A mathematical model for the inhibitory effects of lignin in enzymatic hydrolysis of lignocellulosics

Reversing lignin inhibition on enzymatic hydrolysis through regulating supramolecular assembly

The non-productive binding of enzymes to lignin is a well-documented barrier to efficient enzymatic hydrolysis. While significant attention has been paid to the influence of lignin's chemical structure on enzyme-lignin interactions, the role of its supramolecular assembly in this process has largely been overlooked. In this study, we regulated the supramolecular assembly of ethanol organosolv lignin (EOL) through solvent exchange dialysis and investigated its impact on the enzymatic hydrolysis of cellulose. Our findings reveal that tuning the supramolecular assembly of lignin significantly influenced its physicochemical properties, particularly particle size and hydrophobicity. Larger lignin particles, formed through self-assembly during dialysis, exhibited reduced hydrophobicity. Notably, the size of the lignin aggregates exercised very distinct effects on the performance of enzymatic hydrolysis. Certain large lignin assembled particles even reversed its inhibitory effects on the enzymatic hydrolysis of Avicel. EOL-M1 with the smallest particle size decreased 72 h glucose yields from 70.5 % to 62.9 %, whereas EOL-M100 with the largest particle size increased 72 h glucose yields to 82.3 %. Langmuir adsorption isotherms analysis, and X-ray photoelectron spectroscopy (XPS) analysis demonstrated that lignin with larger particle sizes and lower hydrophobicity reduced the enzyme-binding capacity. Furthermore, this phenomenon was consistently validated with three different lignin samples obtained from organosolv and kraft pulping processes. This study demonstrated the unusual functions of lignin particles in the enzymatic saccharification process. It also provides new insights into the mechanism underlying the influences of lignin on enzyme-lignin interactions and bioprocessing at large.

Enhanced production of sugars and UV-shielded lignin/PAN fiber mats from chemi-mechanical pulps

This study investigated poplar pretreatments by chemi-mechanical pulping (CMP) under different beating degrees and alkali concentrations. The enzyme-mediated hydrolysis of pretreated poplar was enhanced by deacetylation and delignification. Meanwhile, the remaining lignin residues were used to produce lignin/polyacrylonitrile (PAN) fiber mats by electrospinning. These mats exhibited excellent mechanical and UV-blocking performance when the lignin was obtained from pulps under milder alkali concentrations (5 g/L). ³¹P nuclear magnetic resonance (³¹P NMR) and two-dimensional heteronuclear single-quantum correlation nuclear magnetic resonance (2D HSQC NMR) data revealed that increasing the beating degree at low alkali concentration during the CMP process led to the cleavage of β-O-4' interunit linkages and re-condensation in lignin, releasing several phenolic groups. Lignin with more linear β-O-4' interunit linkages and lesser phenolic groups, obtained from treatment of CMP with lower alkali concentration (5 g/L) and beating degree (20°SR), resulted in the corresponding lignin/PAN fiber mats exhibiting better mechanical performance. Further, lignin, along with the increased phenolic-OH and COOH, and p-hydroxybenzoate (PB) units with a more extended conjugate structure, derived from CMP under lower alkali concentration (5 g/L) and higher beating degree (45°SR), led to a stronger ultraviolet (UV) absorption in the corresponding lignin/PAN mats. To summarize, this study reports a mild and low-pollution biomass pretreatment method (CMP) that can efficiently regulate the lignin structure and exhibit efficient anti-ultraviolet properties. The corresponding UV-blocking fiber mats can be potentially used as materials for wearable fabrics.

