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High titer ethanol production from simultaneous enzymatic saccharification and fermentation of aspen at high solids: A comparison between SPORL and dilute acid pretreatments q J.Y. Zhu a,⇑ , R. Gleisner a , C.T. Scott a , X.L. Luo b , S. Tian c a USDA Forest Service, Forest Products Laboratory, Madison, WI, USA b Key Laboratory Pulp and Paper Engineering, South China University of Technology, Guangzhou, China c College of Life Science, Capital Normal University, Beijing, China a r t i c l e i n f o Article history: Received 16 May 2011 Received in revised form 6 July 2011 Accepted 15 July 2011 Available online 22 July 2011 Keywords: (Woody) Biomass Pretreatment High solids Simultaneous saccharification and fermentation (SSF) Cellulosic ethanol titer a b s t r a c t Native aspen (Populus tremuloides) was pretreated using sulfuric acid and sodium bisulfite (SPORL) and dilu te sulf uric acid alon e (DA) . Simu ltane ous enzyma tic sacchari ficati on and fermenta tion (SSF) was conducted at 18% solids using commer cial enzymes with cellulase loadings rangin g from 6 to 15 FPU/g glucan and Saccharomyces cerevisiae Y5. Compared with DA pretreatment, the SPORL pretreatment reduced the energy required for wood chip size-reduction, and reduced mixing energy of the resultant substrate for solid liquefaction. Approximately 60% more ethanol was produced from the solid SPORL substrate (211 L/ton woo d at 59 g/L with SSF efficien cy of 76%) than fro m the solid DA substrate (133 L/ton wo od at 35 g/ L wi th SSF ef fic iency 47 %) at a cell ul ase lo ad ing of 10 FP U/ g gl uc an af ter 12 0 h. Wh en the cell ul as e loa din g was inc rea sed to 15 FPU /g glu can on the DA sub str ate , the eth ano l yie ld still remained lower than the SPORL substr ate at 10 FPU/g glucan. Published by Elsevier Ltd. 1. Introduction Many technologic al barriers remain for the economical production of etha nol fromlignoce llul osic biom ass. Effi cien t and high titer ethanol production thro ugh the enz yma tic sacch arifi catio n and fermentation of cellulose is a promising process strategy. Despite re- cent progress in the development of lignocellulosic pretreatments (Sun and Chen g, 20 02; Zhu and Pa n, 2010) and in enzy matic hy dro- lys is of cel lulo se (Ma nsfi eldet al. , 19 99;Zhan g and Ly nd , 2004), fe w studies hav e dem ons trated efficient high titer (>40 g/L) ethanol production. Furthermore, there have been only limited investiga- tions on the simultaneous enzymatic saccharification and fermentation (SSF) at high-solids loadings. Most of the reported studies resulted in low conversion efficiencies unless very high cellulase loading levels were used ( Jorgensen et al., 2007; Lu et al., 2010; Zhang et al., 2010). For example, an enzyme load ing of 21.5 FPU/g glu can (7 FPU/g subs trate ) pro duc ed an ethanol con cen trati on of 25 g/L wi th an SS F ef ficien cy of ap pr oximate ly 68 % us ing steam ex- pl od ed co rn stover at 200 °C,at a so li dsloa di ngof 20 % (Zh ang et al. , 2010). Oth er example s inclu de an SS F effi cie nc y les s tha n 50%usin g hot water pretreated wheat straw at 205 °C that was hydrolyzed and fer mented at 20% solids using a cell ul aseloadin g of 11 .8 FPU /g glucan ( Jorgensen et al., 2007), and a saccharification efficiency of approximately 70% using a dilute acid (DA) pretreated corn stover that was enzymatically saccharified at 20% solids with a cellulase loading of 15FPU/g glucan (Dasari and Berson, 2007). In addition, it was shown that cellulose conversion decreased rapidly with fur- ther increases in substrate solids loading ( Zhang et al., 2010). Many factors affect enzymatic saccharification kinetics and fermentation efficiency at high-solids loadings. Product (sugar) inhi- bition can significan tly affect enzymatic saccharification at high sugar concentration s (Holtzapple et al., 1990). However, this effect can be eliminated by usin g SSF to maintai n a low sugar concen tra- tion thro ugh out the ferm entation pro cess. Inte nsiv e mixing at high-solids may also play an important role in the saccharification ra te of cel lulos e. Di ffe re nt mi xer s, suc h as a peg mi xer ( Zhan g et al., 2009), helical ribbon mixers ( Zhang et al., 2010), and a tumbling re act or ( Jorg ense n et al., 2007), have been inve stiga ted. But mixi ng ene rgy consum ption was not reported. This makes it diffi cul t to assess mix ing effic ienc y and pro cess ene rgy req uire ments for enz y- matic saccharification. The cellulose accessibility of the pretreated substrate is an important factor affecting enzymatic saccharification efficie ncy . In one study, a high ly dige stibl e orga nos olv pre - treated poplar wood subs trate hydrolyzed at 20% soli ds usin g 0960-8524/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.biortech.2011.07.047 q This work was conducted on official US government time by Zhu, Gleisner, and Scott while Luo and Tian were visiting scienti sts at the USDA Forest Products Laboratory. ⇑ Corresponding author. E-mail address: [email protected] (J.Y. Zhu). Bioresource Technology 102 (2011) 8921–8929 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

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