Novel microalgal based ingredients

Anita Ljubic*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesisResearch

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Abstract

Microalgae are excellent sources of various bioactive compounds such as omega-3-fatty acids, carotenoids and vitamins. With greater consumer preferences toward naturally derived, non-animal ingredients over synthetically produced, it is expected that the market demand will continue to grow in the future. Microalgal based ingredients represent reliable and sustainable substitutes to high value bioactive ingredients from both animal and plant origin. However, for economically viable production of high value ingredients there are still challenges to be addressed in order to enhance the production of these ingredients. The nutritional quality of microalgae can be improved by optimizing abiotic factors such as irradiance, salinity, nutrient availability, pH or temperature, which may stimulate production of metabolites of interest. Besides maximizing the production of high value compounds, maintaining the functionality of compounds during the cell disruption, extraction and drying process, while obtaining high recoveries, is essential.
The overall aim of this PhD study was to optimize existing technologies and develop new technologies for the production of high value ingredients from microalgae. Effect of the abiotic factors, including light intensity, spectral distribution, UVB radiation and salinity, on the production of bioactive ingredients in 8 selected microalgal species was evaluated. Furthermore, the aim was to identify tools that can be used for enhanced production of high-value lipophilic compounds - omega-3 fatty acids, various carotenoids, α-tocopherol, vitamin D and novel compounds, alkenones. Optimization of the downstream processing included optimizing the process parameters for an efficient cell disruption and metabolite extraction by high-pressure homogenization in combination with enzymatic treatment. Lastly, the effect of the novel swirl flash drying technology on the high value compounds present in the biomass was evaluated in order to assess the suitability of the novel drying technology for microalgae.
Stimulating production of bioactive compounds by manipulating different abiotic factors was shown to be highly dependent on species and target compound. Different patterns were observed across the selected species and relatively weak correlation between the content of different metabolites was observed, which required selection of optimal stress tools for each individual species or individual compound. This research demonstrated that it was possible to manipulate different microalgae to produce various high value compounds by exposing them to different light treatments. Production of some of these compounds in microalgae had not been reported before. The most novel finding with the greatest potential was that exposing Nannochloropsis oceanica, N. limnetica and Dunaliella salina to artificial UVB radiation stimulated production of relatively high levels of the fat-soluble vitamin D3 (up to 2.7 μg/g dry biomass), which is commonly found in foods of animal origin. UVB radiation was also shown to stimulate production of α-tocopherol in N. oceanica making this species an excellent alternative vegan source of vitamin D3, α-tocopherol and omega-3 fatty acids, ingredients commonly found in fish oil. Chlorella minutissima illuminated by green:red LED (50:50) was shown to contain up to 70% more lutein compared to control, making this species highly competitive to other lutein-rich sources. For Rhodomonas salina, a species rich in omega-3 fatty acids EPA and DHA, illumination by red LED in combination with salt stress (40 ppt) resulted in an increase in the relative proportion of omega-3 by 40% (nearly doubled proportion of EPA). Similarly, illumination by green LED in combination with salt stress increased production of β-carotene in R. salina by 75% compared to the control, which indicates that interaction of different abiotic stresses may enhance production of certain metabolites compared to each factor individually. Using individual monochromatic LEDs at lower intensity compared to the white LED of high intensity was demonstrated to be a good alternative for stimulating production of the bioactive compounds at lower energy consumption.
Maximizing production of metabolites requires mild downstream processing conditions in order to avoid any deterioration during processing and preserve high yields of the compounds of interest. High-pressure homogenization in combination with enzymatic treatment aimed at mild, energy efficient extraction of bioactive compounds from Chlorella pyrenoidosa. High-pressure homogenization caused cell disintegration, but not complete rupture, which enhanced extractability of lipids. Incubation with enzymes at a temperature of 45° C had an adverse effect on the content of the heat-sensitive carotenoids, lutein and β-carotene. Enzymatic cocktails of carbohydrases and endoproteases showed no effect on the cell wall rupture, most likely due to the low concentration of added enzymes. Therefore, due to the absence of detailed cell wall composition of C. pyrenoidosa, a screening step testing different doses and types of enzymes is necessary in order to optimize enzyme assisted cell wall degradation for this species.
The novel swirl flash drier, designed and constructed in a previous Ph.D project, was used for drying of C. pyrenoidosa biomass in order to evaluate the potential of the novel drying technique. Besides the novel method, relatively mild and gentle freeze drying, was applied in order to compare their effects on the bioactive content in C. pyrenoidosa. Nutritional quality of C. pyrenoidosa biomass dried by novel swirl flash dryer showed no major difference compared to the freeze dried biomass. Biomass that was treated by high-pressure homogenization prior to drying was shown to have an adverse effect on the high value metabolites such as lutein and α-tocopherol. Cell disruption by high-pressure homogenization increased the exposure of heat-sensitive compounds to high-temperature airstream, which resulted in higher degradation rate. Taking into account the relatively low energy consumption (compared to spray and freeze drying), the novel prototoype swirl flash dryer was shown to be a promising new drying technique for microalgal biomass. Future studies should include testing of several different drying techniques and microalgal species in order to confirm the potential of the swirl flash drying technology for microalgae.
Commercial production of microalgal based ingredients is limited to a rather small number of products. Findings of this Ph.D opened up new possibilities such as exploiting microalgae for vitamin D3 production, which has not been previously discussed. Naturally enriched microalgal biomass can be produced by applying inexpensive methods such as illumination by monochromatic LEDs, which may contribute to a more feasible large scale microalgae production of various ingredients, feed or cosmetics. Furthermore, employing the mild novel drying technology may contribute to the prevention of losses of functional ingredients caused by product degradation during downstream processing.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages101
Publication statusPublished - 2020

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