Steam explosion pretreatment and enzymatic saccharification of duckweed (Lemna minor) biomass

Our previous research has shown that duckweed is potentially an ideal feedstock for the production of biofuels because it can be effectively saccharified enzymatically. Here we report the results of experiments in which duckweed was pre-treated by steam explosion prior to enzyme digestion. A range of temperatures, from 130 to 230 °C with a fixed retention time of 10 min, were employed. The best pretreatment conditions were 210 °C for 10 min; these conditions produced the highest amount of water-soluble material (70%), the greatest levels of starch solubilisation (21%) and hemicellulose and pectic polysaccharides degradation (60%). The use of these steam explosion conditions enabled large reductions in the concentrations of enzymes required for effective saccharification. The amount of Celluclast required was reduced from 100 U (4.35 FPU) g⁻¹ substrate to 20 U g⁻¹ substrate, and additional beta-glucosidase was reduced from 100 to 2 U g⁻¹ substrate.     

A re-look at the biochemical strategies to enhance butanol production

Butanol produced from renewable feedstock is defined as an emerging biofuel and biochemical. Research efforts made during the last three decades on biochemical production of butanol via conventional ABE (acetone-ethanol-butanol) fermentation has tried to bring biobutanol close to competition with petrobutanol. However, each new effort of development has been often countered by new challenges, confining biobutanol production mostly to the laboratory scale. This review provides a systematic, comparative analysis of different steps in biochemical production of butanol and identifies the counteractive aspects and challenges to overcome. A special emphasis is given on process inhibitors, applied detoxification techniques, chemical supplements and research & development in industry in order to enhance and update ABE fermentation and make it cost effective. Biobutanol future lies in utilization of inexpensive cellulose enriched lignocellulosic hydrolysates and hyper-butanol producing bacteria, combined with specific detoxification techniques and followed by efficient continuous fermentation technologies together with in situ product recovery.     

Physicochemical characterization and enzymatic digestibility of Chinese pennisetum pretreated with 1-ethyl-3-methylimidazolium acetate at moderate temperatures

1-ethyl-3-methylimidazolium acetate ([EMIM] AC) pretreatment at moderate temperatures (60 °C and 75 °C) was evaluated for improving hydrolysability of Chinese pennisetum, a leading candidate as an energy crop, for bioethanol production. The pretreatment caused slight carbohydrate and lignin loss but significantly changed the material physicochemical characters, such as crystallinity and surface structure. Both changes exhibited positive effects on improving the enzymatic digestibility of the Chinese pennisetum. It was observed that approximately 90% of the cellulose and 50% of the xylan in the Chinese pennisetum after pretreatment at 75 °C were converted to fermentable monosaccharides by the combined cellulases and endo-xylanase. The results suggested that Chinese pennisetum could be effectively pretreated with ([EMIM] AC) pretreatment at moderate temperatures, and the high hydrolysis yield of fermentable sugars from pretreated Chinese pennisetum could be achieved by the synergistic action of accessory xylanase in enzymatic hydrolysis by cellulases.     

Improvement of enzymatic saccharification of Populus and switchgrass by combined pretreatment with steam and wet disk milling

To reduce the recalcitrance of lignocellulosic biomass for subsequent biological processing, we pretreated energy crop feedstocks with mild steam treatment (ST; 130 and 150 °C for 60 min) and wet disk milling (WDM). We tested two phylogenetically different, but typical energy crop feedstocks: Populus trichocarpa and switchgrass (Panicum virgatum). WDM after ST facilitated the fibrillation of both types of biomass, resulting in an increase of specific surface area, improved enzymatic saccharification yield, and decrease in cellulose crystallinity. After steam treatment at 150 °C followed by 17 cycles of WDM, enzymatic hydrolysis resulted in almost complete glucan to glucose conversion in both feedstocks.     

