ENSILING CORN STOVER WITH ENZYMES AS A FEEDSTOCK PRESERVATION METHOD FOR BIOCONVERSION

Open Access
Author:
Chen, Qin
Graduate Program:
Agricultural and Biological Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 08, 2009
Committee Members:
  • Thomas Lehman Richard, Dissertation Advisor
  • Thomas Lehman Richard, Committee Chair
  • Ali Demirci, Committee Member
  • Ming Tien, Committee Member
  • James Landis Rosenberger, Committee Member
  • Corey W Radtke, Committee Member
Keywords:
  • corn stover
  • enzyme
  • laccase
  • ensilage
  • harvest date
  • lignin depolymerization
  • pretreatment
  • bioconversion
  • modeling.
Abstract:
Energy sustainability and environmental protection are two great challenges that face humanity. Biofuels from lignocellulosic biomass have been recognized as a potential solution for both of these interrelated issues. However, there is a serious bottleneck to economical and efficient ethanol production: the recalcitrance of lignocellulosic biomass due to lignin. This bottleneck has to be solved before cellulosic biofuels can play a significant role in a renewable energy society. To produce biofuels sustainably from lignocellulosic biomass, it will be necessary to store preserve large amounts of feedstock from seasonally harvested fields. Wet storage, and specifically ensilage, could serve as a promising platform for biological pretreatment, since saccharification of the cell wall occurs naturally by organic acids and amended enzymes during the ensilage process. In this study, the impact of the interaction between corn stover harvest seasons and cell wall degrading enzymes was investigated. These investigations included both experimental studies and model simulations of the impacts of feedstock, storage conditions and enzymes, including cellulase and hemicellulase or laccase, on the characteristics of stover silage. The objectives were to obtain a low pH, minimize dry matter losses, and create beneficial biochemical changes in the stover that would facilitate downstream pretreatment and bioconversion. In temperate climates, the corn stover harvest can extend from early fall to early winter, during which time the chemical composition of stover varies significantly and influences the ensilage process. Identifying the optimum harvest period can help maximize utilization of stover as a feedstock for bioethanol. The first investigations explored the effects of harvest date and enzyme addition along with possible interactions on the characteristics of corn stover silage. Corn stover was harvested five times in 2005 and eight times in 2006 throughout early fall and early winter. Samples 500g were subsequently ensiled at 37ºC with and without the enzyme treatments at both field moisture and 60% moisture (w.b.). Dry matter loss, pH, water soluble carbohydrate and monosaccharides were analyzed on days 0, 1, 7, and 21. Samples were also subjected to reduced severity dilute acid pretreatment to quantify the conversion to simple sugars. Results demonstrated that harvest date had a significant impact on the quality of stover silage for bioconversion. The moisture content of corn stover, cob and corn were significantly influenced by harvest date, and at later harvest dates, moisture addition was critical for obtaining high quality silage. Results indicated that early fall was the best harvest time in terms of pH, dry matter, water soluble carbohydrate and monosaccharide as well as xylan conversion percentage. With respect to corn stover silage, the addition of enzymes significantly enhanced the positive effects. The presence of lignin in ligninocellulosic biomass constrains and challenges the improvement of bioconversion techniques. A second set of investigations were performed to explore the influence of the lignin-degrading enzyme, laccase, on enzymatic ensiled corn stover. Tetramethylammonium hydroxide (TMAH) thermochemolysis and Gas Chromatography - Mass Spectroscopy (GC-MS) results documented molecular signals of lignin decomposition in laccase-treated stover. Cellulose conversion through enzymatic hydrolysis improved with an increase in the laccase loading rate. This enhanced cellulose digestibility is believed to result from better exposure of cellulose to cellulase through structural changes of lignin, which makes more cellulase available for cellulose hydrolysis. The findings suggest that ensilage might provide a platform for biological pretreatment platform, partially hydrolyzing cellulose and hemicellulose into soluble sugars during the enzymatic ensilage process, and thus facilitating laccase penetration into complex biomass to enhance lignin degradation. These results serve as a first step to understanding the addition of multiple enzyme combinations during ensilage to maximize the utilization of corn stover as a biofuel and biochemical feedstock. The final quality of enzymatically ensiled corn stover was significantly affected by its initial chemical composition, microbial population dynamics, enzyme activities, and thermal and physical conditions of silage fermentations. A comprehensive experimental study to better understand how the interactions of these factors govern silage quality requires a large number of trials and intensive analysis. A predictive enzymatic ensilage model, which was initially developed in a series of papers by Pitt and his colleagues in the late 1980s, was enhanced and then applied to simulate the dynamic behavior of pH, water soluble carbohydrates (WSC), cellulose, and the effects of enzyme additives on the major biochemical and microbial changes during the ensilage process. Estimated final pH, WSC and cellulose concentrations are in agreement with enzymatic silage experimental results. The simulation results also demonstrated that the cellulose loading rate had a significant positive effect on the change of WSC. Results showed enzyme additives in the silage process enhanced the stability of long term storage. The optimal experimental conditions to obtain a high quality enzymatic corn stover silage can be achieved by adjusting the cellulase loading rate and operation temperature. The enhanced model could serve as a guide in designing silage systems (with or without enzyme additives) for large amounts of plant-based biomass. In conclusion, this study demonstrated harvest seasons and cell wall degrading enzymes have strong effects on the characteristics of stover silage, and that ensilage technology was an effective preservation and pretreatment strategy for bioconversion of corn stover biomass. In order to further improve ensilage as a partial substitute for expensive and energy-intensive thermal and chemical pretreatment technologies, future work should focus on using biological strategies to deconstruct the recalcitrant structure of lignocellulosic biomass. If successful, such efforts could thereby improve bioconversion efficiency dramatically.