Integrated biorefinery of aquatic fern Azolla filiculoides for enhanced extraction of phenolics, protein, and lipid and methane production from the residues

Abstract Azolla filiculoides is an aquatic fern native to tropical regions, and because of its unique composition, it was subjected to integrated biorefineries for biochemicals and bioenergy production. The compositional analysis showed that its raw biomass contained 41.4 % carbohydrates, comprising galactan, glucan, and arabinan, 12 % lignin, and 10.3 % inorganic matter. Lipids, proteins, and phenolic compounds were extracted from the biomass and analyzed by different methods to enhance extraction yield and estimate their nutritional values. The residues remained after the (sequential) extraction of the major biochemicals was subjected to biogas production. A maximum lipid yield of 18.1 % was obtained from the biomass, which was rich in palmitic acid and eicosapentaenoic acid (EPA). EPA, an omega-3 fatty acid with wide applications as a food supplement, accounted for 39.8 % of total fatty acids, corresponding to 72 mg/g biomass. Two protein extraction methods, i.e., ultrasound-assisted water followed by sodium hydroxide extraction and sequential extraction by hot trichloroacetic acid, were applied. Using the first method at the optimum conditions, up to 24.4 % protein was obtained, while a maximum 11.9 % water-soluble protein was isolated by applying ultrasound-assisted extraction. In the amino acid profile of the fern protein, glutamic acid was the most abundant component with 20.5 %, followed by threonine. Up to 37.9 % of total amino acids were composed of essential amino acids that showed a high nutritional value of the fern protein. The total phenolic content of 27.2 mg gallic acid equivalent per 100 g of biomass was obtained. In the HPLC analysis, chlorogenic acid, luteolin, caffeic acid, and tannic acid were detected in which caffeic acid had the highest concentration of 18.5 mg/100 g raw material. Tannic acid removal was the most effective pretreatment for enhanced biomethane production. A maximum cumulative methane yield of 250.1 mL/g-VS was obtained from tannic acid-free biomass, which was 12 % higher than that of the raw biomass.