Separate hydrolysis and co-fermentation for improved xylose utilization in integrated ethanol production from wheat meal and wheat straw

Abstract

BackgroundThe commercialization of second-generation bioethanol has not been realized due to several factors, including poor biomass utilization and high production cost. It is generally accepted that the most important parameters in reducing the production cost are the ethanol yield and the ethanol concentration in the fermentation broth. Agricultural residues contain large amounts of hemicellulose, and the utilization of xylose is thus a plausible way to improve the concentration and yield of ethanol during fermentation. Most naturally occurring ethanol-fermenting microorganisms do not utilize xylose, but a genetically modified yeast strain, TMB3400, has the ability to co-ferment glucose and xylose. However, the xylose uptake rate is only enhanced when the glucose concentration is low.ResultsSeparate hydrolysis and co-fermentation of steam-pretreated wheat straw (SPWS) combined with wheat-starch hydrolysate feed was performed in two separate processes. The average yield of ethanol and the xylose consumption reached 86% and 69%, respectively, when the hydrolysate of the enzymatically hydrolyzed (18.5% WIS) unwashed SPWS solid fraction and wheat-starch hydrolysate were fed to the fermentor after 1 h of fermentation of the SPWS liquid fraction. In the other configuration, fermentation of the SPWS hydrolysate (7.0% WIS), resulted in an average ethanol yield of 93% from fermentation based on glucose and xylose and complete xylose consumption when wheat-starch hydrolysate was included in the feed. Increased initial cell density in the fermentation (from 5 to 20 g/L) did not increase the ethanol yield, but improved and accelerated xylose consumption in both cases.ConclusionsHigher ethanol yield has been achieved in co-fermentation of xylose and glucose in SPWS hydrolysate when wheat-starch hydrolysate was used as feed, then in co-fermentation of the liquid fraction of SPWS fed with the mixed hydrolysates. Integration of first-generation and second-generation processes also increases the ethanol concentration, resulting in a reduction in the cost of the distillation step, thus improving the process economics.

Integrated approach for selecting efficient Saccharomyces cerevisiae for industrial lignocellulosic fermentations: Importance of yeast chassis linked to process conditions

“…Ko et al (2016) reported for the SXA-R2 P-E strain, expressing the XI pathway, in oak hardwood hydrolysate with approximately 6 g/L of acetic acid, an ethanol yield of 0.43 g/g. Studies using wheat straw as feedstock with industrial recombinant yeast strains expressing also the XR-XDH pathway have reported ethanol yields from 0.39 g/g (KE6-12 strain) up to 0.48 g/g, with TMB3400 (Erdei et al, 2012(Erdei et al, , 2013. While these results were obtained in fed-batch fermentation, the ethanol yields obtained in this work stand among those.…”

Prefermentation improves ethanol yield in separate hydrolysis and cofermentation of steam-pretreated wheat straw

“…Fed-batch SHCF of steam-pretreated wheat straw has previously been shown to enhance xylose utilization [22,23]. However, regardless of chosen prefermentation configuration a single addition of enzymatic hydrolyzate elicited higher xylose utilization and ethanol yields than corresponding fedbatch cofermentations with two additions (Table 3).…”

“…Both strategies have advantages, and the choice is strain and feedstock dependent. Modifications to the fundamental strategies have been implemented to improve fermentation performance and substrate utilization [22][23][24]. Fed-batch design has been implemented to promote coconsumption in SHCF [22,23].…”

“…Modifications to the fundamental strategies have been implemented to improve fermentation performance and substrate utilization [22][23][24]. Fed-batch design has been implemented to promote coconsumption in SHCF [22,23]. Fed-batch designs, where a glucose-rich feed supported the xylose utilization, improved the overall ethanol yields, and lowered xylitol production in co-fermentation of steam-pretreated wheat straw with strains of xylose-fermenting S. cerevisiae [23].…”

Enhanced bioethanol production from wheat straw hemicellulose by mutant strains of pentose fermenting organisms Pichia stipitis and Candida shehatae

“…The microbial strain selected for the fermentation of pentose sugars has a large effect on ethanol yield (Anuj et al 2011). Therefore, naturally xylose-fermenting yeasts such as Candida shehatae and Pichia stipitis have been widely studied because of their ability to ferment xylose into ethanol (Borbala et al 2012). Pichia stipitis is considered a promising strain because it can ferment a wide range of sugars, including cellobiose (Nigam 2001).…”