Bioconversion of carboxylic acids derived from Kraft black liquor into lipids by Yarrowia lipolytica

Annually, around 1.3 billion metric tonnes of Kraft black liquor (KBL) are generated, commonly managed through evaporation before combustion, which contributes to environmental concerns. Therefore, implementing a comprehensive strategy for producing high-value-added compounds is essential for enhancing sustainability and resource efficiency in industrial processes. This study evaluated lignin recovery and single-cell oil (SCO) generation by the oleaginous yeast Yarrowia lipolytica employing KBL as a renewable feedstock, analysing the impact of C/N ratio, nitrogen source, and flask cultivation with cell recycling. Initially, KBL underwent acid precipitation to partially purify the stream and recover a valuable product such as lignin. The resultant liquid fraction of KBL after lignin recovery (LRKBL) exhibited a 21% viscosity reduction and a 65% decrease in chemical oxygen demand. The stream rich in carboxylic acids (5.9 ± 0.3 g/L acetic acid and 1.8 ± 0.2 g/L propionic acid) was used as the Y. lipolytica cultivation substrate. In the fermentation stage, the optimal C/N ratio for cell growth and biolipid accumulation was found to be 20, with a biomass yield (Y X/S ) of 0.56 ± 0.03 g/g and a lipid yield (Y L/S ) of 0.21 ± 0.01 g/g. The synergistic effect of organic and inorganic nitrogen sources significantly enhanced lipid production, achieving a concentration of 1.52 ± 0.06 g/L. Implementing a sequencing batch culture approach markedly increased lipid content from 38 ± 1% in the initial culture cycle to 52 ± 1% after four cycles, resulting in a twofold increase in lipid concentration. The SCO lipid profile under optimal conditions contained mainly oleic, linoleic, palmitic and stearic acids, making it suitable to produce biodiesel. Therefore, this integrated strategy offers a potential solution for waste management and biofuel production. • Lignin recovery reduced Kraft black liquor COD by 65% and viscosity by 21%. • Y. lipolytica efficiently converted acetic and propionic acids from KBL into lipids. • (NH 4 ) 2 SO 4 and yeast extract were optimal nitrogen sources, giving Y L/S of 0.21 g/g. • Microbial lipid yield in sequencing batch cultures markedly rose from 38% to 52%. • Properties of biodiesel derived from SCO complied with international regulations.