<b>ENHANCING EXTRACTION, BIOACTIVITY AND BIOACCESSIBILITY OF PHENOLIC COMPOUNDS FROM MICROALGAE BY COLD PLASMA AND CONJUGATION</b>

<p dir="ltr">Microalgae are fast-growing microorganisms require significantly less water, land, and nutrients for biomass production compared to terrestrial plants, making them an emerging solution for global food insecurity and natural resource depletion. Microalgae are a rich source of diverse bioactive compounds and thus have great potential for production of functional foods and nutraceuticals. However, recovery of bioactive compounds from microalgae is often limited by their rigid cell wall, which hinders solvent penetration and release of intracellular compounds during extraction. Moreover, despite their high nutritional value, these compounds often known suffer from low stability and bioaccessibility.</p><p dir="ltr">The goal of this thesis is to improve the bioprocessing of microalgae and increase their potential for functional food applications. To achieve this goal, the thesis has two specific objectives with a particular focus on phenolic compounds derived from Chlorella vulgaris: (1) Enhancing phenolic extraction by treating microalgae with atmospheric cold plasma, a non-thermal technology featuring a wide range of reactive species which can induce physicochemical changes in biological materials; and (2) improving the antioxidant capacity and bioaccessibility of phenolic compounds through protein conjugation.</p><p dir="ltr">Atmospheric pin-to-plate cold plasma was generated at different input voltages (140–200 V) to Chlorella vulgaris biomass for 5–20 min. Plasma-treated microalgae showed clear cell wall disruption and more porous microstructure, as revealed by scanning electron microscopy. Plasma treatment also significantly increased the extraction yield of phenolic compounds by 43–68% and the extracts had higher antioxidant activity, in terms of both free radical-scavenging activity (ABTS; 26–49%) and ferric reducing antioxidant power (FRAP; 37–51%). However, increasing plasma input voltage and treatment time did not result in significant improvement; therefore, mild plasma treatment at 140 V for 10 min was considered most suitable.</p><p dir="ltr">The extracted phenolic compounds were further conjugated with protein isolated from Chlorella vulgaris at different phenol-protein ratios (2.5–10%). Spectroscopic analyses, including fluorescence, UV-visible, and Fourier transform infrared, confirmed the successful formation of phenol-protein conjugates at all the ratios tested. The conjugates exhibited up to 644% higher antioxidant capacity (ABTS scavenging activity) and also 75–149% higher bioaccessibility evaluated by in vitro digestion, and the largest improvement was observed with the conjugate containing 2.5% phenolic compounds.</p><p dir="ltr">In summary, this thesis presents two integrated approaches to recover bioactive compounds from microalgae with high yield and biological efficacy. The combination of cold plasma and protein conjugation technologies enable efficient extraction and effective stabilization of phenolic compounds. The findings of thesis can guide microalgae producers to better valorize microalgal biomass and advance the use of microalgae-derived compounds for product development in the food and nutraceutical industries.</p><p><br></p>