Valorization of microalgal biomass to value-added products using integrated supercritical CO2 extraction and sub-critical hydrothermal liquefaction

The study depicts the use of different extraction techniques to utilize each fraction of microalgal consortium biomass (Chlorella sp. and Phormidium sp.) for its conversion to value-added products (VAPs). The algal biomass was rich in proteins (30.8%), lipids (28.1%), carbohydrates and essential amino acids (36.29% of total protein). The lipid yield obtained after supercritical CO2 extraction (SFE) was found to be 3.0 ± 0.4% compared to the conventional soxhlet extraction (28.2 ± 2.3%) with chloroform: methanol (1:1 v/v). However, SFE extracted lipid showed higher Polyunsaturated fatty acids (PUFAs) content (20.68%) with the presence of eicosapentaenoic acid, docosahexaenoic acid, α-linolenic acid, linoleic acid and arachidonic acid. The PUFA content in lipid obtained after soxhlet extraction was only 3.48%. The residual biomass after SFE showed recovery of 17.1 ± 0.7% proteins and 27.1 ± 0.9 mg g−1 of total carotenoids. Subcritical-hydrothermal liquefaction (HTL) of the microalgae at 230 °C and 2.7 MPa produced 30.7% biocrude (dried biomass basis) containing aldehyde/ketone (37.14%), aromatics (27.63%) and alkanes/alkenes/alkynes (15.3%) along with other compounds. This fraction consisted of VAPs targeting multiple industries-food (11%), fuel (22%), pharmaceutical (56%) and cosmeceutical (12%). A novel approach including sequential SFE-HTL process showed a lower biocrude yield (8.6%) but showed upgradation in biocrude quality having 50% aromatic compound of which >48% were pyrazines derivatives (flavoring agents for the food industry) and 20.8% pharmaceutically valuable compounds, besides extracting PUFAs from SFE. Hence, a new microalgal valorization approach has been developed by integrating two extraction techniques to produce multiple VAPs to target a minimum waste generation approach.