Extraction and characterization of lignins from cashew apple bagasse obtained by different treatments

Lignins were extracted from different hydrolysates from the cashew apple bagasse (CAB). The hydrolysates, in turn, were obtained by pretreating the CAB with diluted acid (DA), acid/alkali (AA) and alkaline hydrogen peroxide (AHP). Lignin removal levels with AA and AHP were of 98.0% and 96.9%, resulting in high extraction yields and highly thermostable lignins. The lowest lignin yield was observed with the acid-only pretreatment. The FT-IR results showed that there was no significant difference between the main structures of the lignin isolated from the DA and AHP pretreatments, when compared to sugarcane lignin (SCL), the only exception being the lignin from AA. An 1H–13C HSQC NMR analysis demonstrated that AA pretreatment reduced the syringyl (S) and hydroxyphenyl (H) units, but increased the condensed guaiacyl (G) units, which may contribute to the higher thermal stability of the isolated lignins and to the absence of attached residues of carbohydrates. The detailed structural properties of the lignins extracted from CAB will enable the efficient utilization of these macromolecules within the biorefinery concept, with a positive impact on the economy.