Lignocellulosic ethanol production by starch-base industrial yeast under PEG detoxification

Abstract

Cellulosic ethanol production from lignocellulosic biomass offers a sustainable solution for transition from fossil based fuels to renewable alternatives. However, a few long-standing technical challenges remain to be addressed in the development of an economically viable fermentation process from lignocellulose. Such challenges include the needs to improve yeast tolerance to toxic inhibitory compounds and to achieve high fermentation efficiency with minimum detoxification steps after a simple biomass pretreatment. Here we report an in-situ detoxification strategy by PEG exo-protection of an industrial dry yeast (starch-base). The exo-protected yeast cells displayed remarkably boosted vitality with high tolerance to toxic inhibitory compounds, and with largely improved ethanol productivity from crude hydrolysate derived from a pretreated lignocellulose. The PEG chemical exoprotection makes the industrial S. cerevisiae yeast directly applicable for the production of cellulosic ethanol with substantially improved productivity and yield, without of the need to use genetically modified microorganisms.Cellulosic ethanol from lignocellulosic biomass through biological fermentation has been recognized as a sustainable transportation fuel due to the most abundant carbohydrate content of the broadly distributed non-food feedstock 1-3 . Very high gravity (VHG) fermentation, referring to the fermentation of high sugar concentrations, offers the advantages of improved overall ethanol productivity (producing ethanol in 10-15 vol%), reduced capital cost, and reduced energy input compared to processes at normal gravity 4 . This technology represents a major progress toward cost-competitive production of cellulosic ethanol. With lignocellulosic biomass as the feedstock, a pretreatment process is typically necessary to generate monomeric sugars from the polysaccharide components of the biomass for the subsequent fermentation process. However, typical pretreatment processes of lignocellulosic materials inevitably generate degradation compounds, e. g., acetic and formic acids, furfural, and 5-hydroxymethylfurfural (HMF) and phenolic compounds [5][6][7] . The residue of these compounds often exists in fermentation broth and functions as toxic inhibitors [8][9][10] . To achieve fermentable sugars in a high-concentration for VHG fermentation, the biomass loading ratio during pretreatment must be increased to a considerably high level, which typically results in high concentrations of inhibitors in the fermentation broth. These inhibitors often significantly reduce the rates of yeast metabolism and the final ethanol titers in the subsequent fermentation step 11,12 . The detrimental effect of the inhibitors remains one of the major barriers to the development of an economically viable process for cellulosic ethanol production [13][14][15] . To overcome the issues related to the inhibitory compounds in the lignocellulosic hydrolysates, some techniques on detoxifying the hydrolysates by removing the toxic chemical residues have been repo...

Bioconversion of pomegranate residues into biofuels and bioactive lipids

“…The low μ max values in the present study may be due to the cell growth inhibition by ethanol and/or inhibitory compounds generated during PRs pretreatment. It has been reported that the presence in the fermentation medium of phenolics at 2.0 g/L negatively affects both cell viability of S. cerevisiae and ethanol production (Liu et al, 2016). In the present study, the initial concentration of phenolic compounds was about 2.0 g/L and remained almost constant during fermentation of S. cerevisiae .…”

Evaluation of buriti endocarp as lignocellulosic substrate for second generation ethanol production

“…Additionally, it is possible to observe that 28.99% of the reducing sugars present in the medium were not consumed. Although the pretreatment processes of the lignocellulosic biomass employed in this work are widely used strategies, they have the disadvantage of generating toxic compounds to fermenting organisms, such as phenolic compounds, guaiacol, levulinic acid, furfural and 5-hydroxymethyl furfural (Varanasi et al, 2013;Liu et al, 2016). The presence of such substances may have inhibited the activity of S. cerevisiae, making it impossible to deplete the substrate offered, since the samples were not detoxified to remove these substances.…”

Evaluation of buriti endocarp as lignocellulosic substrate for second generation ethanol production

“…It is possible to observe in the fermentation experiment with Saccharomyces cerevisiae that 28.99% of the reducing sugars present in the medium were not consumed. The pretreatment processes of lignocellulosic biomasses employed in this work are widely used strategies; however, they have the disadvantage of generating toxic compounds to fermenting organisms, such as phenolic compounds, guaiacol, levulinic acid, furfural and 5-hydroxymethylfurfural ( Varanasi et al, 2013 ; Liu et al, 2016 ). Although the pre-treated endocarp was rinsed with distillate water before enzymatic scarification, Zhang et al (2010) indicate that this method is not efficient for total removal of toxic compounds impregnated in the biomass.…”