Renewable Biofuels

Increasing the economic feasibility of biorefineries could strengthen the case for industrial production of biofuels which again could lower the environmental impact of current fossil fuel usage. Lignocellulose-degrading enzymes are derived from certain fungi and bacteria, which are not only difficult to culture at industrial scale but also exhibit low specific activity and low titer concentration. Accordingly, new technologies to improve the performance of lignocellulolytic enzymes have been investigated heavily during the last years. In this paper we will discuss the mechanism of lignocellulose degradation and the action of lignocellulolytic enzymes. We will further examine the latest developments for improving the production of lignocellulose-degrading enzymes from microbial production strains. Finally, we will discuss future strategies for cellulase production and evaluate their benefits and drawbacks.

Cellulase production by two filamentous fungi Trichoderma reesei RUT-C30 and novel fungal strain, Aspergillus saccharolyticus, on pretreated corn stover was investigated. Cellulase production was followed by the hydrolysis of wet-exploded corn stover (WECS) and wet-exploded loblolly pine (WELP) by on-site produced enzyme cocktail containing cellulase from T. reesei RUT-C30 and β-glucosidase from A. saccharolyticus. The sugar yields by using the on-site enzyme cocktail were compared with the commercial enzyme preparations, Celluclast 1.5 L and Novozym 188 at two substrate concentrations, 5 and 10 % (w/w), and enzyme loading at 5 and 15 FPU/g glucan for WECS and WELP. Highest sugar yields were obtained at 5 % (w/w) substrate concentration and 15 FPU/g glucan for both feedstocks, WECS and WELP. Glucose yields of 81 and 88 % were obtained from on-site and commercial enzymes, respectively, from WECS, and 55 and 58 % were achieved from on-site and commercial enzymes, respectively, from WELP.

In-house production of cellulases from filamentous fungi is widely used, but their hydrolytic efficiency compared to commercial enzymes is limited. We studied the effect of supplementing in-house cellulases produced by Trichoderma reesei RUT-C30 and a novel strain Aspergillus saccharolyticus with different types of commercial enzymes for the efficient hydrolysis of wet-exploded loblolly pine. Cellic^®Ctec 2, Cellic^®Htec2, and Novozym 188 were used as the commercial base enzymes for supplementing the in-house produced enzymes. Compared to non-supplemented in-house enzyme preparation, commercial enzymes (Cellic^®Ctec2) added in the same amount as FPU and CBU resulted in 68 % higher glucose yield using wet-exploded loblolly pine (WELP) at a 20 % DM concentration. Highest saccharification yield was achieved by supplementation of the in-house produced cellulases with Cellic^®Htec2 compared to Cellic^®Ctec2 and Novozym 188. Optimal glucose, xylose, and mannose yields, 85 %, 92 %, and 86 %, respectively, were achieved by using in-house enzymes (15 FPU/g cellulose) supplemented with commercial hemicellulase (7.5 FPU/g cellulose). These results showed that supplementing in-house enzymes with commercial enzymes can be advantageous and work for lowering the overall cost of enzymes in a biorefinery.

Enzymatic hydrolysis with on-site produced enzymes and released more than 70 % and up to 70 % of total sugars in corn stover and loblolly pine, respectively, after 72 h. HPLC analysis was used for sugar yield analysis. The results showed that the higher content of lignin present in loblolly pine poses as barrier in cellulase access and indicated the slower rate of the hydrolysis of cellulose to monomeric sugars.