Efficient evaluation of cellulose digestibility by Trichoderma reesei Rut-C30 cultures in online monitored shake flasks

Background Pretreated lignocellulosic biomass is considered as a suitable feedstock for the sustainable production of chemicals. However, the recalcitrant nature of cellulose often results in very cost-intensive overall production processes. A promising concept to reduce the costs is consolidated bioprocessing, which integrates in a single step cellulase production, cellulose hydrolysis, and fermentative conversion of produced sugars into a valuable product. This approach, however, requires assessing the digestibility of the applied celluloses and, thus, the released sugar amount during the fermentation. Since the released sugars are completely taken up by Trichoderma reesei Rut-C30 and the sugar consumption is stoichiometrically coupled to oxygen uptake, the respiration activity was measured to evaluate the digestibility of cellulose. Results The method was successfully tested on commercial cellulosic substrates identifying a correlation between the respiration activity and the crystallinity of the substrate. Pulse experiments with cellulose and cellulases suggested that the respiration activity of T. reesei on cellulose can be divided into two distinct phases, one limited by enzyme activity and one by cellulose-binding-sites. The impact of known (cellobiose, sophorose, urea, tween 80, peptone) and new (miscanthus steepwater) compounds enhancing cellulase production was evaluated. Furthermore, the influence of two different pretreatment methods, the OrganoCat and OrganoSolv process, on the digestibility of beech wood saw dust was tested. Conclusions The introduced method allows an online evaluation of cellulose digestibility in complex and non-complex cultivation media. As the measurements are performed under fermentation conditions, it is a valuable tool to test different types of cellulose for consolidated bioprocessing applications. Furthermore, the method can be applied to identify new compounds, which influence cellulase production.