Effective fermentation of sugarcane bagasse whole slurries using robust xylose-capable Saccharomyces cerevisiae

The pre-treatment of lignocellulose material toward cellulosic bioethanol production releases microbial inhibitors that severely limit the fermentation ability of Saccharomyces cerevisiae. This study evaluated to what degree robust xylose capable strains may improve the fermentability of non-detoxified sugarcane bagasse (SCB) slurries derived from steam explosion (StEX), and further compared this to slurries derived from ammonia fibre expansion (AFEX) pre-treatment. Initial screening in separate hydrolyses and co-fermentation processes using StEx-SCB hydrolysates identified S. cerevisiae TP-1 and CelluXTM4 with higher xylose consumption (≥ 88%) and ethanol concentrations (≥ 50 g/L). Subsequent fermentations compared StEx and AFEX pre-treated SCB material under industrially relevant fed-batch pre-hydrolysis simultaneous saccharification and co-fermentation (PSSCF) conditions, which resulted in only 3 g/L differences in ethanol titres for StEx and AFEX PSSCF fermentations. The study achieved non-detoxified whole-slurry co-fermentation using StEx pre-treated SCB, with higher ethanol yields than previously reported, by utilising robust xylose-capable strains.

A Whole-Slurry Fermentation Approach to High-Solid Loading for Bioethanol Production from Corn Stover

Corn stover is the most produced byproduct from maize worldwide. Since it is generated as a residue from maize harvesting, it is an inexpensive and interesting crop residue to be used as a feedstock. An ecologically friendly pretreatment such as autohydrolysis was selected for the manufacture of second-generation bioethanol from corn stover via whole-slurry fermentation at high-solid loadings. Temperatures from 200 to 240 °C were set for the autohydrolysis process, and the solid and liquid phases were analyzed. Additionally, the enzymatic susceptibility of the solid phases was assessed to test the suitability of the pretreatment. Afterward, the production of bioethanol from autohydrolyzed corn stover was carried out, mixing the solid with different percentages of the autohydrolysis liquor (25%, 50%, 75%, and 100%) and water (0% of liquor), from a total whole slurry fermentation (saving energy and water in the liquid–solid separation and subsequent washing of the solid phase) to employing water as only liquid medium. In spite of the challenging scenario of using the liquor fraction as liquid phase in the fermentation, values between 32.2 and 41.9 g ethanol/L and ethanol conversions up to 80% were achieved. This work exhibits the feasibility of corn stover for the production of bioethanol via a whole-slurry fermentation process.

Steam Explosion Pretreatment of Beechwood. Part 1: Comparison of the Enzymatic Hydrolysis of Washed Solids and Whole Pretreatment Slurry at Different Solid Loadings

Steam explosion is a well-known process to pretreat lignocellulosic biomass in order to enhance sugar yields in enzymatic hydrolysis, but pretreatment conditions have to be optimized individually for each material. In this study, we investigated how the results of a pretreatment optimization procedure are influenced by the chosen reaction conditions in the enzymatic hydrolysis. Beechwood was pretreated by steam explosion and the resulting biomass was subjected to enzymatic hydrolysis at glucan loadings of 1% and 5% employing either washed solids or the whole pretreatment slurry. For enzymatic hydrolysis in both reaction modes at a glucan loading of 1%, the glucose yields markedly increased with increasing severity and with increasing pretreatment temperature at identical severities and maximal values were reached at a pretreatment temperature of 230 °C. However, the optimal severity was 5.0 for washed solids enzymatic hydrolysis, but only 4.75 for whole slurry enzymatic hydrolysis. When the glucan loading was increased to 5%, glucose yields hardly increased for pretreatment temperatures between 210 and 230 °C at a given severity, and a pretreatment temperature of 220 °C was sufficient under these conditions. Consequently, it is important to precisely choose the desired conditions of the enzymatic hydrolysis reaction, when aiming to optimize the pretreatment conditions for a certain biomass.

