Simultaneous Saccharification and Fermentation of Pretreated Eucalyptus grandis Under High Solids Loading

A comparative account of glucose yields and bioethanol production from separate and simultaneous saccharification and fermentation processes at high solids loading with variable PEG concentration

A process strategy to aid in optimal enzymatic hydrolysis through the addition of polyethylene glycol (PEG6000) was tested for separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF). Pretreated wheat straw at 30% solids (w/w) loading was enzymatically hydrolyzed with 0, 0.5, 1, 1.5, 2 and 2.5% of PEG6000 through SHF and SSF. During SHF, bioethanol concentration of 107.5 g/L (2.5% PEG6000) was achieved. SSF ethanol concentration were about 113 g/L at 1.5% PEG6000 addition. A technoeconomic feasibility showed a return on investment (ROI) of 8.13% using 0.5% PEG6000 for SHF (96 h) and 12.25% ROI for SSF control (72 h). Life cycle assessment for the various scenarios indicated higher environmental gains for best cases of SSF over SHF. The study shows the SSF approach (0% PEG6000; 72 h) facilitates higher process efficiencies; technoeconomic gains and high environmental sustainability for future scale-up and commercial realization.

Comparison of ethanol yield from pretreated lignocellulo-starch biomass under fed-batch SHF or SSF modes

The ethanol yields from lignocellulo-starch biomass (peels of sweet potato, elephant foot yam, tannia, greater yam and beet root) by fed-batch separate hydrolysis and fermentation (F-SHF) and simultaneous saccharification and fermentation (F-SSF) using Saccharomyces cerevisiae were compared. Fed-batch saccharification of steam or dilute sulphuric acid pretreated biomass enhanced the reducing sugar yield which resulted in high RS consumption, volumetric ethanol productivity and ethanol yield during the first 24 h fermentation under F-SHF mode, while continuous production and utilization of reducing sugars occurred up to 72 h in F-SSF. Dilute sulphuric acid pretreated residues under F-SHF gave higher ethanol yield (34-43 g/L) and productivity (274-346 ml/kg dry biomass) than steam pretreatment (27-36 g/L and 223-295 ml/kg respectively), while F-SSF was superior for steam pretreated peels of sweet potato, elephant foot yam and tannia giving ethanol yields from 281 to 302 ml/kg. Glucose and xylose were present in all the hydrolysates with a preponderance of glucose and fermentation resulted in significant reduction in glucose levels in both F-SHF and F-SSF. Higher levels of total soluble phenolics and hydroxymethyl furfural were observed in the hydrolysates from dilute sulphuric acid pretreatment and yeast assimilated/detoxified part of the inhibitors, while only trivial amounts of furfural were present due to the low xylose content in the hydrolysates. Continuous formation led to higher accumulation of inhibitors in F-SSF despite supplementation with the detoxification mix comprising Tween 20, polyethylene glycol and sodium borohydride. F-SHF of dilute sulphuric acid pretreated biomass could be considered as a comparatively advantageous process where only one time feeding of enzyme cocktail and yeast was adopted compared to multiple feeds of enzymes and yeast along with other additives such as detoxification mix or nutrient solution in F-SSF.