This article highlights our approach to converting biomass into ethanol using agricultural residues or coproducts as the substrate. Pectin-rich residues, such as those from processing of citrus, apples, and sugar beets, contain significant quantities of carbohydrates. These residues are available in relatively large amounts and are stockpiled in processing plants which may decrease transportation and collection costs. Ongoing research is designed to evaluate the feasibility of building a rural-based biorefinery coupled with existing agricultural processes. Our studies using pectin-rich sugar beet pulp as the processing coproduct for fuel ethanol production with engineered bacteria as the biocatalyst will be briefly described as the foundation for a rural-based biorefinery. 
 
 Lignocellulosic Biomass Conversions 
 Corn-to-ethanol plants currently produce most U.S. ethanol and are located predominantly in the Upper Midwest. Environmental concerns, realization that fossil fuel is limited, and increasing discomfort with our dependence on foreign oil may significantly increase the demand for ethanol. Lignocelluose constitutes the bulk of renewable biomass, is available in a wide geographic area, and may be grouped into four major categories: dedicated energy crops, urban wastes, forest related products, and agricultural residues and coproducts. 
 	
 Most lignocellulose is resistant to degradation and presents a challenging process hurdle. Ideally, the cellulose and hemicellulose components would be solubilized and fermented to ethanol, while the high BTU value lignin could provide energy for ethanol recovery. Lignocellulose is usually subjected to some form of pretreatment (dilute acid hydrolysis, steam or ammonia explosion, for example) in order to render the cellulose components accessible to enzymatic degradation. Many researchers have contributed to making a lignocellulose-based ethanol process that is economically feasible. In addition, companies such as BC International, Iogen/Petro Canada, Arkenol, Masada Resource Group, Bioresearch Engineering, and others are in various stages of lignocellulosic biomass-to-ethanol projects. 
 
 A Biorefinery from Pectin-rich Residues 
 A significant benefit of using agricultural residues for a biorefinery is that several products are generated from an already collected, partially processed biomass stream (Figure 1). According to the Roadmap for Biomass Technologies in the United States (http://www.bioproducts-bioenergy.gov/pdfs/FinalBiomassRoadmap.pdf), a biorefinery is "a processing and conversion facility that (1) efficiently separates its biomass raw material into individual components and (2) converts these components into marketplace products, including biofuels, biopower, and conventional and new bioproducts. As we all realize, ethanol must be made cheaply in order to compete with gasoline. Unfortunately, the cost of a gallon of gasoline ignores the true cost of using petroleum-based fuels including: environmental damage, military actions, imbalance of trade, etc. If additional higher-value products could be manufactured from a single biomass stream - or fractionated stream - the overall costs of ethanol production should decrease. As corn-to-ethanol producers already realize, a surplus of coproducts produces new opportunities as well as challenges, and can greatly alter the overall economics of their process. 
 	
 Our recent studies have used pectin-rich sugar beet pulp as a substrate for production of fuel ethanol. Sugar beet pulp is produced annually on the order of 400 million tons, with U.S. production fluctuating between 20-30 million wet tons for the past 20 years. Production of livestock feed from beet pulp can be an economically marginal part of beet sugar processing because of low feed value and relatively high drying costs, with 20 percent to 40 percent of overall energy costs of sugar beet processing devoted to dehydration and pelletizing pulp. The market for ethanol production from whole sugar beets may be self-limiting due to competing value as food, similar to corn-based fermentations. However, conversion of parts remaining after processing is an attractive option for production of fuel ethanol and for environmental bioremediation. Another attraction of this approach is that a significant number of agricultural processing plants, such as sugar production from beets, are farmer/industry cooperatives. Farmers and rural communities would see a much greater benefit if they are able to produce the biofuels or bioproducts themselves, either on the farm or as owners in a local production plant.
 
 Recombinant Bacteria as Biocatalysts

 Our laboratory at the University of Georgia is evaluating microorganisms isolated from the environment and engineered bacteria capable of fermenting acidic sugars for the production of ethanol and specialty chemicals from pectin-rich biomass (Figure 2). Environmental isolates from diverse sites ranging from agricultural processing sites to the gut of a lignocellulose digesting aquatic insect are being explored for novel organisms and enzymes for biomass conversions. Related projects involve using chemical pretreatments to help decrease the amount of enzymes required for biomass degradation. Another aspect of our research collaborations is to identify specialty chemicals coproduced with ethanol, and those that may be produced from a separate processing stream at the same plant.
 	
