Optimizing drying techniques in euryhaline microalgae to reveal strain-specific phenolic signatures and amplified antioxidant potency

Abstract

This research investigated the impact of three distinct drying modalities—freeze-drying (FD), oven drying (OD), and shade drying (SD)—used for desiccating microalgal biomass on phenolic composition and antioxidant properties in a range of euryhaline microalgal strains, including Spirulina subsalsa (B6), Anabaena sp. (B7), and Chlorella sp. (B8), as well as freshwater strains Spirulina NCIM (SN), Chlorella NCIM (CN), and commercially cultivated Spirulina (BR). Irrespective of drying methods, a notably diminished quantity of aggregate polyphenols in both free and bound states was found within the freshwater and commercially produced strains, compared with their euryhaline counterparts. Amongst the euryhaline strains, Anabaena out with significantly higher phenol and flavonoid (p < 0.05) contents under FD and OD, followed by Chlorella and Spirulina. Among the drying methods evaluated, OD demonstrated the best preservation of bound phenols and the highest DPPH and superoxide scavenging activities, which were strongly correlated (r = 0.88 and r = 0.86, respectively) with the former. FD exhibited higher total reducing power and higher free phenolics, showing a robust positive correlation (r = 0.97). Nitric oxide scavenging activities correlated positively with total flavonoids, with high correlation coefficients observed under FD (r = 0.97) and OD (r = 0.95). Furthermore, euryhaline microalgal strains showed significantly higher antioxidant potential than commercial and freshwater strains (p < 0.05). Amongst the euryhaline strains, B7 demonstrated markedly enhanced DPPH scavenging activity, especially under OD (63.06%) attributed to increased bound phenols. These findings provide invaluable insights into the confluence of strain typology and drying methodologies vis-à-vis the bioactive properties of euryhaline microalgae, imparting potential utility across diverse industries.