Corn stover hydrolysis

Selig M, Adney W, Himmel M, Decker S. (2009). The impact of cell wall acetylation on corn stover hydrolysis by cellulolytic and xylanolytic enzymes. Cellulose, 16(4), 711-722. 

Recent studies have proven ethanol to be an ideal liquid fuel for transportation and renewable lignocellulosic biomass to be an attractive feedstock for ethanol fuel production by fermentation (1,2). The major fermentable sugars from hydrolysis of lignocellulosic biomass, such as rice and wheat straw, sugarcane bagasse, corn stover, corn fiber, softwood, hardwood, and grasses, are D-glucose and D-xylose except that softwood... 

Corn stover, like lignocellulosic materials in general, is resistant to enzymatic hydrolysis, because of both the tight network in the lignocellulose complex and the crystalline structure of the native cellulose. These difficulties can be overcome by employing a suitable pretreatment (7). 

Index Entries Ammonia fiber explosion corn stover enzymatic hydrolysis simultaneous saccharification and fermentation moisture content residence time. 

A major obstacle to the commercialization of enzymatic hydrolysis of biomass is the high cost of the enzymes. One way to reduce this cost is to use much less enzyme per unit of biomass hydrolyzed. Previous work has shown that the effective enzymatic hydrolysis of AFEX-treated biomass at enzyme loadings as low as 5 filter paper units (FPU)/g of dry biomass was achieved by adjusting the pretreatment parameters (6). The objectives of the present study, were to determine the best AFEX conditions for pretreatment of corn stover, to employ different enzyme loading levels (60,15, and 7.5, FPU/g of glucan), and to compare the enzymatic hydrolysis results and ethanol yield. 

Fig. 6. Normalized hydrolysis profile for corn stover treated at 60% moisture content (dwb), 90°C, and 1 1 ammonia loading ratio, with enzyme loading of 60 FPU/g of glucan.

Note that all the runs (with one exception) with longer treatment time showed lower ethanol yield. However, the hydrolysis results of these runs all showed greater glucan and xylan conversion. As mentioned previously, perhaps during longer treatment times some inhibitory materials were produced that in turn reduced the yield of the fermentation. Based on these findings, 5 min is currently considered the best residence time for the AFEX treatment of corn stover. 

Furthermore, enzymatic hydrolysis of the corn stover treated under optimal AFEX conditions showed almost 98% glucan conversion and 80% xylan conversion vs 29 and 16% for untreated com stover, respectively (at an enzyme loading of 60 FPU/g of glucan). Unlike acidic pretreatments, AFEX does not generate sugar monomers. The cellulase mixture in our study was developed for hydrolysis of acid-pretreated materials and has about 1% by weight xylanase activity (J. Cherry, personal communication, Nov. 2002). Enzyme cocktails with enhanced xylanase activity would presumably completely hydrolyze AFEX-treated xylans. 

Initial Evaluation of Simple Mass Transfer Models to Describe Hemicellulose Hydrolysis in Corn Stover... 

Figures 1 and 2 present some representative results for the application of Eq. 1 to describe data for hydrolysis of hemicellulose in corn stover in only water by batch and flowthrough systems, respectively. In Figs. 1 and 2, xylan conversion is calculated as the initial mass of xylan minus the mass of xylan at a time t all divided by the initial mass of xylan. Although the data are not presented, acid-catalyzed systems behaved similarly.

Fig. 3. Data and biphasic mass transfer leaching model predictions of xylan concentrations in solids and solution vs time for water-only batch tube hydrolysis of corn stover at 180°C and 10% solids.

Xylan remaining in solid residues was also analyzed. As seen in Fig.5, increasing velocity significantly increased xylan removal, especially in the first 8 min. For example, xylan removal for operation at 2.8,5.2, and 10.7 cm/ min was 60, 70, and 82% for hot water only pretreatment of corn stover at 200°C after 8 min. However, after 16 min, the differences in xylan removal were less for all velocities run, suggesting that fluid velocity has less impact on the overall degree of hemicellulose hydrolysis. 

The present study investigated the inhibition of Saccharomyces cerevisiae by the liquid hydrolysate and the kinetics of enzymatic hydrolysis of the solid components produced by the pretreatment of aspen wood and corn stover by liquid hot water and hot carbonic acid. Inhibition of yeast was determined by measuring the rate of glucose consumption by yeast growing in hydrolysates produced at various reaction severities. The enzymatic hydrolysis rates of pretreated solids was determined by measuring rates of sugar accumulation of enzyme-digested pretreated solids. 

Selected High-, Medium-, and Low-Severity Yeast Inhibition/ Enzymatic Hydrolysis Experiments on Aspen Wood and Corn Stover... 

Fig. 5. Rate and yield of enzymatic hydrolysis vs reaction severity of low-, medium-, and high-severity pretreatment for corn stover samples.

Dilute H2S04 is a well-studied pretreatment system that has been shown to be effective but expensive. Limited studies on carbonic acid have shown that this mild acid offers some benefit compared to liquid hot water (4,5), but that performance is generally less effective than optimized dilute H2S04. Laboratory investigations of carbonic acid pretreatment have shown that pretreatment effectiveness is primarily a function of time and temperature, and that high C02 pressure enhances hydrolysis on some substrates such as corn stover (8) but offers little benefit on aspen wood (6,7). Thus, for certain substrates such as aspen wood, lower pressure values are likely to offer performance similar to higher pressures. To date, no study integrating carbonic acid pretreatment, enzymatic hydrolysis, and fermentation has been carried out to determine the overall ethanol yield compared to similar. 

Aden, A., Ruth, M., Ibsen, K., Jechura, J., Neeves, K., et al., Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis for Corn Stover NREL Report No. TP-510-32438 available online at URL http //www.nrel.gov/publications/ National Renewable Energy Laboratory Golden, CO, June, 2002, 2002 p. 154. 

For example, biomimetic catalysis for hemicellulose hydrolysis in corn stover have been reported [206]. In fact, efficient and economical hydrolysis of plant cell wall... 

Process Design and Economics for Biochemical Conversion ofLignocel-lulosic Biomass to Ethanol Dilute-Acid Pretreatment and Enzymatic Hydrolysis of Corn Stover, National Renewable Energy Laboratory, Golden, CO. 

Figure 7. Hydrolysis of corn stover by binary mixtures of P. funiculosum Cel7A and A. celluloivticus EL Open circles demonstrate the hydrolysis kinetics of the recombinant enzyme produced in A. awamori and closed circles demonstrate the kinetics of Cel7A enzyme purified from the native P. funiculosum host. Each cellobiohydrolase was loaded at 27,8 mg/g cellulose in the presence of celluloivticus rEIcd loaded at 1.13 mg/g cellulose (95 5 molar ratio of cellobiohydrolase to endoglucanase). DSA at pH 5.0 (20 mM acetate/sodium acetate), 38/ C, with pretreated corn stover loaded at 4.4% (w/v) total solids for...

---