Simultaneous saccharification and co-fermentation for bioethanol production using corncobs at lab, PDU and demo scales

Abstract

Background: While simultaneous saccharification and co-fermentation (SSCF) is considered to be a promising process for bioconversion of lignocellulosic materials to ethanol, there are still relatively little demo-plant data and operating experiences reported in the literature. In the current work, we designed a SSCF process and scaled up from lab to demo scale reaching 4% (w/v) ethanol using xylose rich corncobs.

Results: Seven different recombinant xylose utilizing Saccharomyces cerevisiae strains were evaluated for their fermentation performance in hydrolysates of steam pretreated corncobs. Two strains, RHD-15 and KE6-12 with highest ethanol yield and lowest xylitol yield, respectively were further screened in SSCF using the whole slurry from pretreatment. Similar ethanol yields were reached with both strains, however, KE6-12 was chosen as the preferred strain since it produced 26% lower xylitol from consumed xylose compared to RHD-15. Model SSCF experiments with glucose or hydrolysate feed in combination with prefermentation resulted in 79% of xylose consumption and more than 75% of the theoretical ethanol yield on available glucose and xylose in lab and PDU scales. The results suggest that for an efficient xylose conversion to ethanol controlled release of glucose from enzymatic hydrolysis and low levels of glucose concentration must be maintained throughout the SSCF. Fed-batch SSCF in PDU with addition of enzymes at three different time points facilitated controlled release of glucose and hence co-consumption of glucose and xylose was observed yielding 76% of the theoretical ethanol yield on available glucose and xylose at 7.9% water insoluble solids (WIS). With a fed-batch SSCF in combination with prefermentation and a feed of substrate and enzymes 47 and 40 g l-1 of ethanol corresponding to 68% and 58% of the theoretical ethanol yield on available glucose and xylose were produced at 10.5% WIS in PDU and demo scale, respectively. The strain KE6-12 was able to completely consume xylose within 76 h during the fermentation of hydrolysate in a 10 m3 demo scale bioreactor.

Conclusions: The potential of SSCF is improved in combination with prefermentation and a feed of substrate and enzymes. It was possible to successfully reproduce the fed-batch SSCF at demo scale producing 4% (w/v) ethanol which is the minimum economical requirement for efficient lignocellulosic bioethanol production process.

Figure 1

Screening of S. cerevisiae strains in corncobs hydrolysate. Xylose consumption, xylitol and ethanol yields, ethanol concentration in corncobs hydrolysate after 96 h of fermentation in anaerobic shake flasks. KE4-22 is the parental strain of AD2-10 and KE6-12. AD1-13 is the parental strain of RHA-15, RHC-15 and RHD-15. A: xylose consumed (%), B: xylitol yield on consumed xylose (g g^-1), C: ethanol yield (%, based on maximum theoretical ethanol yield on available glucose and xylose), D: ethanol concentration (g l^-1) at the end of 96 h.

Figure 2

Screening of S. cerevisiae strains in corncobs whole slurry. Glucose (diamonds) and xylose (squares) consumption, ethanol (circles) and xylitol (crosses) production in SSCF at 7.5% WIS content, 5 g l^-1 of yeast loading and 5 FPU gWIS^-1 of enzyme loading using KE6-12 (a) and RHD-15 (b).

Figure 3

Model SSCF. Glucose (diamonds) and xylose (squares) consumption, ethanol (circles) and xylitol (crosses) production in a model SSCF in corncobs hydrolysate with 5 g l^-1 of KE6-12 at lab scale using a feed of glucose solution (a) and at PDU using a feed of liquid fraction after enzymatic hydrolysis (b). Amount of glucose fed is corresponding to 7.5% WIS content.

Figure 4

Fed-batch SSCF with prefermentation and split addition of enzyme at PDU. Glucose (diamonds) and xylose (squares) co-consumption, ethanol (circles) and xylitol (crosses) production using corncobs whole slurry at 7.9% WIS, 6 g l^-1 of KE6-12 with 3 FPU gWIS^-1 of enzyme loading at each time points of 2 h, 24 h and 48 h.

Figure 5

SSCF with prefermentation and fed-batch addition of substrate and enzyme. Glucose (diamonds) and xylose (squares) co-consumption, ethanol (circles) and xylitol (crosses) production using corncobs whole slurry at 10.5% WIS, 5 g l^-1 of KE6-12 and 15 FPU gWIS^-1 of enzyme loading. Split addition of substrate at 0 h, 5 h, 27 h, 49 h and enzyme solution at 2 h, 24 h, 48 h, 72 h, 96 h in PDU (a). Fed-batch addition of substrate for 48 h and split addition of enzyme solution at 2 h, 24 h, 48 h, 72 h, 96 h in demo scale (b).

Figure 6

Fed-batch fermentation in corncobs hydrolysate at demo scale. Glucose (diamonds) and xylose (squares) consumption, ethanol (circles) and xylitol (crosses) production using 5 g l^-1 of KE6-12. Corncobs hydrolysate corresponding to 6% WIS content was fed for 24 h.