Enzymatic Hydrolysis of Pretreated Corn Stover at High Solids Loadings

Biochemically produced cellulosic ethanol holds promise for the future of renewable liquid transportation fuels. Cellulosic ethanol must demonstrate an economic and large-scale viability in order to realize its full potential, and therefore intermediate processes such as enzymatic cellulose digestion must be more fully understood. In this study, the digestibility of dilute sulfuric acid pretreated corn stover was explored with respect to pretreatment severity, particle size, and regimen of pH adjustment before enzymatic hydrolysis. A series of experiments were performed at 20% insoluble solids weight, which is considered a high solids concentration because the material has an appreciable yield stress. Conversion of cellulose to glucose and cellobiose by 20 mg protein/ g cellulose GC220 enzyme was assessed over a period of 7 days. Each sample consisted of 60 g of slurry in 125-mL cylindrical bottles rotated on mechanized rollers at 4 rpm incubated at 48.5°C. Enzymatic hydrolysis of pretreated insoluble solids suspended in DI water and citrate buffer was performed in duplicate with and without mechanical size reduction. GC220 enzyme is most effective at a pH near 5, thus necessitating the buffer in the insoluble solids samples. Pretreated whole biomass slurries include soluble and insoluble solids and have a pH around 1.5 – 2.0. Ammonium hydroxide was mixed into the whole slurry samples to increase the pH before enzyme addition. Results showed that increasing extent of pretreatment predicts increasing digestibility of the pretreated insoluble biomass samples in both cases. Despite particle size reduction of 10-40%, there was little difference in conversion between the two different particle size distribution materials. The results indicate that pretreatment is effective and the well-established correlation between smaller particle size and high digestibility is not a simple causal relationship. In all cases, whole slurry is more difficult to break down with enzymes than insoluble solids suspended in the equivalent amount of water, but ultimately this challenge will need to be overcome to efficiently convert biomass to fuels. This series of experiments begins to show the effect that process-relevant conditions have on cellulose digestibility. Further investigation into these operations can lead to modifications and improvements that may help cellulosic ethanol become the liquid fuel of the future.

The Journal of Young Investigators is not affiliated with the US Department of Energy. This paper was written by a student intern with the Department of Energy and does not constitute a declarative position of either the Department of Energy or the Journal of Young Investigators.
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