Steam Explosion Pretreatment of Beechwood. Part 2: Quantification of Cellulase Inhibitors and Their Effect on Avicel Hydrolysis

Biomass pretreatment is a mandatory step for the biochemical conversion of lignocellulose to chemicals. During pretreatment, soluble compounds are released into the prehydrolyzate that inhibit the enzymatic hydrolysis step. In this work, we investigated how the reaction conditions in steam explosion pretreatment of beechwood (severity: 3.0–5.25; temperature: 160–230 °C) influence the resulting amounts of different inhibitors. Furthermore, we quantified the extent of enzyme inhibition during enzymatic hydrolysis of Avicel in the presence of the prehydrolyzates. The amounts of phenolics, HMF, acetic acid and formic acid increased with increasing pretreatment severities and maximal quantities of 21.6, 8.3, 43.7 and 10.9 mg/gbeechwood, respectively, were measured at the highest severity. In contrast, the furfural concentration peaked at a temperature of 200 °C and a severity of 4.75. The presence of the prehydrolyzates in enzymatic hydrolysis of Avicel lowered the glucose yields by 5–26%. Mainly, the amount of phenolics and xylose and xylooligomers contributed to the reduced yield. As the maximal amounts of these two inhibitors can be found at different conditions, a wide range of pretreatment severities led to severely inhibiting prehydrolyzates. This study may provide guidelines when choosing optimal pretreatment conditions for whole slurry enzymatic hydrolysis.

Biogas Production from Physicochemically Pretreated Grass Lawn Waste: Comparison of Different Process Schemes

Various pretreatment methods, such as thermal, alkaline and acid, were applied on grass lawn (GL) waste and the effect of each pretreatment method on the Biochemical Methane Potential was evaluated for two options, namely using the whole slurry resulting from pretreatment or the separate solid and liquid fractions obtained. In addition, the effect of each pretreatment on carbohydrate solubilization and lignocellulossic content fractionation (to cellulose, hemicellulose, lignin) was also evaluated. The experimental results showed that the methane yield was enhanced with alkaline pretreatment and, the higher the NaOH concentration (20 g/100 gTotal Solids (TS)), the higher was the methane yield observed (427.07 L CH4/kg Volatile Solids (VS), which was almost 25.7% higher than the BMP of the untreated GL). Comparing the BMP obtained under the two options, i.e., that of the whole pretreatment slurry with the sum of the BMPs of both fractions, it was found that direct anaerobic digestion without separation of the pretreated biomass was favored, in almost all cases. A preliminary energy balance and economic assessment indicated that the process could be sustainable, leading to a positive net heat energy only when using a more concentrated pretreated slurry (i.e., 20% organic loading), or when applying NaOH pretreatment at a lower chemical loading.

Biogas Production from Sunflower Head and Stalk Residues: Effect of Alkaline Pretreatment

Sunflower residues are considered a prominent renewable source for biogas production during anaerobic digestion (AD). However; the recalcitrant structure of this lignocellulosic substrate requires a pretreatment step for efficient biomass transformation and increased bioenergy output. The aim of the present study was to assess the effect of alkaline pretreatment of various parts of the sunflower residues (e.g., heads and stalks) on their methane yield. Experimental data showed that pretreatment at mild conditions (55 °C; 24 h; 4 g NaOH 100 g−1 total solids) caused an increase in the biochemical methane potential (BMP) of both heads and stalks of the sunflower residues as determined in batch tests. The highest methane production (268.35 ± 0.11 mL CH4 g−1 volatile solids) was achieved from the pretreated sunflower head residues. Thereafter; the effect of alkaline pretreatment of sunflower head residues was assessed in continuous mode; using continuous stirred-tank reactors (CSTRs) under two operational phases. During the first phase; the CSTRs were fed with the liquid fraction produced from the pretreatment of sunflower heads. During the second phase; the CSTRs were fed with the whole slurry resulting from the pretreatment of sunflower heads (i.e., both liquid and solid fractions). In both operating phases; it was observed that the alkaline pretreatment of the sunflower head residues had a negligible (phase I) or even a negative effect on biogas production; which was contradictory to the results of the BMP tests. It seems that; during alkaline pretreatment; this part of the sunflower residues (heads) may release inhibitory compounds; which induce a negative effect on biogas production in the long term (e.g., during continuously run digesters such as CSTR) but not in the short-term (e.g., batch tests) where the effect of the inoculum may not permit the inhibition to be established.