Enzymatic removal of inhibitory compounds from lignocellulosic hydrolysates for biomass to bioproducts applications

The physicochemical pretreatment is an important step to reduce biomass recalcitrance and facilitate further processing of plant lignocellulose into bioproducts. This process results in soluble and insoluble biomass fractions, and both may contain by-products that inhibit enzymatic biocatalysts and microbial fermentation. These fermentation inhibitory compounds (ICs) are produced during the degradation of lignin and sugars, resulting in phenolic and furanic compounds, and carboxylic acids. Therefore, detoxification steps may be required to improve lignocellulose conversion by microoganisms. Several physical and chemical methods, such as neutralization, use of activated charcoal and organic solvents, have been developed and recommended for removal of ICs. However, biological processes, especially enzyme-based, have been shown to efficiently remove ICs with the advantage of minimizing environmental issues since they are biogenic catalysts and used in low quantities. This review focuses on describing several enzymatic approaches to promote detoxification of lignocellulosic hydrolysates and improve the performance of microbial fermentation for the generation of bioproducts. Novel strategies using classical carbohydrate active enzymes (CAZymes), such as laccases (AA1) and peroxidases (AA2), as well as more advanced strategies using prooxidant, antioxidant and detoxification enzymes (dubbed as PADs), i.e. superoxide dismutases, are discussed as perspectives in the field.

Enzymatic Hydrolysis of Sugarcane Bagasse in Aqueous Two-Phase Systems (ATPS): Exploration and Conceptual Process Design

The enzymatic conversion of lignocellulosic material to sugars can provide a carbon source for the production of energy (fuels) and a wide range of renewable products. However, the efficiency of this conversion is impaired due to product (sugar) inhibition. Even though several studies investigate how to overcome this challenge, concepts on the process to conduct the hydrolysis are still scarce in literature. Aqueous two-phase systems (ATPS) can be applied to design an extractive reaction due to their capacity to partition solutes to different phases in such a system. This work presents strategies on how to conduct extractive enzymatic hydrolysis in ATPS and how to explore the experimental results in order to design a feasible process. While only a limited number of ATPS was explored, the methods and strategies described could easily be applied to any further ATPS to be explored. We studied two promising ATPS as a subset of a previously high throughput screened large set of ATPS, providing two configurations of processes having the reaction in either the top phase or in the bottom phase. Enzymatic hydrolysis in these ATPS was performed to evaluate the partitioning of the substrate and the influence of solute partitioning on conversion. Because ATPS are able to partition inhibitors (sugar) between the phases, the conversion rate can be maintained. However, phase forming components should be selected to preserve the enzymatic activity. The experimental results presented here contribute to a feasible ATPS-based conceptual process design for the enzymatic conversion of lignocellulosic material.

Micromorphological changes and mechanism associated with wet ball milling of Pinus radiata substrate and consequences for saccharification at low enzyme loading

In this work, substrates prepared from thermo-mechanical treatment of Pinus radiata chips were vibratory ball milled for different times. In subsequent enzymatic hydrolysis, percent glucan conversion passed through a maximum value at a milling time of around 120min and then declined. Scanning electron microscopy revealed breakage of fibers to porous fragments in which lamellae and fibrils were exposed during ball milling. Over-milling caused compression of the porous fragments to compact globular particles with a granular texture, decreasing accessibility to enzymes. Carbon-13 NMR spectroscopy showed partial loss of interior cellulose in crystallites, leveling off once fiber breakage was complete. A mathematical model based on observed micromorphological changes supports ball milling mechanism. At a low enzyme loading of 2FPU/g of substrate and milling time of 120min gave a total monomeric sugar yield of 306g/kg of pulp which is higher than conventional pretreatment method such as steam exploded wood.

Improved saccharification of steam-exploded Pinus radiata on supplementing crude extract of Penicillium sp

Commercially available enzymes do not contain all the necessary softwood-specific accessory enzymes to obtain high saccharification efficiency. In this work, six saprophytic fungi obtained from Pinus radiata plantation site were screened for the putative softwood-specific accessory enzyme, β-mannanase. A Penicillium sp. was found to produce β-mannanase in both solid (31.6 units/g of dry biomass) and liquid (117 units/g of dry biomass) cultures using locust bean gum as an inducer after 2 weeks of incubation. The saccharification of steam-exploded Pinus radiata was 7.8 % w/w improved when the crude extract of Penicillium sp. was added to a mixture of commercial enzymes.

