The potential of microbial processes for lignocellulosic biomass conversion to ethanol: A review

Background: This paper assesses the feasibility of a single- or multi-stage process entirely based on microbial cultures, with no or minimal non-biological pretreatment and with no external enzyme addition, for the conversion of lignocellulosic materials into ethanol. The process considered involves three distinct microbial processes, which can possibly combined in one single reaction stage: (a) lignin hydrolysis; (b) cellulose and hemicelluloses hydrolysis; and (c) glucose fermentation to ethanol. This paper critically reviews the literature on the three microbial processes and compares the rates of microbial processes with those of the alternative physico-chemical pretreatment processes. Results: There is a large number of microbial species that can perform each of the three processes required for the conversion of lignocellulosic biomass to ethanol, although only one species has been unquestionably reported, so far, to be able to hydrolyse lignin under anaerobic conditions; another challenge is controlling the anaerobic fermentation of glucose to ethanol with mixed cultures; the rates of the microbial processes reported so far in the literature are generally lower than the rates obtained with physico-chemical pretreatments. Conclusions: While in principle the whole process from lignocellulosic biomass to bioethanol can be carried out with existing, non-engineered microorganisms, there is a need for further research to obtain rates and yields which are commercially attractive.