Although macroalgae, i.e., seaweed, accumulate low levels of lipids, their polar nature, high PUFA-levels and co-existence with e.g., pigments, and enzymes in membranes provide a risk for oxidation. At the same time, seaweed is enriched in a diverse pool of antioxidants, but to date, very little is known about how the anti- to pro-oxidant balance in seaweed translates into oxidation progression, and subsequently into quality chances. Throughout the Sweaweed, BlueGreen, Blue Food and CirkAlg projects, we have explored oxidation in red and green seaweed species during storage and downstream processing, but also the role of seaweed in preventing oxidation of other marine lipids. Here, selected results on sea lettuce (Ulva fenestrata) lipid oxidation will be presented.
When oven dried (40°C) U. fenestrata biomass was stored ≤370 days, there was a moderate development of aldehydes (MDA, HHE, HNE) parallel to a great loss of unsaturated fatty acids and ascorbic acid. Storage in light compared to semi-light or darkness stimulated fatty acid loss as well as pigment bleaching. In later studies with extended storage times (550 days in darkness) and varying drying temperatures (40 or 60°C), significant changes in volatile organic compounds (VOCs), sensory profiles and pigments were recorded. Among VOCs, hexanal, pentanal, heptanal, benzaldehyde, and 2,4-heptadienal dominated, but revealed different kinetics. Drying at 60°C preserved green colour and reduced storage-induced VOC-development better than 40°C-drying. Seaweed and ocean odour decreased over time, while rosehip soup, grass and hay odours increased. Oxidation-derived aldehydes and furans were also detected in pH-shift-produced protein extracts from U. fenestrata, reflecting the risk of biomass disintegration, pH-alterations and/or dilution of endogenous antioxidants. Extracts also had lower grassy odour but higher bitterness than non-processed biomass when evaluated in emulsions. In all the above studies, different strategies have been evaluated to mitigate lipid oxidation, from which results will be highlighted.
To conclude, lipid oxidation progresses during storage and processing of seaweed, but more knowledge is needed about its consequences for different end-uses of the seaweed biomass -e.g., as a flavour enhancers, colourants or nutrient carriers.