Extraction of phenolic compounds from açaí processing residue (Euterpe oleracea Mart.) as a strategy for adding value in the context of biorefineries

Biorefineries based on agro-industrial lignocellulosic residues are promising alternatives for producing renewable energy and value-added bioproducts. Contributing to the full use of biomass and, consequently, to the economic feasibility of the process. In this context, this study aimed to identify and quantify the phenolic compounds present in extracts obtained sequentially with water and anhydrous ethanol from lignocellulosic residues of açaí seeds and fibers. The extraction processes were performed using a Soxhlet apparatus, following the National Renewable Energy Laboratory (NREL) method. The aqueous and ethanolic extracts were analyzed by liquid chromatography using a UPLC system coupled to a photodiode array (PDA) detector and a mass spectrometer with electrospray ionization (ESI). An Acquity Premier UPLC BEH C18 column (1.7 µm, 2.1 × 55 mm) was used at 25 °C, with a flow rate of 0.3 mL/min and a gradient elution using Milli-Q water containing 0.1% formic acid and methanol. Sixteen external standards were evaluated, including phenolic acids and flavonoids. The total extractives content was 15.56 ± 0.74% (w/w) of the dry lignocellulosic biomass. Nine compounds were quantified in the aqueous extract: procyanidin B1 (909.52 μg/g), procyanidin B2 (680.24 μg/g), (+)-catechin (592.79 μg/g), (-)-epicatechin (302.14 μg/g), protocatechuic acid (218.27 μg/g), p-hydroxybenzoic acid (48.15 μg/g), ferulic acid (23.39 μg/g), syringic acid (5.85 μg/g), and p-coumaric acid (4.28 μg/g). Eight compounds were quantified in the ethanolic extract: procyanidin B1 (210.74 μg/g), procyanidin B2 (147.70 μg/g), (+)-catechin (127.94 μg/g), protocatechuic acid (77.10 μg/g), (-)-epicatechin (31.14 μg/g), p-hydroxybenzoic acid (15.69 μg/g), gallic acid (9.06 μg/g), and p-coumaric acid (0.37 μg/g). The higher concentrations observed in the aqueous extract can be interpreted considering the sequential extraction, since ethanol extraction was performed after water extraction. Therefore, the results do not necessarily indicate that the compounds are more soluble in water. Procyanidins B1 and B2 were the main compounds in both extracts, followed by (+)-catechin. (-)-Epicatechin also showed a notable concentration, particularly in the aqueous extract. These compounds belong to the proanthocyanidin and flavan-3-ol classes, respectively. The large number of phenolic groups present in this material is associated with its high antioxidant potential, highlighting an opportunity to add value to açaí residues as a biorefinery byproduct. The recovery of phenolic compounds could be part of a pathway for producing high-value coproducts in an açaí-processing biorefinery. This could contribute to the economic feasibility of fully using the residue, which could later be used to develop new products or fuels.