INDUSTRIALLY ROBUST ENZYMES AND MICROORGANISMS FOR PRODUCTION OF SUGARS AND ETHANOL FROM AGRICULTURAL BIOMASS
Location: National Center for Agricultural Utilization Research
Title: CHEMICAL COMPOSITION AND RESPONSE TO DILUTE-ACID PRETREATMENT AND ENZYMATIC SACCHARIFICATION OF ALFALFA, REED CANARYGRASS, AND SWITCHGRASS
Submitted to: Biomass and Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 24, 2006
Publication Date: September 22, 2006
Citation: Dien, B.S., Jung, H.G., Vogel, K.P., Casler, M.D., Lamb, J.F., Iten, L.B., Mitchell, R., Sarath, G. 2006. Chemical composition and response to dilute-acid pretreatment and enzymatic saccharification of alfalfa, reed canarygrass, and switchgrass. Biomass and Bioenergy. 30:880-891.
Interpretive Summary: Production of ethanol from corn and other grain crops fulfills approximately 3% of the U.S. automotive fuel needs. Converting energy crops to ethanol may also be beneficial because they can be grown on otherwise marginal crop lands, thereby greatly increasing the land available for producing biomass. In this paper, three energy crops were investigated as a feedstock for ethanol: alfalfa, reed canarygrass, and switchgrass. Recoverable sugar yields were measured for each species using traditional conversion technologies. Further novel discoveries resulting from this work included: glucose can be more efficiently recovered from earlier than later harvested biomass, alfalfa (a dicot) required higher severity pretreatment conditions than the monocots (reed canarygrass and switchgrass), and especially for younger maturity plants, pretreatments need to take into account the presence of soluble sugars. Fructose was found to be particularly sensitive to pretreatment conditions required for releasing structural carbohydrates. The information presented in this paper is expected to be of value to farmers wishing to gain further value from land not suitable for growing grain crops and to ethanol producers looking for future feedstocks to supplement grains.
Alfalfa, reed canarygrass, and switchgrass; perennial herbaceous species that have potential as biomass energy crops in temperate regions; were evaluated for their bioconversion potential as energy crops. Each forage species was harvested at two or three maturity stages and analyzed for carbohydrates, lignin, protein, lipid, organic acids, and mineral composition. The biomass samples were pretreated with dilute sulfuric acid at two temperatures (121 deg C and 150 deg C). The pretreated residues were subsequently subjected to enzymatic saccharification using a commercial cellulase preparation. Acid-released sugars and cellulase degradability of the biomass samples were determined. More mature biomass samples of all forage species contained less protein and mineral constituents than immature harvests. Alfalfa stems contained the most protein of the biomass samples, whereas reed canarygrass had more minerals and less lignin than the other biomass samples. Carbohydrate concentration overlapped among the biomass samples, but the two grasses contained more storage carbohydrates (starch and/or fructans) than the alfalfa stems. Cell wall polysaccharides of the grasses were richer in non-glucose residues than alfalfa stems, but cell wall glucose content of the biomass samples was similar. The alfalfa stem samples required a higher acid loading to achieve the same final pretreatment pH as the grasses. The 150 deg C dilute-acid pretreatment method resulted in higher yields of glucose than observed with the 121 deg C pretreatment for almost all the forage samples; however, yield of non-glucose sugars was generally reduced at the higher pretreatment temperature. The loss in yield was due to the degradation of fructose (present in the biomass samples as free fructose, sucrose, and fructans) at the higher pretreatment temperature. Efficiency of glucose release was negatively correlated with Klason lignin content of the biomass samples.