Evidence for antimicrobial activity in hemp hurds and lignin-containing nanofibrillated cellulose materials

Antimicrobial activity is one of the most well-studied properties of hemp, with numerous contradictions and anecdotal information. However, most of the studies on antimicrobial activity of hemp have been focused on extractives, obtained by use of polar or nonpolar solvents such as petroleum ether, acetone, methanol, and ethanol, etc. This work compared the antimicrobial activity of unprocessed hemp hurds powder, processed hemp hurds-lignin containing nano-fibrillated cellulose (LNFC), and their ethanol extractives. The hemp hurd shives were pulped (defibrillated) to obtain fibers by employing a hydrothermal, carbonate and kraft pulping process. The obtained cellulose fibers were further processed into LNFC using intensive mechanical processing. LNFC films were prepared using a solvent casting method. The extractives of hemp hurd powder and differently treated pulps were obtained using ethanol and benzene. The obtained extractives were used to treat selected sterilized paper discs and hydrothermal pulped LNFC films. Extractives from the hemp hurds were characterized for the presence of antimicrobial active compounds using GC–MS. The LNFC films and extractives-treated paper substrate were tested against E. coli for their antimicrobial activity. The extractives-treated paper showed a significant reduction in bacterial growth and resulted in a zone of bacterial inhibition up to 1.85 mm in disk diffusion assays. The antimicrobial activity of extractives was confirmed by doing a colony-forming assay, which showed a bacterial inhibition by 98% colony forming units (CFU). However, no significant antimicrobial activity of unprocessed hemp hurd powder or LNFC films was observed. On the contrary, treating LNFC films with obtained extractives led to a reduction of CFU by 99.7%. These results of using hemp extractives and LNFC as antimicrobial coatings creates great potential for valorizing the industrial hemp residues for sustainable antimicrobial applications.