Bioconversion of Corn Stover into Value-Added Chemicals: Dilute Sulfuric Acid Pretreatment, Xylo-Oligosaccharides Production, and Lactic Acid Fermentation

Abstract

In the first part of this dissertation, dilute-acid pretreatment of corn stover was investigated under high-solids condition using a percolation reactor. High-solids condition in a pretreatment process can lead to high productivity and low liquid throughput. The latter reduces the cost of process energy as well as the load of wastewater treatment. The effects of temperature, acid concentration, acid flow rate, and reaction time on the glucan digestibility and recovery of hemicellulose sugars were investigated. The experimental results were assessed to determine the optimum ranges of reaction conditions. The xylose yield was affected sensitively by the flow rate under a given reaction condition. This behavior appears to be related to sugar decomposition, mass transfer resistance and the fact that acid is neutralized by the buffering components of the biomass. Further improvement of this process was sought with modifications in the operation mode of the percolation reactor. The reactor was preheated under atmospheric pressure to remove moisture that may cause autohydrolysis and subsequent sugar decomposition. In addition, liquid throughput was minimized to the extent that only one reactor void volume of liquid was collected. This was done to attain high xylose concentration. For the substrates treated under the optimum reaction and operating conditions, near quantitative glucan digestibility was obtained while the decomposition of carbohydrates was suppressed to an extremely low level. The digestibility tests done on the pretreated samples indicated that the digestibility is related to the extent of xylan removal. The second part of this dissertation dealt with enzymatic production of xylo-oligosaccharides (XOs, also known as xylo-oligomers) from corn stover and cobs. The process started with a pretreatment known as soaking in aqueous ammonia (SAA). The pretreated feedstock containing digestible xylan was then directly subjected to selective enzymatic hydrolysis for the production of XOs. The complex substrate purification step required in the conventional method is not necessary in this process. This simplifies the overall process, and more importantly, improves the process economics. In the subsequent stage, fractionation and refining of XOs were accomplished by charcoal adsorption followed by ethanol elution, with XOs being collected in relatively high yields. Xylanolytic hydrolysis of the SAA treated corn stover has shown high digestibility of the remaining glucan. As a feedstock for production of XOs, corn cobs are superior to corn stover because of high xylan content and density. The high packing density of corn cobs reduced water use in both SAA treatment and enzymatic hydrolysis, which eventually led to high XOs concentration. In the last part of the dissertation, production of lactic acid by simultaneous saccharification and co-fermentation (SSCF) was investigated. The SAA treated and water washed corn stover was the substrate of this investigation. The microorganism used for the co-fermentation was Lactobacillus pentosus ATCC 8041 (CECT-4023). When the SSCF was conducted in batch mode based on 3% (w/w) glucan loading, the carbohydrates (both cellulose and hemicellulose) in the treated corn stover were effectively converted to lactic acid and acetic acid, the maximum lactic acid yield reaching 92 % of the theoretical maximum based on the total of carbohydrates (glucose, xylose, and arabinose). Small amount of acetic acid was also produced from the pentoses through the phosphoketolase pathway. The impacts of enzyme loading, inocula size, yeast extract concentration, and clarified corn steep liquor (cCSL) concentration were investigated in relation to lactic acid production following a statistical experimental design. The statistical analysis showed that enzyme and yeast extract were the most important factors affecting the final lactic acid yield. In contra