Evaluation of dilute acid and alkaline pretreatments,enzymatic hydrolysis and fermentation of napiergrass for fuel ethanol production

Napiergrass (Pennisetum purpureum Schum.) is a promising low cost raw material which does not compete with food prices, has attractive yields and an environmentally friendly farming. Dilute sulfuric acid pretreatment of napiergrass was effective to obtain high yields of sugars and low level of degradation by-products from hemicellulose. Detoxification with Ca(OH)₂ removed inhibitors but showed sugars loss. An ethanol concentration of 21 g/L after 176 h was found from the hydrolyzate using Pichia stipitis NBRC 10063 (fermentation efficiency 66%). An additional alkaline pretreatment applied to the solid fraction remaining from the diluted acid pretreatment improved the lignin removal. The highest cellulose hydrolysis values were found with the addition of β-glucosidase and PEG 6000. The simultaneous hydrolysis and fermentation of the cellulosic fraction with Saccharomyces cerevisiae, 10% (w/v) solid concentration, β-glucosidase and PEG 6000, showed the highest ethanol concentration (24 g/L), and cellulose hydrolysis values (81%). 162 L ethanol/t of dry napiergrass were produced (overall efficiency of 52%): 128 L/t from the cellulosic fraction and 34 L/t from the hemicellulosic fraction.     

Enhanced substrate degradation and methane yield with maleic acid pre-treatments in biomass crops and residues

Organic acids are envisaged as alternative catalysts to strong mineral acids, in pre-treatment of ligno-cellulosic biomass for anaerobic digestion (AD). To evaluate this hypothesis, an untreated control and four pre-treatments (25 °C for 24 h) involving two levels of maleic acid (34.8 and 69.6 kg m⁻³), alone and combined with sulphuric acid (4 kg m⁻³), were studied in three agricultural substrates: Arundo (aka giant reed), Barley straw and B133 fibre sorghum. Methane production was assessed in a batch AD assay (35 °C for 51 days) with 4 g L⁻¹ of volatile solid (VS) load. Fibre composition and structure were investigated through chemical analysis and Fourier transform infrared (FTIR) spectrometry. Arundo and B133 that were the most and least recalcitrant substrate, respectively, staged the highest and lowest increase in methane with high maleic acid: +62% over 218 cm³ g⁻¹ of VS in untreated Arundo; +36% over 284 cm³ g⁻¹ of VS in untreated B133. Barley straw showed an intermediate behaviour (+41% over 269 cm³ g⁻¹ of VS). H₂SO₄ addition to maleic acid did not improve CH₄ output. The large increase in methane yield determined by pre-treatments was reflected in the concurrent decrease of fibre (between 14 and 39% depending on fibrous component). Based on FTIR spectra, bands assigned to hemicellulose and cellulose displayed lower absorbance after pre-treatment, supporting the hypothesis of solubilisation of structural carbohydrates and change in fibre structure. Hence, maleic acid was shown a suitable catalyst to improve biodegradability of ligno-cellulosic biomass, especially in recalcitrant substrates as Arundo.     

An oil palm-based biorefinery concept for cellulosic ethanol and phytochemicals production: Sustainability evaluation using exergetic life cycle assessment

In this study, thermo-environmental sustainability of an oil palm-based biorefinery concept for the co-production of cellulosic ethanol and phytochemicals from oil palm fronds (OPFs) was evaluated based on exergetic life cycle assessment (ExLCA). For the production of 1 tonne bioethanol, the exergy content of oil palm seeds was upgraded from 236 MJ to 77,999 MJ during the farming process for OPFs production. Again, the high exergy content of the OPFs was degraded by about 62.02% and 98.36% when they were converted into cellulosic ethanol and phenolic compounds respectively. With a total exergy destruction of about 958,606 MJ (internal) and 120,491 MJ (external or exergy of wastes), the biorefinery recorded an overall exergy efficiency and thermodynamic sustainability index (TSI) of about 59.05% and 2.44 per tonne of OPFs' bioethanol respectively. Due to the use of fossil fuels, pesticides, fertilizers and other toxic chemicals during the production, the global warming potential (GWP = 2265.69 kg CO₂ eq.), acidification potential (AP = 355.34 kg SO₂ eq.) and human toxicity potential (HTP = 142.79 kg DCB eq.) were the most significant environmental impact categories for a tonne of bioethanol produced in the biorefinery. The simultaneous saccharification and fermentation (SSF) unit emerged as the most exergetically efficient (89.66%), thermodynamically sustainable (TSI = 9.67) and environmentally friendly (6.59% of total GWP) production system.     