Factors affecting cellulose hydrolysis based on inactivation of adsorbed enzymes

The rate of enzymatic hydrolysis of cellulose reaction is known to decrease significantly as the reaction proceeds. Factors such as reaction temperature, time, and surface area of substrate that affect cellulose conversion were analyzed relative to their role in a mechanistic model based on first order inactivation of adsorbed cellulases. The activation energies for the hydrolytic step and inactivation step were very close in magnitude: 16.3 kcal mol(-1) for hydrolysis and 18.0 kcal mol(-1) for inactivation, respectively. Therefore, increasing reaction temperature would cause a significant increase in the inactivation rate in addition to the catalytic reaction rate. Vmax,app was only 20% or less of the value at 72 h compared to at 2h as a result of inactivation of adsorbed cellulases, suggesting prolonged hydrolysis is not an efficient way to improve cellulose hydrolysis. Hydrolysis rate increased with corresponding increases in available substrate surface binding area.

Relative extents of activity loss between enzyme-substrate interactions and combined environmental mechanisms

Enzymatic hydrolysis of biomass undergoes a significant decrease in rate, which is often attributed to activity loss of enzyme during the incubation. Activity loss due to both interaction with substrate (for example inactivation of adsorbed enzyme) and all combined environmental mechanisms in a substrate free buffer solution were compared in this study. Enzyme-substrate interactions contributed more towards the overall activity loss than did the combined environmental sources as evidenced from three independent metrics. (1) Relative extents of inactivation were higher for enzyme-substrate interactions than for environmental mechanisms. (2) Apparent half-lives (1.37-11.01 h) following interaction with substrate were relatively small compared to environmental inactivation, which was 21.5h. (3) The inactivation rate constant for enzyme-substrate interactions (0.56 h(-1)) was 46 times higher than that of environmental inactivation (0.0123 h(-1)). These results suggest enzyme-substrate interaction is the main cause of cellulase activity loss and contributes significantly to the slow rate of hydrolysis.

Influence of enzyme loading on enzymatic hydrolysis of cardboard waste and size distribution of the resulting fiber residue

Enzymatic hydrolysis of lignocellulosic biomass to sugars alters the properties of the cellulosic fibers. Several process variables, including enzyme loading, play an important role in these changes. Many physical properties of fibers are affected: their length and width, porosity, specific surface area, and degree of fibrillation, for instance, may undergo dramatic changes when subjected to enzymatic degradation. In this study, the influence of enzyme loading on the fiber size was investigated using milled cardboard waste as the raw material. The effect of cellulases and hemicellulases on the monosaccharide production and the resulting fiber size was studied using commercial enzyme products. It was shown that the cellulase loading largely determined the amount of sugars produced. The fiber length was reduced during the course of hydrolysis, although the size reduction was not especially dramatic. Based on the SEM images, no significant damage to the fiber surfaces occurred during the process.

Enzymatic hydrolysis of pretreated waste paper--source of raw material for production of liquid biofuels

Enzymatic hydrolysis of waste paper is becoming a perspective way to obtain raw material for production of liquid biofuels. Reducing sugars solutions that arise from the process of saccharification are a precursors for following or simultaneous fermentation to ethanol. Different types of waste paper were evaluated, in terms of composition and usability, in order to select the appropriate type of the waste paper for the enzymatic hydrolysis process. Novozymes® enzymes NS50013 and NS50010 were used in a laboratory scale trials. Technological conditions, which seem to be the most suitable for hydrolysis after testing on cellulose pulp and filter paper, were applied to hydrolysis of widely available waste papers - offset paper, cardboard, recycled paper in two qualities, matte MYsol offset paper and for comparison again on model materials. The highest yields were achieved for the cardboard, which was further tested using various pretreatment combinations in purpose of increasing the hydrolysis yields.