Effect of harvest date on Arundo donax L. (giant reed) composition, ensilage performance, and enzymatic digestibility

Advances in microbial pretreatment for biorefining of perennial grasses

Perennial grasses are potentially abundant sources of biomass for biorefineries, which can produce high yields with low input requirements, and many added environmental benefits. However, perennial grasses are highly recalcitrant to biodegradation and may require pretreatment before undergoing many biorefining pathways. Microbial pretreatment uses the ability of microorganisms or their enzymes to deconstruct plant biomass and enhance its biodegradability. This process can enhance the enzymatic digestibility of perennial grasses, enabling saccharification with cellulolytic enzymes to produce fermentable sugars and derived fermentation products. Similarly, microbial pretreatment can increase the methanation rate when the grasses are used to produce biogas through anaerobic digestion. Microorganisms can also increase the digestibility of the grasses to improve their quality as animal feed, enhance the properties of grass pellets, and improve biomass thermochemical conversion. Metabolites produced by fungi or bacteria during microbial pretreatment, such as ligninolytic and cellulolytic enzymes, can be further recovered as added-value products. Additionally, the action of the microorganisms can release chemicals with commercialization potential, such as hydroxycinnamic acids and oligosaccharides, from the grasses. This review explores the recent advances and remaining challenges in using microbial pretreatment for perennial grasses with the goal of obtaining added-value products through biorefining. It emphasizes recent trends in microbial pretreatment such as the use of microorganisms as part of microbial consortia or in unsterilized systems, the use and development of microorganisms and consortia capable of performing more than one biorefining step, and the use of cell-free systems based on microbial enzymes. KEY POINTS: • Microorganisms or enzymes can reduce the recalcitrance of grasses for biorefining • Microbial pretreatment effectiveness depends on the grass-microbe interaction • Microbial pretreatment can generate value added co-products to enhance feasibility.

Enhancement of Biomass Conservation and Bioethanol Production of Sweet Sorghum Silage by Constructing Synergistic Microbial Consortia

The efficient storage of materials before bioethanol production could be key to improving pretreatment protocol and facilitating biodegradation, in turn improving the cost-effectiveness of biomass utilization. Biological inoculants were investigated for their effects on ensiling performance, biodegradability of silage materials, and final bioethanol yield from sweet sorghum. Two cellulolytic microbial consortia (CF and PY) were used to inoculate silages of sweet sorghum, with and without combined lactic acid bacteria (Xa), for up to 60 days of ensiling. We found that the consortia notably decreased pH and ammonia nitrogen content while increasing lactic acid/acetic acid ratios. The microbes also functioned in synergy with Xa, significantly reducing lignocellulose content and improving biomass preservation. First-order exponential decay models captured the kinetics of nonstructural carbohydrates and suggested high water-soluble carbohydrate (grams per kilogram dry matter [DM]) preservation potential in PY-Xa (33.48), followed by CF-Xa (30.51). Combined addition efficiently improved enzymatic hydrolysis and enhanced bioethanol yield, and sweet sorghum treated with PY-Xa had the highest ethanol yield (28.42 g L-1). Thus, combined bioaugmentation of synergistic microbes provides an effective method of improving biomass preservation and bioethanol production from sweet sorghum silages. IMPORTANCE Ensiling is an effective storage approach to ensure stable year-round supply for downstream biofuel production; it offers combined facilities of storage and pretreatment. There are challenges in ensiling sweet sorghum due to its coarse structure and high fiber content. This study provides a meaningful evaluation of the effects of adding microbial consortia, with and without lactic acid bacteria, on changes in key properties of sweet sorghum. This study highlighted the bioaugmented ensiling using cellulolytic synergistic microbes to outline a cost-effective strategy to store and pretreat sweet sorghum for bioethanol production.