Upgraded biofuel from residue biomass by Thermo-Catalytic Reforming and hydrodeoxygenation

Research is focused on the utilisation of waste or residue biomass for bioenergy conversion. A promising conversion technology for the production of liquid biofuels from residue biomass is a process called Thermo-Catalytic Reforming (TCR^®​) which is a combination of prior thermal treatment of the biomass at mild temperatures (intermediate pyrolysis) followed by a second catalytic treatment step at elevated temperatures (reforming). This article focuses on the conversion of TCR^® liquids from digestate as a feedstock for subsequent hydrocarbon production. The generated bio-oil showed a lower heating value of 34.0 MJ kg⁻¹ with an oxygen content of 7.0% and a water content of 2.2%. The bio-oil was hydrodeoxygenated using an industrial NiMo–Al₂O₃ catalyst at temperatures of 503 K–643 K and a pressure of 14 MPa. The hydrodeoxygenated bio-oil reached a lower heating value of 42.3 MJ kg⁻¹ with an oxygen content below 0.8 mg kg⁻¹ and water content of 30 ppm. Product yields and catalyst life give confidence that upgrading of the TCR^®​ bio-oil offers a suitable option to meet the high standards of common fuels.     

Characterization,pretreatment and saccharification of spent seaweed biomass for bioethanol production using baker's yeast

Seaweeds are marine macroalgae found abundantly and viewed as potential source of phycocolloids to produce biofuel. In this study, seaweed spent biomass obtained from alginate production industry and biomass obtained after pigment extraction were found to contain a considerable amount of phycocolloids. These two spent biomasses were investigated for the production of ethanol. In this study, the red seaweed spent biomass of Gracilaria corticata var corticata showed higher content of polysaccharide (190.71 ± 30.67 mg g⁻¹ dry weight) than brown seaweed spent biomass (industrial) (136.28 ± 30.09 mg g⁻¹ dry weight). Hydrolysis of spent biomasses with different concentrations of sulfuric acid (0.1%, 0.5% and 1%) was also investigated. Brown seaweed spent biomass and red seaweed spent biomass exhibited high amount of sugar in 0.5% and 1% sulfuric acid treatment, respectively. Proximate and ultimate composition of seaweed spent biomasses were analysed for energy value. The FT-Raman spectra exhibited similar stretches for both acid hydrolysed spent biomasses with their respective standards. Ethanol produced through a fermentation process using spent hydrolysates with baker's yeast at pH 5.3 was found to be significant. The ethanol yield from brown seaweed spent biomass and red seaweed spent biomass was observed to be 0.011 g g⁻¹ and 0.02 ± 0.003 g g⁻¹ respectively, when compared with YPD (0.42 ± 0.03 g g⁻¹) and d-galactose (0.37 ± 0.04 g g⁻¹) as standard on day 4. The present study revealed the possibility of effective utilization of spent biomass from seaweed industry for ethanol production.     

Ethanologens vs. acetogens: Environmental impacts of two ethanol fermentation pathways

Bioconversion production of ethanol from cellulosic feedstock is generally proposed to use direct fermentation of sugars to ethanol. Another potential route for ethanol production is fermentation of sugars to acetic acid followed by hydrogenation to convert the acetic acid into ethanol. The advantage of the acetogen pathway is an increased ethanol yield; however, using an acetogen requires the additional hydrogenation, which could substantially affect the life cycle global warming potential of the process. Assuming a poplar feedstock, a cradle to grave Life cycle assessment (LCA) is used to evaluate the environmental impacts of an acetogen based fermentation pathway. An LCA of a fermentation pathway that uses ethanologen fermentation is developed for comparison. It is found that the ethanologen and acetogen pathways have Global Warming Potentials (GWP) that are 92% and 46% lower than the GWP of gasoline, respectively. When the absolute GWP reduction compared to gasoline is calculated using a unit of land basis, the benefit of the higher ethanol yield using the acetogen is observed as the two pathways achieve similar GWP savings. The higher ethanol yield in the acetogen process plays a crucial role in choosing a lignocellulosic ethanol production method if land is a limited resource.     

Effect of doping pretreated corn stover conditions on yield of bioethanol in immobilized cell systems

The surface characteristics of immobilized yeast before and after adding CO₂-laser pretreated corn stover (LPCS) substrates were investigated using bioethanol production. Response surface methodology (RSM), based on the Box–Behnken design (BBD) for experiments, was used to optimize the doping condition. An optimum experimental condition was obtained at pH 4.5, 2.08% yeast concentration, and 0.20% LPCS substrates. Under this condition, doping LPCS increased the yield of bioethanol from 53% to 84%, which matched the predicted value. After doping LPCS, the results of inverted microscope (IM) and atomic force microscopy (AFM) illustrated that the immobilized gel beads changed from rod-like in shape with a smooth surface to a larger rod-like ultrastructure with a rougher surface. The yield was relatively stable within 28 d, with a downward trend subsequently appearing.     