 One recent project used sugar beet pulp for fermentation to ethanol by engineered bacteria provided by Dr. Lonnie Ingram at the University of Florida. These engineered bacteria contain the ethanol pathway genes from another bacterium, Zymomonas mobilis. Sugar beet pulp was used as collected at the processing plant in the pelletized (10 percent moisture), pressed (75 percent moisture), and wet (90 percent moisture) forms. All forms performed well in initial studies, with the pressed pulp yields slightly higher than the others. 
 
 Although several of the engineered bacteria possessed enzymes for biomass degradation, low levels of ethanol were 
 produced (3-12 grams/liter, depending upon the bacterium and form of pulp). When fermentations were amended with commercially available fungal enzyme preparations, the ethanol concentrations increased accordingly. Using 122 g dry weight pulp/L present at the start of fermentations and 1 percent v/v of pectinase and 1 percent cellulase, over 40 g ethanol/L was produced after 48 hours of fermentation using a recombinant strain (Figure 3). Using a second approach of starting at a 10% solids concentration followed by pulp addition (122 g dry weight pulp final solids), 38 g ethanol /L was produced by 120 hours with 1 percent v/v cellulase and only 0.5 percent v/v pectinase. These recombinant strains could not metabolize cellobiose, and an additional 0.27% v/v of cellobiase was required to reach 40 g/L ethanol (Figure 3). Currently a cellobiose metabolizing strain is being evaluated in similar studies. An ethanol concentration of 40 g/L was our target for cost effective distillation, however, some believe that lower concentrations of ethanol may be economically recovered with the economics of the overall process allowing flexibility. With no chemical or physical pretreatments and an enzyme loading of 0.57 percent v/v cellulase, 0.28 percent v/v pectinase, and 0.28 percent v/v cellobiase, recombinant E. coli generated about 30 g ethanol/L. 
 	
 We have also evaluated chemical pretreatment of the pulp such as ammonia pressurization and depressurization (APD) in collaboration with Dr. Bruce Dale at Michigan State University, in which the pulp was exploded by sudden evaporation of ammonia in a reactor vessel. ADP substantially increased hydrolysis efficiency of the cellulose component, comparable to other types of biomass. Sugar beet pulp was the first pectin-rich biomass to be ADP processed and initial experiments showed that when hemicellulose and pectin-degrading enzymes were added, treated pulp hydrolysis was no better than untreated controls. Initial studies with dilute acid hydrolysis in our laboratory and others, coupled with the ADP results indicate that chemical pretreatments of pectin-rich residues has yet to be optimized. 
 
 Ongoing Studies 
 In order to further decrease the amount of externally supplied fungal enzymes in beet pulp fermentations, we are engineering bacteria for production of pectin-degrading enzymes. In collaboration with Dr. Ingram, we are evaluating engineered Escherichia coli strains for pectin-rich biomass conversions by adding Erwinia chrysanthemi secretion pathway genes and genes for pectin degradation. Two new recombinant derivatives of E. coli, LY41 and LY40, containing the casAB operon for cellobiose utilization and the pelE gene for pectate lyase activity, show promise for cost efficient pectin-rich biomass conversions for fuel ethanol production.
 	
 Another aspect of our research collaborations is to identify specialty chemicals coproduced with ethanol, and those that may be produced from a separate processing stream at the same plant. Flavor enhancers, antimicrobials, and high quality absorbent cellulose, are just a few of the products that may be produced from a beet-pulp-to-ethanol processing facility. 
 	
 Finally, a collaboration with researchers at the University of Georgia, Oak Ridge National Laboratories and Michigan State University may help to determine the fate of lignocellulosic carbon in small stream ecosystems while providing novel microorganisms and their enzymes for potential industrial applications. EP

University of Georgia researcher Dr. Joy Doran Peterson highlights her team's approach to converting biomass into ethanol using pectin-rich residues, such as those from processing of citrus, apples, and sugar beets.

Pectin-rich Biorefinery for Production of Ethanol and Specialty Chemicals

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Upcoming Events

2024 International Fuel Ethanol Workshop & Expo

Biodiesel Summit: Sustainable Aviation Fuel & Renewable Diesel

Carbon Capture & Storage Summit

North American SAF Conference & Expo

2025 International Biomass Conference & Expo

Pectin-rich Biorefinery for Production of Ethanol and Specialty Chemicals