Ensiling Improved the Colonization and Degradation Ability of Irpex lacteus in Wheat Straw

To develop a non-thermal method to replace steam autoclaving for white-rot fungi fermentation, Irpex lacteus spawn was inoculated in wheat straw (WSI) or ensiled WS (WSI) at varying ratios of 10%, 20%, 30%, 40%, and 50%, and incubated at 28 °C for 28 days to determine the effects of the ensiling and inoculation ratio on the colonization and degradation ability of Irpex lacteus in wheat straw (WS). The results demonstrate that ensiling effectively inhibited the growth of aerobic bacteria and molds, as well as other harmful microorganisms in WS, which created a favorable condition for the growth of I. lacteus. After the treatment of I. lacteus, the pH of EWSI decreased to below 5, while that of WSI, except for the feedstocks of WSI-50%, was around 7, indicating that I. lacteus colonized well in the ensiled WS because the substrates dominated by I. lacteus are generally acidic. Correspondingly, except for the molds in WSI-50% samples, the counts of other microorganisms in WSI, such as aerobic bacteria and molds, were significantly higher than those in EWSI (p < 0.05), indicating that contaminant microorganisms had a competitive advantage in non-ensiled substrates. Incubation with I. lacteus did not significantly affect the cellulose content of all samples. However, the NDS content of EWSI was significantly higher than that of WSI (p < 0.05), and the hemicellulose and lignin contents were significantly lower than the latter (p < 0.05), except for the NDS and hemicellulose contents of WSI-50% samples. Correlation analysis revealed a stronger negative correlation between NDS content and the contents of hemicellulose, cellulose, and lignin in EWSI, which could be caused by the destruction of lignin and hemicellulose and the conversion from structural carbohydrates to fungal polysaccharides or other compounds in NDS form. Even for WSI-50% samples, the sugar yield of WS treated with I. lacteus improved with an increasing inoculation ratio, but the ratio was not higher than that of the raw material. However, the sugar yield of EWSI increased by 51-80%, primarily owing to the degradation of lignin and hemicellulose. Above all, ensiling improves the colonization ability of I. lacteus in WS, which promotes the degradation of lignin and hemicellulose and the enzymic hydrolysis of cellulose, so combining ensiling and I. lacteus fermentation has promising potential in the pretreatment of WS.

Exploring the Addition of Herbal Residues on Fermentation Quality, Bacterial Communities, and Ruminal Greenhouse Gas Emissions of Paper Mulberry Silage

This study aimed to investigate the influence of herbal residues on the fermentation quality and ruminal fermentation of paper mulberry silage. Clove, mint, and purple perilla residues were used as additives. Silage treatments were designed as control (no additives), 5% of clove, 5% of mint, and 5% of purple perilla. After 21 and 75 days of fermentation, the fermentation characteristics, bacterial communities, and ruminal greenhouse gas emissions in vitro incubation of paper mulberry were analyzed. The results showed that the used herbal residues could reduce the protein losses in paper mulberry silage based on the lower contents of ammoniacal nitrogen and nonprotein nitrogen. Compared with control, higher lactic acid and propionic acid contents were observed in the silages treated with mint and purple perilla but with a higher acetic acid content in clove treatment. Real-time sequencing technology (single-molecule real-time) revealed that Lactobacillus was the dominant bacteria in all silages at the genus level, whereas the bacterial abundance in the treated silages differed greatly from control at the species level. Lactobacillus hammesii abundance was the highest in control, whereas Lactobacillus acetotolerans was the first predominant in the treated silages. All the additives enhanced the digestibility of in vitro dry matter significantly. However, purple perilla decreased the production of total gas, methane, and carbon dioxide. The findings discussed earlier suggested that herbal residues have potential effects in improving fermentation quality, reducing protein loss, and modulating greenhouse gas emissions in the rumen of paper mulberry silage by shifting bacterial community composition.

Comparison of three ionic liquids pretreatment of Arundo donax L. For enhanced photo-fermentative hydrogen production

Ionic liquids (ILs) pretreatment has been regarded as a promising green way to treat lignocellulosic biomass. 1-Butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF₄), 1-allyl-3-methylimidazolium chloride ([Amim]Cl), and 1-Butyl-3-methylimidazolium Hydrogen Sulfate ([Bmim]HSO₄) with different loadings (2, 4, 8, and 16 g/L) were adopted to pretreat the Arundo donax L.. 16 g/L [Bmim]HSO₄ pretreated Arundo donax L. obtained the highest sugar yield of 7.9 g/L during the enzymatic hydrolysis and hydrogen yield of 106.1 mL/g TS during the photo-fermentation, which were 68.8 % and 35.3 % higher than those of untreated Arundo donax L., respectively. Moreover, volatile fatty acids (VFAs) distribution revealed that acetic acid was the main by-product during hydrogen production process with ILs pretreated Arundo donax L.. Besides, the relationship between sugar yield and hydrogen yield was the closest based on scatter matrix analysis. This study helps to understand of correlation between ILs pretreatment with the behavior of bioenergy production.

