Sulphite addition during steam pretreatment enhanced both enzyme-mediated cellulose hydrolysis and ethanol production

Sulphite addition during steam pretreatment of softwoods under acidic, neutral and alkaline conditions was assessed to try to minimize lignin condensation. Although pretreatment under neutral/alkaline conditions resulted in effective lignin sulphonation, non-uniform size reduction was observed. In contrast, acidic sulphite steam treatment at 210 °C for 10 min resulted in homogenous particle size reduction and water-insoluble component that was 62% carbohydrate and 33% lignin. This carbohydrate-rich substrate was readily hydrolyzed and fermented which indicated the lack of fermentation inhibitors in the steam-pretreated whole slurry. The use of high solid loading (25% w/v) resulted in a hydrolysis yield of 58% at an enzyme loading of 40 mg protein/g glucan and efficient fermentation (46.6 g/L of ethanol). This indicated that the addition of acidic sulphite at the steam pretreatment of softwoods improved both the enzymatic hydrolysis and fermentation of steam-pretreated whole slurries.

Whole sugar 2,3-butanediol fermentation for oil palm empty fruit bunches biorefinery by a newly isolated Klebsiella pneumoniae PM2

Effective utilization of cellulose and hemicelluloses is essential to sustainable bioconversion of lignocellulose. A newly isolated xylose-utilizing strain, Klebsiella pneumoniae PM2, was introduced to convert the biomass "whole sugars" into high value 2,3-butanediol (2,3-BDO) in a biorefinery process. The fermentation conditions were optimized (30°C, pH 7, and 150 rpm agitation) using glucose for maximum 2,3-BDO production in batch systems. A sulfite pretreated oil palm empty fruit bunches (EFB) whole slurry (substrate hydrolysate 119.5 g/L total glucose mixed with pretreatment spent liquor 80 g/L xylose) was fed to strain PM2 for fermentation. The optimized biorefinery process resulted in 75.03 ± 3.17 g/L of 2,3-BDO with 0.78 ± 0.33 g/L/h productivity and 0.43 g/g yield (87% of theoretical value) via a modified staged separate hydrolysis and fermentation process. This result is equivalent to approximately 135 kg 2,3-BDO and 14.5 kg acetoin precursors from 1 ton of EFB biomass without any wastage of both C₆ and C₅ sugars.

Why can hydrothermally pretreating lignocellulose in low severities improve anaerobic digestion performances?

A lignocellulosic residue, rice straw, was hydrothermally pretreated for the whole slurry anaerobic digestion. In contrast to the unpretreated rice straw, 110-120 °C pretreatment promoted biogas yield by 35%-38%, while only 14% promotion happened on the pretreatment at 180 °C. To understand why this improvement happened at lower severities, the pretreated rice straw at 90 °C, 120 °C, and 180 °C were selected for the further investigation, in which the liquor and solid fraction were separated for digestion, and compared with the whole slurry digestion. Results indicated more methane was released from the derived liquor of 180 °C than that of 90 °C and 120 °C, however, solid fraction did not exhibit significantly different methane yields (187.77-193.91 mL/g TS). These results suggested that the released soluble fraction from pretreatment could facilitate the methanogenesis. Furthermore, the released inherent soluble fraction in rice straw was mainly responsible for higher biogas yield at lower temperatures. Pretreatment at higher temperatures disintegrated the rice straw recalcitrance more, and intensified the release of soluble fraction accordingly. Consequently, the methanogenesis of whole slurry could be promoted at the initial digestion; the hydrolysis/acidification of the solid fraction in whole slurry was weakened greatly, which resulted in a lower biogas yield. This can also be proved by the evolution of dominant bacteria and archaea in the anaerobic digestion of whole slurry, separated solid and liquor fraction.

