Insights into structure and bioactive potential of solvent-extracted lignins from enzymatic hydrolysis residues

To enhance the competitiveness of biorefineries based on sugars production, the interest in the valorization of hydrolysis lignin residue is becoming increasingly essential. Nevertheless, hydrolysis lignin residue (Hyd-Lig) faces limitations in its valorization—for instance, as bioactive agent due to different factors, including poor solubility, carbohydrates contamination, and chemical and structural variability. In this study, alkaline and organosolv (acetone, dioxane or methanol) approaches were evaluated for extraction of lignin from a hydrolysis residue generated during steam explosion and subsequent saccharification of poplar. The extracted lignins were characterized according to purity, chemical, and structural features by analytical standard methods and FTIR, SEC, 13C and 1H–13C NMR. Antioxidant (Trolox Equivalent Antioxidant Capacity, TEAC) and antibacterial (against Staphylococcus aureus) properties were also evaluated. The extracted lignin recovery yield using alkaline conditions (Alk-Lig) was 39.1 %, whereas using acetone (Ace-Lig), dioxane (Dio-Lig), and methanol (Met-Lig) were 24.1 %, 19.1 % and 13.1 %, respectively. Compared to Hyd-Lig, with a total lignin content of 63.9 %, extracted lignins displayed a higher purity (total lignin of 93.9–95.9 %). In addition, extracted lignins displayed a removal of lateral chains (aryl-β ether) and, therefore presented a higher phenolic content (around of 210–238 mg GAE/g lignin) compared to Hyd-Lig sample (42 mg GAE/g lignin). Then, the solubility of extracted lignins was improved, and higher antioxidant (TEAC values between 0.055 and 0.155) and antibacterial activities (Log R values between 0.5 and 1.2), compared to Hyd-Lig sample (TEAC and Log R of 0.022 and 0.02, respectively), were obtained. This work shows the necessity of extracting lignin from hydrolysis residues to enhance its applicability as bioactive agent and enable its potential valorization.