Insight into the contribution of endogenous carotenoids of photoautotrophic microalgae to the oxidative stability of n-3 LC-PUFA

Promovendus/a: Robbe Demets

Promotor(en): Prof. dr. ir. Imogen Foubert, Prof. dr. ir. Ann Van Loey, Mevrouw Lore Gheysen

Both fish and fish oil are currently the most important sources of long-chain omega-3 polyunsaturated fatty acids (n‑3 LC‑PUFA). A sufficient intake of these fatty acids is associated with multiple health benefits. Especially the prevention of cardiovascular diseases and a good visual and cognitive development are generally recognized. Sustainability issues resulting from the finite character of the fish stock however pose a threat on solely relying on fish as traditional n‑3 LC‑PUFA source. As the demand for n‑3 LC‑PUFA sources is growing due to a rising population and an increased health benefit awareness, this problem only became more prominent and has driven the search for novel n‑3 LC‑PUFA sources. Among these, microalgae have already shown their potential. Microalgae are unicellular, aquatic organisms. At the base of the aquatic food chain, they serve as the primary producer of n‑3 LC‑PUFA. Cultivating them would generate a new supply of n‑3 LC‑PUFA rather than depleting an existing one, and adding them to food products (food enrichment) would offer vegan opportunities to the health-conscious consumer to guarantee the sufficient n‑3 LC‑PUFA intake for which the global fish stock cannot account. Moreover, whereas fish (oil) is known to be prone to the chemical deterioration process of lipid oxidation (n‑3 LC‑PUFA in particular are very susceptible due to their structure), former research has shown that certain microalgae and with them enriched food products led to oxidatively stable products. Lipid oxidation is undesirable since the nutritional value of n‑3 LC‑PUFA is lost and it results in the formation of off-flavors and potentially toxic compounds. The reason for certain microalgae to be quite resistant against lipid oxidation of their n‑3 LC‑PUFA can be found in the presence of carotenoids working as antioxidants. Obtaining further insight into these carotenoids’ antioxidative role and capacity was the main objective of the current PhD research. Although the PhD research was more fundamentally than practically oriented, understanding this could reveal important points of interest for developing n‑3 LC‑PUFA rich food products with microalgae or microalgal compounds.

Throughout the PhD research, the antioxidative role of microalgal carotenoids was studied in both aqueous suspensions, i.e. water mixed with microalgal biomass, and bulk oil systems, i.e. fish oil mixed with microalgal biomass or with microalgal oil extracted from the microalgal biomass. Both systems clearly showed the importance of the carotenoids for obtaining n‑3 LC‑PUFA stability. Their degradation upon storage exposed their antioxidative role. Moreover, the impact of their quantity on top of their presence was revealed: suspensions with two different microalgae having clear differences in carotenoid content led to oxidative stability differences, and bulk oils with different carotenoid/n‑3 LC-PUFA ratios led to a clear distinction between oxidatively stable and oxidatively unstable. The latter also showed that especially the initial carotenoid quantity is of importance. Apart from that, also the type of carotenoid and the type of system mattered since clear differences in degradation profile were shown among these. Within these, unequivocal conclusions were however harder to make.

Apart from the more fundamentally oriented conclusions, also some practical insights for using microalgae for food enrichment were gained. First of all, since illumination is able to promote lipid oxidation in the presence of so-called photosensitizers that are also abundantly present in the investigated microalgae, its impact was considered. Since it was found to only be minor, the potential of using oxidatively stable microalgae for enriching food products that are stored inside transparent containers was still valid. Secondly, approaching microalgae as an alternative source of antioxidants (i.e. the carotenoids) on top of their n‑3 LC‑PUFA enrichment, showed that this had no potential: adding pure microalgal biomass to fish oil did not result in oxidative stability despite a detectable carotenoid transfer. The microalgal carotenoids themselves do possess this potential, yet high (initial) amounts are required that are one order of magnitude above the usual food industry antioxidant additions.

Finally, it could be stated that one microalga is not the other. In realizing their advantage of giving rise to oxidatively stable n‑3 LC‑PUFA rich food products, the amount of carotenoids is crucial. For applying n‑3 LC‑PUFA rich microalgae in future food products however, several challenges still need to be addressed regarding production costs, legislation and consumer acceptance. Nevertheless, their potential in overcoming both the sustainability and stability issues n‑3 LC‑PUFA rich products face, remains clear.