Enzymatic Hydrolysis of Softwood Derived Paper Sludge by an In Vitro Recombinant Cellulase Cocktail for the Production of Fermentable Sugars

Techno-economic analysis of biomass value-added processing informed by pilot scale de-ashing of paper sludge feedstock

Paper sludge biomass represents an underutilized feedstock rich in pulped and processed cellulose which is currently a waste stream with significant disposal cost to industry for landfilling services. Effective fractionation of the cellulose from paper sludge presents an opportunity to yield cellulose as feedstock for value-added processes. A novel approach to cellulose fractionation is the sidehill screening system, herein studied at the pilot-plant scale. Composition analysis determined ash removal and carbohydrate retention of both sidehill and high-performance benchtop screening systems. Sidehill screening resulted in greater carbohydrates retention relative to benchtop screening (90% vs 66%) and similar ash removal (95% vs 98%). Techno-economic analysis for production of sugar syrup yielded a minimum selling price of $331/metric ton of sugar syrup including disposal savings, significantly less than a commercial sugar syrup without fractionation. Sensitivity analysis showed that screening conditions played a significant role in economic feasibility for cellulosic yield and downstream processes.

Optimisation of β-Glucosidase Production in a Crude Aspergillus japonicus VIT-SB1 Cellulase Cocktail Using One Variable at a Time and Statistical Methods and its Application in Cellulose Hydrolysis

Pulp and paper mill sludge (PPMS) is currently disposed of into landfills which are reaching their maximum capacity. Valorisation of PPMS by enzymatic hydrolysis using cellulases is an alternative strategy. Existing commercial cellulases are expensive and contain low titres of β-glucosidases. In this study, β-glucosidase production was optimised by Aspergillus japonicus VIT-SB1 to obtain higher β-glucosidase titres using the One Variable at a Time (OVAT), Plackett Burman (PBD), and Box Behnken design (BBD)of experiments and the efficiency of the optimised cellulase cocktail to hydrolyse cellulose was tested. β-Glucosidase production was enhanced from 0.4 to 10.13 U/mL, representing a 25.3-fold increase in production levels after optimisation. The optimal BBD production conditions were 6 days of fermentation at 20 °C, 125 rpm, 1.75% soy peptone, and 1.25% wheat bran in (pH 6.0) buffer. The optimal pH for β-glucosidase activity in the crude cellulase cocktail was (pH 5.0) at 50 °C. Optimal cellulose hydrolysis using the crude cellulase cocktail occurred at longer incubation times, and higher substrate loads and enzyme doses. Cellulose hydrolysis with the A. japonicus VIT-SB1 cellulase cocktail and commercial cellulase cocktails resulted in glucose yields of 15.12 and 12.33 µmol/mL glucose, respectively. Supplementation of the commercial cellulase cocktail with 0.25 U/mg of β-glucosidase resulted in a 19.8% increase in glucose yield.