Optimizing feedstock selection for biofuel production in Hawaii: CuO oxidative lignin products in C4 grasses

Global interest in renewable fuels is rapidly growing with particular emphasis on local energy growth and creating new energy feedstocks, specifically liquid fuel sources. However, the interactive effect of plant species/variety and growth environment on plant structural components, which may influence conversion efficiency and thus play an important role in optimizing the production of biofuels, is not fully understood. In this study cupric oxide (CuO) extractable lignin, which extracts and quantifies lignin-derived monomers, was determined for 25 cultivated and naturalized tropical perennial C₄ grass varieties of napiergrass and Guinea grass that were under assessment for suitability as feedstocks for liquid fuel generation. Principal component analysis of CuO extractable lignin-derived monomers showed differences in composition between napiergrass and Guinea grass, as well as environmental differences within many, but not all napiergrass varieties. Among the samples tested, the greatest differences in lignin composition occurred specifically in vanillyl lignin structures. A wide range (1–2.5) in the ratio of cinnamyl to vanillyl structures (C/V), which often relates to enzymatic degradability in natural systems, was also found. It is expected that the observed CuO lignin differences representing the structure and bonding of lignin will relate to ease of chemical and enzymatic conversion and the effectiveness of pretreatment and conversion in biofuel application. We hypothesize that the C/V ratio of feedstock will positively relate to conversion efficiency and if supported, then compositional lignin metrics such as the C/V ratio could be a predictor to select for more easily degradable biomass for biofuel production.     

Cob biomass supply for combined heat and power and biofuel in the north central USA

Corn (Zea mays L.) cobs are being evaluated as a potential bioenergy feedstock for combined heat and power generation (CHP) and conversion into a biofuel. The objective of this study was to determine corn cob availability in north central United States (Minnesota, North Dakota, and South Dakota) using existing corn grain ethanol plants as a proxy for possible future co-located cellulosic ethanol plants. Cob production estimates averaged 6.04 Tg and 8.87 Tg using a 40 km radius area and 80 km radius area, respectively, from existing corn grain ethanol plants. The use of CHP from cobs reduces overall GHG emissions by 60%–65% from existing dry mill ethanol plants. An integrated biorefinery further reduces corn grain ethanol GHG emissions with estimated ranges from 13.9 g CO₂ equiv MJ⁻¹ to 17.4 g CO₂ equiv MJ⁻¹. Significant radius area overlap (53% overlap for 40 km radius and 86% overlap for 80 km radius) exists for cob availability between current corn grain ethanol plants in this region suggesting possible cob supply constraints for a mature biofuel industry. A multi-feedstock approach will likely be required to meet multiple end user renewable energy requirements for the north central United States. Economic and feedstock logistics models need to account for possible supply constraints under a mature biofuel industry.     

Hydrogen,ethanol and cellulase production from pulp and paper primary sludge by fermentation with Clostridium thermocellum

Pulp and paper industry primary sludge being largely composed of lignocellulosic fibres, it could be used as carbon source by bacteria having cellulolytic capability. The aim of this study was to evaluate the use of cellulose contained in this type of sludge for Clostridium thermocellum to produce ethanol, hydrogen and cellulases. In an ATCC 1191 medium containing 5 kg m⁻³ dry primary sludge from recycled paper mill, batch culture reached stationary phase after 2 days. All of the available cellulose was hydrolysed after 60 h of incubation, with a final pH of 5.83. Metabolites produced after 60 h of fermentation were acetate (8.50 mol m⁻³), ethanol (11.30 mol m⁻³), lactate (8.75 mol m⁻³), formate (0.27 mol m⁻³), hydrogen (11.20 mol m⁻³) and carbon dioxide (18.41 mol m⁻³). Cellulase activity was detected in the supernatant after 36 h, with a maximal activity of 0.25 U cm⁻³ at 72 h. Pulp and paper primary sludge appeared to be a readily usable substrate for C. thermocellum at this concentration, yielding both potential biofuels (hydrogen and ethanol) as well as active cellulases.     

