioxidative carbonyl-amine reaction product formation from the d18:0 amine group and oxidation product carbonyls. Some of the formed imine structures could be tentatively identified for the first time. Application of d18:0 for increasing the stability of DHA-rich oils showed promise, but further research is needed for a more comprehensive understanding of the effect.

This thesis elucidated the role of lipid structure and different antioxidant strategies on the stabilization of DHA. Also, an analytical approach for comprehensive lipid oxidation product analysis from a small lipid amount was introduced. The findings can be utilized in future research on lipid oxidation and antioxidant strategies, as well as for formulating DHA-rich oils with improved stability.