Enhancement of biomass conservation and enzymatic hydrolysis of rice straw by dilute acid-assisted ensiling pretreatment

To reduce the cost of lignocellulosic pretreatment, rice straw was ensiled with dilute formic acid (FA, 0, 0.2, 0.4, and 0.6%) for 3, 6, 9, 15 and 30 days, and evaluated its effects on fermentation dynamics, lignocellulosic degradation and enzymatic hydrolysis. The results showed that the application of FA, especially at 0.6% level, reduced total fermentation losses of the resulting silages, as evidenced by low dry matter loss, ammonia nitrogen and ethanol content. Meanwhile, the 0.6% FA application promoted hemicellulose removal (232.41 vs 187.52 g/kg DM) and xylose production (0.35 vs 2.80 g/kg DM). The glucose yield and cellulose convertibility of rice straw increased after 30 days of ensiling, and further enhanced by the 0.6% FA application. In conclusion, the 0.6% FA-assisted ensiling pretreatment improved both biomass preservation, hemicellulose removal and enzymatic hydrolysis of rice straw, which is beneficial to the subsequent biofuel production chain.

Evaluation of the effect of feruloyl esterase-producing Lactobacillus plantarum and cellulase pretreatments on lignocellulosic degradation and cellulose conversion of co-ensiled corn stalk and potato pulp

The effects of feruloyl esterase-producing Lactobacillus plantarum A1, cellulase, or their combination on the fermentation characteristics, carbohydrate composition, and enzymatic hydrolysis of mixed corn stalk and potato pulp silage were investigated. Two mixture ratios were used: a weight ratio of rehydrated corn stalk to potato pulp of 35:1 (HD) and a weight ratio of dry corn stalk to potato pulp of 5:11 (LD). No advantage was observed with the addition of strain A1 alone for lignocellulosic degradation and cellulose conversion, while its combination with cellulase enhanced the lignocellulosic degradation and preserved more fermentable carbohydrates in co-ensiled corn stalk and potato pulp. The enzymatic hydrolysis results indicated a potential benefit of pretreatment for biogas production, as the co-ensiled HD ratio mixture without additive treatment showed high glucose yield after enzymatic hydrolysis following 60 d of fermentation.

Effect of alkaline pretreatment on photo-fermentative hydrogen production from giant reed: Comparison of NaOH and Ca(OH)2

Giant reed was the first time used for photo-fermentative hydrogen production with HAU-M1 bacteria. Effects of NaOH and Ca(OH)₂ pretreatments of giant reed on structural changes, enzymatic digestibility, hydrogen production, and energy conversion efficiency were evaluated. Compared to Ca(OH)₂ pretreatment, NaOH pretreatment removed more dry matter and lignin at the same loading. The highest glucose yield (44.9%) of NaOH pretreatment was 1.74-fold higher than that of Ca(OH)₂ pretreatment. 20% NaOH pretreated giant reed biomass achieved the highest hydrogen yield (98.3 mL/g TS), which was 20% and 70% higher than the highest level of Ca(OH)₂ pretreated (20% Ca(OH)₂) and untreated giant reed, respectively. Only giant reed biomass pretreated with 20% NaOH resulted in a significant (p < 0.05) increase (25%) in energy conversion efficiency.

Ensiling characteristics, physicochemical structure and enzymatic hydrolysis of steam-exploded hippophae: Effects of calcium oxide, cellulase and Tween

To find a comprehensive way to enhance the utilizability of steam-exploded hippophae, calcium oxide (CaO) preimpregnation, cellulase-added storage and saccharification with addition of Tween 20 were investigated in this study. Both CaO preimpregnation and cellulase addition promoted the ensiling fermentation of anaerobically stored steam-exploded hippophae indicated by lower cellulose proportion and higher organic acids content, but led to the decrease of saccharification yield by 11.83% and 46.77-51.22%, respectively. When taking into account of organic acids being utilizable energy source, storing with addition of cellulase enhanced the utilizability of the materials in whole. Moreover, the addition of Tween 20 enhanced saccharification yield of the steam-exploded hippophae by 26.69-45.25%. Additionally, FTIR and XRD spectra clearly illustrated the structural alteration during storage. It is concluded that storing with addition of cellulase and hydrolyzing with addition of Tween 20 can enhance the utilizability of steam-exploded hippophae.