Surfactant mediated enhanced glycerol uptake and hydrogen production from biodiesel waste using co-culture of Enterobacter aerogenes and Clostridium butyricum

In the present study, Tween 80, a non-ionic surfactant, has been used for enhanced hydrogen production by crude glycerol bioconversion using co-culture of Enterobacter aerogenes and Clostridium butyricum. The purpose of introducing the surfactant was to decrease the crude glycerol viscosity, so that apparent solubility and bioavailability of glycerol could be improved at the expenses of pretreatment steps. Experiments were planned using central composite design (CCD); crude glycerol and Tween 80 concentrations were optimized whereas, hydrogen production, glycerol utilization and viscosity of the media were considered as responses. The response surface for quadratic model showed, Tween 80 concentration had significant effect (p < 0.05) on all the three responses. Using the optimized conditions at 17.5 g/L crude glycerol and 15 mg/L Tween 80, hydrogen production reached a maximum of 32.1 ± 0.03 mmol/L of medium. The increase in hydrogen production was around 1.25-fold in presence of Tween 80 in comparison to its absence with 25.56 ± 0.91 mmol/L production. Selected optimum conditions were also validated against absence of crude glycerol (4.69 ± 0.76), with pretreated crude glycerol (20.06 ± 0.51) and across mono-culture system (15.43 ± 0.79 to 22.14 ± 0.94). Introduction of Tween 80 to the fermentation medium improved the glycerol utilization rate, resulting in increased hydrogen production and eliminated pretreatment steps.     

Periodic peristalsis enhancing the high solids enzymatic hydrolysis performance of steam exploded corn stover biomass

Enzymatic hydrolysis beyond 15% solid loading offers many advantages such as increased sugar and ethanol concentrations and decreased capital cost. However, difficult mixing and handling limited its industrialized application. A novel intensification method, periodic peristalsis, had been exploited to improve the high solids enzymatic hydrolysis performance of steam exploded corn stover (SECS). The optimal steam explosion conditions were 200 °C and 8 min, under which glucan and xylan recovery was 94.3% and 64.8%, respectively. Glucan and xylan conversions in periodic peristalsis enzymatic hydrolysis (PPEH) were 28.0–38.5% and 25.0–36.0% higher than those in static state enzymatic hydrolysis with solid loading increasing from 1% to 30%, respectively, while they were 1.0–11.2% and 3.0–9.2% higher than those in incubator shaker enzymatic hydrolysis (ISEH). Glucan and xylan conversion in PPEH at 21% solid loading reached 71.2% and 70.3%, respectively. Periodic peristalsis also facilitated fed-batch enzymatic hydrolysis of which SECS was added completely before transition point. Results presented that PPEH shortened the transition point time from solid state to slurry state, decreased the viscosity of hydrolysis mixture, and reduced the denaturation effect of enzymes compared with ISEH, and hence improve the high solids enzymatic hydrolysis efficiency.     

Second-generation bioethanol of hydrothermally pretreated stover biomass from maize genotypes

Twelve maize genotypes, were agronomically evaluated and their stover hydrothermally pretreated in a temperature range of 210–225 °C to assess the effects of genotype and pretreatment severity on stover recalcitrance toward bioethanol conversion. Maize genotypes exhibited significant variation for biomass yield and all agronomic evaluated, while among all cell wall constituents measured in the unpretreated stover, only ash content showed differences among genotypes. The pretreatment severities assayed impacted most stover compositional traits, and the glucose recovered after enzymatic hydrolysis displayed a similar profile among genotypes with similar genetic background. Harsher pretreatment conditions maximized the potential cellulosic bioethanol production (208–239 L/t), while the mildest maximized the bioethanol from the hemicellulosic hydrolysates (137–175 L/t). Consequently, when both pentose and hexose sugars were considered, the total potential bioethanol produced at the lowest and highest pretreatment temperatures was similar in all genotypes (292–358 L/t), indicating that the lowest temperature (210 °C) was the optimal among all assayed. Importantly, the ranking of genotypes for bioethanol yield (L/ha) closely resembled the ranking for stover yield (t/ha), indicating that breeding for biomass yield would increase the bioethanol production per hectare regardless of the manufacturing process. Similarly, the genetic regulation of corn stover moisture is possible and relevant for efficient energy production as biomass moisture has a potential impact on stover transportation, storage and processing requirements. Overall, these results indicate that local landrace populations are important genetic resources to improve cultivated crops, and that simultaneous breeding for production of grain and stover bioethanol is possible in corn.     

