ADVANCED CONVERSION TECHNOLOGIES FOR SUGARS AND BIOFUELS: SUPERIOR FEEDSTOCKS, PRETREATMENTS, INHIBITOR REMOVAL, AND ENZYMES
Location: Bioenergy Research Unit
Title: Hydrothermal pretreatment of sugarcane bagasse using response surface methodology improves digestibility and ethanol production by SSF
Submitted to: Journal of Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 24, 2011
Publication Date: November 12, 2011
Citation: da Cruz, S.H., Dien, B.S., Nichols, N.N., Saha, B.C., Cotta, M.A. 2012. Hydrothermal pretreatment of sugarcane bagasse using response surface methodology improves digestibility and ethanol production by SSF. Journal of Industrial Microbiology and Biotechnology. 39:439-447.
Interpretive Summary: Sugar bagasse is the discarded stalks generated once sugar is removed from sugarcane. Brazilian sugar and ethanol producers currently burn the bagasse to generate steam to run their process. It should be possible to convert it also to ethanol because more sugarcane bagasse is collected than needed for steam production. In this paper, a process was evaluated for converting sugarcane bagasse into ethanol. The biomass was treated with liquid hot-water as a pretreatment in order to open up the stalks and expose the carbohydrates. The pretreated biomass was next mixed with enzymes that convert the carbohydrates into simple sugars and a yeast that ferments these sugars into ethanol. The yeast used in this study was especially suited for these types of fermentations because it gives higher ethanol yields compared to industrial yeast strains; it was developed by National Center for Agricultural Utilization Research (NCAUR) scientists. The ethanol yield per gram of sugarcane bagasse was evaluated for varying pretreatment conditions and a statistical approach was applied to determine the optimal reaction conditions. One problem when biomass is pretreated with hot water is that chemicals are formed during the reaction that inhibits the yeast fermentation. This potential problem was avoided by measuring inhibitor concentrations and ensuring the pretreatment condition selected did not generate excess amounts of these chemicals. We also used a novel method developed by NCAUR scientists that is able to remove these inhibitors prior to fermentation. The optimal pretreatment selected was treating bagasse at 190 deg C for 17.2 min. This paper will be of interest to sugarcane refiners throughout the world and of general interest to those interested in commercializing cellulosic ethanol.
Sugarcane bagasse was characterized as a feedstock for production of ethanol using hydrothermal pretreatment. Reaction temperature and time were varied between 160-200 deg C and 5-20 min, respectively, using a response surface experimental design. The liquid fraction was analyzed for soluble carbohydrates and furan aldehydes. The solid fraction was analyzed for structural carbohydrates and Klason lignin. Pretreatment conditions were evaluated based upon enzymatic extraction of glucose and xylose and conversion to ethanol using a simultaneous saccharification and fermentation scheme. The severity of the pretreatment should be sufficient to drive enzymatic digestion and ethanol yields, however, sugar losses and especially sugar conversion into furans needs to be minimized. As expected, furfural production increased with pretreatment severity and specifically xylose release. However, provided that the severity was kept below a general severity factor of 4.0, production of furfural was below an inhibitory concentration and carbohydrate contents were preserved in the pretreated whole hydrolysate. There were significant interactions between time and temperature for all the responses except cellulose digestion. The models were highly predictive for cellulose digestibility (r**2=0.8737) and for ethanol production (r**2=0.9632), but less so for xylose extraction. Both cellulose digestion and ethanol production increased with severity, however, high levels of furfural generated under more severe pretreatment conditions favor lower severity pretreatments. The optimal pretreatment condition that gave the highest conversion yield of ethanol, while minimizing furfural production, was judged to be 190 deg C and 17.2 min.