Production of bioethanol from unwashed-pretreated rapeseed straw at high solid loading

Reduction in water consumption and increase in ethanol concentration are two main challenges for bioethanol production from lignocellulosic materials. To address the two challenges, the aim of this work was to study the production of bioethanol from unwashed-pretreated rapeseed straw (RS) at high solid loading. RS pretreated with 1% (w w^-1) H₂SO₄ at 160 °C for 10 min resulted in excellent digestibility and fermentability of pretreated RS. The unwashed-pretreated RS was subjected to presaccharification and fed-batch simultaneous saccharification and fermentation (P-FB-SSF) at a final solid loading of 22% (w w^-1). Ethanol concentration and ethanol yield of 53.1 g L^-1 (equivalent to 4.1% (w w^-1) based on fermentation slurry) and 72.4% were obtained, respectively. In total, 92.1 g water g^-1 ethanol was consumed, a much smaller amount than that observed with washing after pretreatment or fermentation performed at lower solid loading.

Improved ethanol production from the slurry of pretreated brewers' spent grain through different co-fermentation strategies

The aim of this work was to bioconvert all sugars in BSG into ethanol using a process scheme that includes the enzymatic hydrolysis of the whole slurry resulting from the pretreatment of BSG with phosphoric and sulfuric acid using previously optimised conditions, followed by the co-fermentation of the mixed sugars. More than 90% of the sugars in raw BSG were recovered in the pretreatment and the subsequent enzymatic hydrolysis of the whole slurry. The co-fermentation of the enzymatic hydrolysates with Escherichia coli was then compared with that the co-culture of Scheffersomyces stipitis and Saccharomyces cerevisiae, which resulted in lower ethanol production. The co-fermentation strategy with a single microorganism (E. coli) when BSG was pretreated with phosphoric acid resulted into the highest ethanol concentration, 39 g/L, which means that 222 L of ethanol can be obtained from a ton of BSG without detoxification requirements.

Improvement of bioethanol production from pomegranate peels via acidic pretreatment and enzymatic hydrolysis

The aim of this study was to improve the ethanol production from pomegranate peels (PPs). Therefore, the effect of enzymatic hydrolysis and different pretreatments on ethanol production by yeasts was examined. There were three different enzyme concentrations (3.6, 7.2, 14.4 FPU/g substrate) tested for enzymatic hydrolysis, and four different PP media, such as WSPP (whole slurry of PP), LFPP (liquid fraction of PP), WSFPP (washed solid fraction of PP) and N-WSFPP (non-washed solid fraction of PP), were prepared. Bioethanol production was monitored for 96 h. Maximum ethanol concentrations were obtained at WSPP medium as 12.69 g/L, 14.35 g/L and 4.23 g/L in Saccharomyces cerevisiae, Kluyveromyces marxianus and Pichia stipitis, respectively. On the other hand, the washing step of biomass increased the kinetic parameters dramatically and the highest theoretical ethanol yields and Y_P/S values were obtained from WSFPP medium in all tested yeasts. Theoretical ethanol yields were 97.8%, 98.7% and 35.5% for S. cerevisiae, K. marxianus and P. stipitis, respectively. Q_p values were observed as 0.98 g/L h, 0.99 g/L h and 0.04 g/L h for the same yeasts. The highest Y_P/S values were detected as 0.50 g/g for S. cerevisiae, 0.50 g/g for K. marxianus and 0.30 g/g for P. stipitis in the washed pomegranate peel biomass.

Can hydrothermal pretreatment improve anaerobic digestion for biogas from lignocellulosic biomass?

Hydrothermally-pretreated rice straw (HPRS) from various pretreatment temperatures was anaerobically-digested in whole slurry. Results indicated promoting pretreatment temperature significantly deconstructed rice straw, and facilitated the conversion of insoluble fractions to soluble fractions. Although 306.6 mL/g TS biogas was maximally yielded in HPRS-90 and HPRS-180, respectively, via digestion in whole slurry, it was only 3% promotion compared to the unpretreated rice straw. HPRS-210 yielded 208.5 mL/g TS biogas, which was 30% reduction with longer lag period of 19.8 d, suggesting serious inhibitions happened. Through slightly increasing organic loading, more serious acidification and reduction on biogas yield, especially at higher pretreatment temperatures, indicated the soluble fractions controlled digestion performances. Pearson correlation analysis suggested negative relationship existed between methane yield and the soluble fractions including soluble carbohydrates, formic acid and furfural. Hydrothermal pretreatment, especially at higher temperature, did not improve anaerobic digestion, thereby, was not recommended, however, lower temperature can be considered potentially.