High temperature enzymatic prehydrolysis prior to simultaneous saccharification and fermentation of steam pretreated corn stover for ethanol production

As a consequence of heightened concern for the greenhouse effect, depleting oil reserves and skyrocketing oil prices, renewable fuels, such as bioethanol, are becoming increasingly important. Cellulosic biomass like wood, agricultural residue and herbaceous material is a huge, cheap natural resource that can be used for ethanol production. Decreasing production costs through technological development is, however, still needed to make ethanol from cellulosics economically feasible. In this study, the effect on overall ethanol yield of an enzymatic prehydrolysis prior to simultaneous saccharification and fermentation (SSF) of steam pretreated corn stover was investigated. Two enzyme mixtures were utilised. A mixture of commercially available enzymes (Novozymes A/S, Baegersvaerd, Denmark) was compared with a developmental thermo-active cellulase complex produced by Roal Oy (Rajamäki, Finland). The thermoactive preparation comprised three essential cellulases and one xylanase enzyme. The prehydrolysis was, under the conditions evaluated, found to have no or negative effect on the overall ethanol yield. Longer prehydrolysis time resulted in a larger decrease in overall ethanol yield than shorter prehydrolysis. Using the experimental thermo-active enzyme mixture was shown to give a higher glucose concentration after prehydrolysis than when the commercial enzyme mixture was used in similar experiments. The highest ethanol concentration, 33.8 g/L, was reached in the SSF with 11.5% water-insoluble substances using the developmental thermo-active cellulase complex, and 1.8 g/L compressed bakerʹs yeast. This concentration corresponded to 80.2% overall ethanol yield based on the glucose content in the raw material. However, if the xylose present in the beer at the end of the SSF could be fermented to ethanol, another 12.6 g ethanol/L could theoretically be produced (0.51 g ethanol/g xylose).