No abstract

An autohydrolysis pretreatment that suppresses lignin repolymerisation helps overcoming the recalcitrance of softwood for enzymatic hydrolysis of its cellulose.

Lignocellulosic biomass is uniquely suited as a sustainable feedstock for the biotechnological production of alternative fuels and chemicals. However, due to the biomass recalcitrance, the enzymatic conversion process is complex and needs to be simplified. To this end, we developed a process, which allows the consolidated bioprocessing of lignocellulose to ethanol in a single multi-species biofilm membrane reactor featuring both aerobic and anaerobic conditions necessary for the simultaneous fungal cellulolytic enzyme production and alcoholic yeast fermentation of the hydrolysis-derived sugars. The general feasibility of the concept was successfully demonstrated by producing ethanol with a 67% yield from undetoxified whole slurry dilute acid pretreated wheat straw by the combined action of Trichoderma reesei, Saccharomyces cerevisiae and Scheffersomyces stipitis. The results achieved underscore the potential of the process as a versatile cheap sugar platform for the production of fuels and chemicals based on lignocellulosic biomass by specifically compiled consortia of industrially proven robust microorganisms. Broader context Biofuels made from lignocellulosic biomass have the potential to be useful elements in the overall approach to tackle the issues of climate change and sustainable energy supply. However, this potential can only be unlocked if they are cost competitive to petroleum and starch and sucrose based biofuels. For the biochemical conversion route comprising the three main steps of physicochemical pretreatment, enzymatic hydrolysis and fermentation, the following objectives have to be addressed: (i) development of an integrated process without washing and detoxication steps, (ii) reduction of cellulase costs, (iii) improving hexose and pentose sugar co-utilization, and (iv) overcoming the plants' recalcitrance. As an alternative to employing one genetically engineered superior biocatalyst capable of both degrading biomass and producing biofuel for consolidated bioprocessing (CBP), we present in this paper a novel microbial consortium based approach to tackle issues (i) to (iii). We developed a membrane biolm reactor, which, as a unique feature, enables the coexistence of aerobic and anaerobic conditions at the same time. This allows the coexistence of the two common "workhorses" of the cellulosic ethanol industry in one system: the aerobic hydrolytic enzyme producing fungus Trichoderma reesei and the anaerobic ethanol producing yeast Saccharomyces cerevisiae.

Lignocellulosic biomass - such as wood, agricultural residues or dedicated energy crops - is a promising renewable feedstock for production of fuels and chemicals that is available at large scale at low cost without direct competition for food usage. Its biochemical conversion in a sugar platform biorefinery includes three main unit operations that are illustrated in this review: the physico-chemical pretreatment of the biomass, the enzymatic hydrolysis of the carbohydrates to a fermentable sugar stream by cellulases and finally the fermentation of the sugars by suitable microorganisms to the target molecules. Special emphasis in this review is put on the technology, commercial status and future prospects of the production of second-generation fuel ethanol, as this process has received most research and development efforts so far. Despite significant advances, high enzyme costs are still a hurdle for large scale competitive lignocellulosic ethanol production. This could be overcome by a strategy termed 'consolidated bioprocessing' (CBP), where enzyme production, enzymatic hydrolysis and fermentation is integrated in one step - either by utilizing one genetically engineered superior microorganism or by creating an artificial co-culture. Insight is provided on both CBP strategies for the production of ethanol as well as of advanced fuels and commodity chemicals.