Extraction of lignin from agricultural residues and biological properties

Abstract

This research aims to extract lignin from agricultural residues, rice straw, and longan peel. Rice straw had a lignin content of about 5.32±0.22% (w/w), while longan peel had a lignin content of 20.82±0.46%. The effect of dewaxing on lignin extraction during the pretreatment of rice straw and longan peel was examined. The results revealed that the lignin content was not different between un-dewaxed and dewaxed samples. After that, alkaline extraction with 40% sodium hydroxide was selected to extract lignin. The conventional method (boiling in a water bath, 4 h) and the autoclaving (121°C for 15 min) were used as physical pretreatment to assist the alkaline process in lignin extraction from rice straw and longan peel. The results revealed that the alkaline lignin yield from rice straw was 11.3 to 12.1% (w/w), while the lignin yield from longan peels was between 5.0 to 6.5%, with no difference in lignin content between both extraction methods. However, the rice straw lignin contained a high ash content as impurities. SEM, FTIR, DLS, and zeta potential characterized the physio-chemical properties of all alkaline lignin. Moreover, the alkaline lignin solubility test revealed that all alkaline was highly dissolved in 1 M NaOH (70-90% solubility), followed by 60% acetone or DMSO and 80% acetone, respectively. The biological activity of alkaline lignin from rice straw and longan peel was determined for the total phenolic compound, antioxidant, antidiabetic (α-amylase inhibition), antibacterial, and cytotoxicity. The total phenolic compound found in lignin from longan peel (123.26 - 155.47 mg GAE/g samples) was higher than rice straw lignin. The antioxidant activity of the lignin solution was represented through the DPPH and ABTS scavenging activity assays and the ferric-reducing power. Alkaline lignin from longan peels showed greater potential activity than lignin from rice straw with the high IC50 value of 32.46±1.41 - 38.54±0.63 mg TE/mL from DPPH, 23.94±0.26 - 28.58±0.10 mg TE/mL from ABTS and 450.76±5.94 - 1014.53±0.52 mg FeSO4•H2O/g sample of FRAP value. Moreover, all alkaline lignin revealed the possibility of inhibition of in vitro α-amylase at a range between 40.3 - 78.5% for rice straw lignin and between 51.6 - 77.4% for longan peel lignin. The antibacterial activities of alkaline lignins were evaluated with the tested bacterial strains such as P. aeruginosa TISTR2370, S. aureus TISTR746, S. epidermidis TISTR518, and B. subtilis TISTR1930. Alkaline lignins from rice straw and longan peel were potentially more effective against P. aeruginosa than the others. The cytotoxicity of all alkaline lignin was determined using HIEC-6 CRL-3266 (human intestinal epithelial cell line) and showed non-toxicity at 6.25-25 µg/mL. This research indicated that the yield and characteristics of lignin were affected by different pretreatments and alkali extractions, which were assisted by conventional and autoclave methods. However, all alkaline lignin derived from rice straw and longan peel showed promising biological properties. Thus, the extracted lignin could be further used as value-added products in pharmaceutical applications.