Structural features and antioxidant activities of degraded lignins from steam exploded bamboo stem

• Steam explosion pretreatment was applied to refine bamboo for biofuel production.
• An increasing steam pretreatment pressure/time facilitated the release of lignins.
• The isolated lignin fractions are homogeneous and have low molecular weights.
• The RSI value is correlated to the phenolic hydroxyl groups in the lignins.
• DPPH test suggested that the lignins may be a potential antioxidant in industry.

A two-step process based on steam explosion pretreatment and alkali/alkaline ethanol post-treatments of bamboo stems was developed to produce bioethanol efficiently. In this study, alkali/alkaline ethanol post-treatments were applied to isolate lignins from steam exploded (1.8 and 2.0 MPa for 5 min, and 2.0 MPa for 8 min) bamboo stems, which resulted in the dissolution of 28.9, 53.3 and 60.1% of acid-insoluble lignins, respectively. The structural features of the isolated lignins were comparatively and comprehensively investigated by FT-IR and NMR techniques. The lignins showed low molecular weights (Mw, 860–1190 g/mol), narrow polydispersities (Mw/Mn, 1.46–1.92), and high content of phenolic hydroxyl groups (0.53–1.79 mmol/g). Antioxidant activities investigation showed that the radical scavenging index (RSI) of the extracted lignins (2.02–5.56) was higher than BHT (0.19) but lower than that of BHA (11.11). The relatively high antioxidant activity of the lignins suggested that they may be a potential antioxidant in food and other industries.

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