Can mild alkaline pretreatment simultaneously enhance the antioxidant capacity of Beta-carotene extracts and biomethane yields in a sustainable Dunaliella salina biorefinery?

This research aims to assess the effect of alkaline pretreatments on the antioxidant potential of β-carotene-rich extracts from the microalga Dunaliella salina and the cumulative biomethane production from its spent biomass, within the framework of a circular economy approach using four biorefineries. A solvent screening was performed, with ethyl acetate achieving the maximum β-carotene extraction yield (5.3% ± 0.03%). Alkaline pretreatments were applied to the initial biomass (direct) and extracts after a extraction with ethyl acetate (indirect), using two matrices: water (W) and a mixture water:ethanol (WE). Direct alkaline pretreatments (D) offered extracts with higher potential than indirect pretreatments (I) in terms of: i) antioxidant capacity, as measured by ABTS^•+ assay (0.69±0.1 and 0.61±0.1 mmolTE/gDW for W-D and WE-D, respectively, and 0.55±0.1 and 0.53±0.1 mmolTE/gDW for W-I and WE-I, respectively) and •OH scavenging activity (1.89±0.2 and 2.05±0.5 mmolTE/gDW for W-D and WE-D, respectively, and 0.48±0 and 1.2±0.3 mmolTE/gDW for W-I and WE-I, respectively), ii) biomethane production from their spent biomass (301±14 mLCH₄/gVS and 289±9.0 mLCH₄/gVS for W-D and WE-D, respectively, compared to 235±57 mLCH₄/gVS without alkaline pretreatment), and iii) sustainability analysis, which includes the assessment of the biomass exploitation for β-carotene extraction and biomethane production. The most sustainable biorefinery was W-D as it achieved the highest biomass exploitation (33.8%), compared to WE-D (29.1%), W-I (33.1%) or WE-I (32.8%). This underscores the novelty and effectiveness of direct alkaline pretreatments for enhancing both antioxidant potential and energy recovery from D. salina biomass in a biorefinery context.