Advances in ethanol production from hardwood spent sulphite liquors

Hardwood spent sulphite liquors (HSSLs) are by-products from the pulping industry rich in pentoses, which are not yet fully exploited for bioprocessing, namely for the production of bioethanol. The sustainable fermentation of pentoses into bioethanol is a challenge to overcome. Besides sugars, HSSLs contains inhibitors that decrease the possibility of bioprocessing of these by-products. Nevertheless, recent studies have brought new insights in using HSSLs for bioethanol production. This paper reviews the results of relevant studies carried out with HSSLs towards bioprocessing to bioethanol. The composition of SSLs was compared and related with the wood origin stressing specificity of microbial inhibitors from HSSL and their anti-microbial effect. The different fermentative processes, the microorganisms used, and the strategies to improve yield and productivity used so far were also reviewed. This review allowed concluding that research is still needed in several areas, including optimization of detoxification processes, fermentation strategies and selection of suitable microbial strains in order to achieve the integration of the different steps needed for HSSLs bioconversion into ethanol thus contributing for sustainability of pulping mills within biorefinary concept.