From wheat straw to bioethanol: integrative analysis of a separate hydrolysis and co-fermentation process with implemented enzyme production

In lignocellulose-to-bioethanol processes, unit-operations are heavily inter-related and single parameters can affect the process efficiency in a multitude of ways. Optimization studies therefore must include a balance-based process analysis instead of focusing on single process aspects only. Here we present the integration and analysis of a separate hydrolysis and co-fermentation (SHCF) process for bioethanol production from pre-treated wheat straw on a representative laboratory scale (90 mL to 4 L). Key features of the process are the enzyme production through Hypocrea jecorina on the feedstock as integrated unit-operation and the conversion of glucose and xylose to ethanol utilizing the genetically and evolutionary engineered Saccharomyces cerevisiae strain IBB10B05 [1; 2].

Different process configurations were analyzed. The highest process yield (Y_{Ethanol-Process} 78.3 g_Ethanol/kg_DM-WS) was reached with batch fungal cultivations and an enzyme loading of 30 FPU/g_DM-WS in the hydrolysis reactions. The resulting enzyme yield was 1.7 FPU/mL. Glucose and xylose conversion efficiencies were 67% and 95%, respectively. Utilizing strain IBB10B05 it was possible to reach an ethanol yield of 0.4 g/g_Glc+Xyl [1]. Enzyme yields, glucose conversion efficiencies and mass losses between the unit-operations were found to exhibit the strongest influence on Y_{Ethanol-Process}. Under comparable conditions (without enzyme production) Y_{Ethanol-Process} was equal to pilot scale plants (136.1 g_Ethanol/kg_DM-WS).

The herein presented bench-top SHCF process represents an effective and simple tool to identify key parameters, bottlenecks and optimization targets already on laboratory scale.

[1] Novy et al., 2014, Biotechnol Biofuels, 7. [2] Klimacek et al., 2014, Microb Cell Fact, 13.