Ferulic acid esterase-producing lactic acid bacteria and cellulase pretreatments of corn stalk silage at two different temperatures: Ensiling characteristics, carbohydrates composition and enzymatic saccharification

The effects of Acremonium cellulase and L. plantarum A1 with ferulic acid esterase activity on corn stalk silage fermentation characteristics, carbohydrate composition and enzymatic saccharification were studied at 25 and 40 °C, respectively. Corn stalk was ensiled without additive (C), Acremonium cellulase (AC), L. plantarum A1 (Lp) and AC + Lp for 60 days. Pretreatment with Lp or AC + Lp promoted the better silage fermentation and the degradation of lignocellulose as indicated by high lactic acid and low pH and lignocellulose content compared to control silages at 25 °C. AC + Lp performed better in reducing lignocellulose and DM loss. In addition, Lp alone enhanced enzymatic saccharification of corn stalk silage. However, the influence of L. plantarum A1 on corn stalk silage was not obvious at 40 °C. Corn stalk ensiled with combined additive is a suitable pretreatment method for subsequent biofuel production at 25 °C, but addition of Acremonium cellulase alone at 40 °C may be a promising method.

Reactor performance and economic evaluation of anaerobic co-digestion of dairy manure with corn stover and tomato residues under liquid, hemi-solid, and solid state conditions

Anaerobic co-digestion of tomato residues, dairy manure, and corn stover at ratios of 20:48:32, 40:36:24, and 60:24:16 (volatile solid basis) were compared for liquid anaerobic digestion (L-AD), hemi-solid state AD (HSS-AD), and solid state AD (SS-AD) systems. The highest methane yield (353.5 L/kg-VS_added) and volumetric methane productivity (24.5 m³_methane/m³_{reactor volume}) were both obtained with 20% tomato residues addition under L-AD and HSS-AD conditions, respectively. Total solid and feedstock mixing ratio affected the degradation of protein and lipids during AD, but not cellulose and hemicellulose. Economic analysis results indicated that capital and labor costs have the dominant effect on total investment. SS-AD of tomato residues, dairy manure, and corn stover at ratios of 20:48:32 (VS basis) has the highest net present value (2.6 million US$) and shortest payback period (10.1 year), which indicated SS-AD was financially attractive under analysis conditions.

The effects of fibrolytic enzymes, cellulolytic fungi and bacteria on the fermentation characteristics, structural carbohydrates degradation, and enzymatic conversion yields of Pennisetum sinese silage

Biological inoculants were tested on Pennisetum sinese for their effects on fermentation characteristics, structural carbohydrates degradation, and enzymatic conversion yields. Pennisetum sinese was ensiled without additive, Lactobacillus plantarum (Lp), Trichoderma reesei (Tr), fibrolytic enzymes (E), and Enterococcus faecium (Y83) for 90 days. Y83 silages had higher LA and lower AA, ammonia-N and DM loss as compared to E and Tr silages. Tr and E had superior effects for degrading lignocellulose while Y83 had intermediate effects. The first-order exponential decay models (R² = 0.928-0.998) predicted nonstructural carbohydrates kinetics and demonstrated high water soluble carbohydrate (g/kg DM) preservation potential in Y83 (21.40), followed by Tr (18.94) and E (16.74). Addition of Y83 improved the conversion efficiency of P. sinese silage than Tr and E, indicated by higher glucose and total reducing sugars yield (22.49 and 36.89 w/w % DM, respectively). In conclusion, Y83 can be exploited for the ensiling lignocellulosic biomass before grass processing.

Effect of glucose and cellulase addition on wet-storage of excessively wilted maize stover and biogas production

In north China, large amounts of excessively wilted maize stover are produced annually. Maize stover wet storage strategies and subsequent biogas production was examined in this study. Firstly, wet storage performances of harvested maize stover, air-dried for different time durations, were evaluated. Results showed that optimal storage performance was obtained when the initial water soluble carbohydrate (WSC) content after air-drying was higher than 8.0%. Therefore, cellulase and glucose were added to the excessively wilted maize stover to achieve the targeted pre-storage WSC levels. Good storage performances were observed in treatments with addition of 76.4 g/kg DM glucose and 12.5 g/kg DM of cellulase; the specific methane yield increased by 23.7% and 19.2%, respectively. However, use of glucose as additive or co-storing with high WSC substrates can serve as economically feasible options to adapt wet storage of excessively wilted maize stover.