Sustainable Production of Bioethanol Using Levulinic Acid Pretreated Sawdust

The sustainability and economic viability of the bioethanol production process from lignocellulosic biomass depend on efficient and effective pretreatment of biomass. Traditional pretreatment strategies implicating the use of mineral acids, alkalis, and organic solvents release toxic effluents and the formation of inhibitory compounds posing detrimental effects on the environment and interfering with the enzymatic saccharification process, respectively. Ionic liquids (ILs) as green solvents were used to overcome this issue, but the deep eutectic solvent as an emerging class of ionic liquids performed better in terms of making the process environmentally and economically viable. The green solvent-based pretreatment strategy applied in the current research was levulinic, acid-based natural deep eutectic solvent (NADES). Three different hydrogen bond acceptors (HBAs)—acetamide, betaine, and choline chloride—in combination with levulinic acid as hydrogen bond donor (HBD) in (HBD: HBA) molar ratio 2:1, were screened for biomass pretreatment. The best deep eutectic solvent was levulinic acid: choline chloride in an optimized molar ratio of 1:0.5, resulting in 91% delignification. The physicochemical parametric optimization of saccharification exhibited maximum enzymatic hydrolysis of 25.87% with 125 mg of pretreated sawdust via simultaneous addition of three thermostable cellulases [i.e., endo-1,4-β-D-glucanase (240 U), exo-1,4-β-D-glucanase (180 U), and β-glucosidase (320 U)] for 5 h of incubation at 75°C. The reducing sugar slurry obtained from the saccharified biomass was then added to a fermentation medium for bioethanol production, and a maximum of 11.82% of production was obtained at 30°C, 72 h, and 180 rpm using a 2.5% 24 h old Saccharomyces cerevisiae seed culture. The current study revealed that the levulinic-based deep eutectic solvent exhibited remarkable delignification, which led to the efficient enzymatic hydrolysis of sawdust and hence bioethanol production. Furthermore, it will prospect new avenues in bioethanol production using a deep eutectic solvent. Deep eutectic solvent overcame the issues posed by ionic liquids: toxicity, expensive and complex preparation, and non-biodegradability.

Enhanced Saccharification of Purple Alfalfa via Sequential Pretreatment with Acidified Ethylene Glycol and Urea/NaOH

Purple Alfalfa is an inexpensive, abundant, readily available lignocellulosic material. This work was attempted to develop an efficient combination pretreatment by sequential HClO4–ethyl glycol–H2O (1.2:88.8:10, w/w/w) extraction at 130 °C in 0.5 h and urea/NaOH (urea 12 wt%, NaOH 7 wt%) soaking at −20 °C for 0.5 h for the pretreatment of purple alfalfa. The porosity, morphology, and crystallinity of pretreated purple alfalfa were characterized with SEM, FM, XRD, and FTIR. This combination pretreatment had a significant influence on hemicellulose removal and delignification. The above changes could enhance cellulose accessibility to enzymes and improve the enzymatic digestibility of cellulose. High yields of reducing sugars from pretreated purple alfalfa were obtained at 93.4%. In summary, this combination pretreatment has high potential application in the future.

Defining the Frontiers of Synergism between Cellulolytic Enzymes for Improved Hydrolysis of Lignocellulosic Feedstocks

Lignocellulose has economic potential as a bio-resource for the production of value-added products (VAPs) and biofuels. The commercialization of biofuels and VAPs requires efficient enzyme cocktail activities that can lower their costs. However, the basis of the synergism between enzymes that compose cellulolytic enzyme cocktails for depolymerizing lignocellulose is not understood. This review aims to address the degree of synergism (DS) thresholds between the cellulolytic enzymes and how this can be used in the formulation of effective cellulolytic enzyme cocktails. DS is a powerful tool that distinguishes between enzymes’ synergism and anti-synergism during the hydrolysis of biomass. It has been established that cellulases, or cellulases and lytic polysaccharide monooxygenases (LPMOs), always synergize during cellulose hydrolysis. However, recent evidence suggests that this is not always the case, as synergism depends on the specific mechanism of action of each enzyme in the combination. Additionally, expansins, nonenzymatic proteins responsible for loosening cell wall fibers, seem to also synergize with cellulases during biomass depolymerization. This review highlighted the following four key factors linked to DS: (1) a DS threshold at which the enzymes synergize and produce a higher product yield than their theoretical sum, (2) a DS threshold at which the enzymes display synergism, but not a higher product yield, (3) a DS threshold at which enzymes do not synergize, and (4) a DS threshold that displays anti-synergy. This review deconvolutes the DS concept for cellulolytic enzymes, to postulate an experimental design approach for achieving higher synergism and cellulose conversion yields.

Editorial Catalysts: Special Issue on “Biomass Derived Heterogeneous and Homogeneous Catalysts”

The replacement of petrol products for environmentally-friendly ones is a reality today, as many governments and international organizations are promoting the implementation of renewable energy sources and natural feedstocks in industrial activity [...]