Sunflower as a biofuels crop: An analysis of lignocellulosic chemical properties

Four accessions of cultivated sunflower (Helianthus annuus) and silverleaf sunflower (Helianthus argophyllus), were each grown in three locations (Georgia, British Columbia, and Iowa) at different planting densities and phenotyped for biomass-related traits and wood biochemistry. In most environments, H. argophyllus produced significantly more biomass than H. annuus. Cell wall chemistry for a subset of plants grown in Georgia and Iowa was assessed using analytical wet chemistry methods to measure lignin and sugar content/composition. The analysis of lignin and the S/G-lignin ratios for a larger number of samples (n > 250) was also assessed by high-throughput pyrolysis Molecular Beam Mass Spectrometry. Average pyMBMS estimated lignin content (i.e., dry weight fraction) for 60 °C dried basal stem samples of H. annuus and H. argophyllus was 29.6% (range, 24.0%–34.6%) and 28.6% (range, 24.6%–33.3%), respectively when averaged across all environments. The average S/G lignin mass ratio was 1.5 (range, 1.0–2.0) for H. annuus and 1.7 (range, 1.0–2.4) in H. argophyllus. Stem samples from these two species only differed statistically for a few cell wall chemistry traits; however, accession level differences within each species were apparent. Cell wall chemistry in both species was significantly affected by both location and planting density, thus demonstrating the need to select for these traits in the environment for which the crop will be produced. Overall, these results show that cultivated sunflower and silverleaf sunflower both possess the necessary phenotypic diversity to facilitate the development of a hybrid sunflower with improved lignocellulosic biofuels traits, namely increased biomass, decreased lignin, and increased glucan.     

Fuel properties of Brassica juncea oil methyl esters blended with ultra-low sulfur diesel fuel

Brassica juncea is a drought-tolerant member of the Brassicaceae plant family with high oil content and a short growing season that is tolerant of low quality soils. It was investigated as a feedstock for production of biodiesel along with evaluation of subsequent fuel properties, both neat and in blends with petroleum diesel fuel. These results were compared against relevant fuel standards such as ASTM D6751, EN 14214, ASTM D975, EN 590, and ASTM D7467. Crude B. juncea oil was extracted from unconditioned seeds utilizing a continuous tubular radial expeller. The oil was then chemically refined via degumming, neutralization and bleaching to render it amenable to direct homogeneous sodium methoxide-catalyzed transesterification. The principal fatty acid detected in B. juncea oil was erucic acid (44.1%). The resulting biodiesel yielded fuel properties compliant with the biodiesel standards with the exception of oxidative stability and kinematic viscosity in the case of EN 14214. Addition of tert-butylhydroquinone and blending with soybean oil-derived biodiesel ameliorated these deficiencies. The fuel properties of B5 and B20 blends of B. juncea oil methyl esters (BJME) in ultra-low sulfur (<15 ppm S) diesel (ULSD) fuel were within the ranges specified in the petrodiesel standards ASTM D975, EN 590 and ASTM D7467 with the exception of derived cetane number in the case of EN 590. This deficiency was attributed to the inherently low cetane number of the certification-grade ULSD, as it did not contain performance-enhancing additives. In summary, this study reports new fuel property data for BJME along with properties of B5 and B20 blends in ULSD. Such results will be useful for the development of B. juncea as an alternative source of biodiesel fuel.     

Relationships between cellulosic biomass particle size and enzymatic hydrolysis sugar yield: Analysis of inconsistent reports in the literature

Cellulosic ethanol made from cellulosic biomass is a promising alternative to petroleum-based transportation fuels. Enzymatic hydrolysis is a crucial step in cellulosic ethanol production. In order to better understand the mechanisms of enzymatic hydrolysis, relationships between cellulosic biomass particle size and enzymatic hydrolysis sugar yield have been studied extensively. However, the literature contains inconsistent reports. This paper presents an analysis of the inconsistent reports on the relationships in the literature. It discusses the differences in the reported experiments from five perspectives (biomass category, particle size definition, sugar yield definition, biomass treatment procedure, and particle size level). It also proposes future research activities that can provide further understanding of the relationships.