Comparison of sodium hydroxide and calcium hydroxide pretreatments of giant reed for enhanced enzymatic digestibility and methane production

NaOH pretreatment with leachate reuse and Ca(OH)₂ pretreatment were compared for improved enzymatic digestibility and biogas production from giant reed, a promising energy crop. The NaOH pretreatment with leachate reuse increased glucose yields during enzymatic hydrolysis by 2.6-fold, and methane yields during anaerobic digestion by 1.4- to 1.6-fold. However, NaOH pretreatment had a negative net benefit (i.e., revenue from increased energy production minus chemical cost). Pretreatment with 7-20% Ca(OH)₂ not only improved glucose yield and methane yield by up to 2.3-fold and 1.4-fold, respectively, but also obtained a net benefit of $1.1-5.8/tonne dry biomass. Thus, Ca(OH)₂ pretreatment was shown to be more feasible than NaOH pretreatment for biogas production from giant reed.

Effects of lactic acid bacteria and molasses additives on the microbial community and fermentation quality of soybean silage

The objective was to study effects of lactic acid bacteria (L) and molasses (M) on the microbial community and fermentation quality of soybean silage. Soybean was ensiled with no additive control (C), 0.5% molasses (0.5%M), 0.5%M+L (0.5%ML), 2%M, 2%M+L (2%ML) for 7, 14, 30 and 60days. The M-treated silages could increase the content of lactic acid and decrease butyric acid than control. Besides, higher crude protein was also observed in M-treated silages. With prolonged ensiling time, there was a reduction of the ratio of lactic acid/acetic acid in the 2%M-treated and 2%ML-treated silages. The combined addition of L and 2%M could enhance the account of desirable Lactobacillus and inhibit the growth of undesirable microorganism such as Clostridia and Enterobacter. In summary, the silage quality of soybean was improved with the addition of L and M.

Ensiling characteristics, structural and nonstructural carbohydrate composition and enzymatic digestibility of Napier grass ensiled with additives

Ensiling characteristics, structural and nonstructural carbohydrate composition and enzymatic digestibility (ED) of Napier grass silage was examined. Napier grass ensiled with no additive control, 0.2% formic acid, 0.4% molasses, and 0.3% fibrolytic enzyme for, 7, 30, 60 and 90days. Additives increased lactic acid, soluble carbohydrate and decreased all of lignocellulosic contents except acid detergent lignin and pH than control. The highest value of nonstructural carbohydrate and large reduction in lignocellulosic contents was observed in formic acid and fibrolytic enzyme silage respectively. The content of glucose and fructose showed rapid drop in the first 7days of ensilage. Ensilage decreased lignocellulosic contents and increased ED compared to fresh material. The ED of formic acid and molasses silage was significantly higher than control and fibrolytic enzyme silages in all tested days. In summery the ensiling quality structural and nonstructural carbohydrate and ED value of mature Napier grass silage improved through additives.

Comparison of liquid hot water and alkaline pretreatments of giant reed for improved enzymatic digestibility and biogas energy production

Liquid hot water (LHW) and alkaline pretreatments of giant reed biomass were compared in terms of digestibility, methane production, and cost-benefit efficiency for electricity generation via anaerobic digestion with a combined heat and power system. Compared to LHW pretreatment, alkaline pretreatment retained more of the dry matter in giant reed biomass solids due to less severe conditions. Under their optimal conditions, LHW pretreatment (190°C, 15min) and alkaline pretreatment (20g/L of NaOH, 24h) improved glucose yield from giant reed by more than 2-fold, while only the alkaline pretreatment significantly (p<0.05) increased cumulative methane yield (by 63%) over that of untreated biomass (217L/kgVS). LHW pretreatment obtained negative net electrical energy production due to high energy input. Alkaline pretreatment achieved 27% higher net electrical energy production than that of non-pretreatment (3859kJ/kg initial total solids), but alkaline liquor reuse is needed for improved net benefit.