Enzymatic hydrolysis of lime-pretreated corn stover and investigation of the HCH-1 Model: inhibition pattern, degree of inhibition, validity of simplified HCH-1 Model

Modelling of enzyme kinetics: cellulose enzymatic hydrolysis case

Enzymes as industrial biocatalysts offer numerous advantages over traditional chemical processes resulting on improvements in process economy and environmental sustainability. Because enzymes are extensively used in different industrial areas, the enzyme kinetics is an important factor for industry as it is able to estimate the extent of substrate conversion under known conditions and evaluate reactor performance. Furthermore, kinetic modelling is useful in the analysis, prediction, and optimization of an enzymatic process. Thus, kinetic modelling is a powerful tool for biochemical reaction engineering. In addition to the aforementioned, in the industrial technology, modelling together with simulation play a key role because they help to understand how a system behaves under specific conditions, and thus they allow saving on costs and lead times. Enzymatic conversion of renewable cellulosic biomass into biofuels is at the heart of advanced bioethanol production. In the production of bioethanol from cellulosic biomass, enzymatic hydrolysis of cellulose to fermentable sugars accounts for a large portion (∼30%) of the total production costs. Therefore, a thorough understanding of enzymatic hydrolysis is necessary to create a robust model which helps designing optimal conditions and economical system. Nevertheless, it is a challenging task because cellulose is a highly complex substrate and its enzymatic hydrolysis is heterogeneous in nature, and thus the whole process of cellulose conversion to glucose involves more steps than classical enzyme kinetics. This chapter describes the bases of enzyme kinetic modelling, focussing on Michaelis-Menten kinetics, and presents the models classification based on the fundamental approach and methodology used. Furthermore, the modelling of cellulose enzymatic hydrolysis is described, also reviewing some model examples developed for cellulose hydrolysis over the years. Finally, the application of enzyme kinetics modelling in food, pharmaceutical and bioethanol industry is presented.

Xylanase inhibition by the derivatives of lignocellulosic material

Hydrolysis of lignocellulosic materials into simple sugar plays an important role in biorefinery. Hemicellulosic sugars from the hydrolysis of lignocellulosic materials could be used in xylitol production. However, xylanase activity during hydrolysis process is affected by activators and inhibitors that may present in the reaction system. The pretreatment process was reported to produce compounds that may affect the enzymatic hydrolysis process, such as furans, aliphatic acid, and aromatics. The purpose of this study was to investigate the inhibition effect of these potential inhibitors on xylanase activity. Three groups of potential inhibitors were evaluated including, furan, aliphatic acid, and hydrolysis-fermentation products. The result showed that ethanol, vanillin, and formic acid gave the highest inhibition effect from each group. Ethanol competed with xylanase competitively. Vanillin showed non-competitive inhibition. Formic acid performed mixed-inhibition by reducing maximum hydrolysis rate and giving varied Michaelis constant values at different concentrations.

Analysis of the Long Time Behavior of Enzymatic Cellulose Hydrolysis Kinetics

Enzymatic hydrolysis of biomass to produce sugars that can be converted to fuels and other valuable chemicals, is viewed as the prime technology for utilization of this renewable resource. To accelerate technology development, models are needed that are able to accurately predict the hydrolysis rate so that reactors can be tailored to the multitude of processing conditions and substrates that can be used. Of particular interest is the ability to predict the long time conversion in the hydrolysis reaction which dictates the maximum possible sugar concentration. It is our aim in this article to develop a simple model which is able to predict the long-term conversion of cellulose to soluble sugars. Drawing on the analogy from the theory of reactions in continuous mixtures, it is shown that analysis of the long time kinetics of hydrolysis by examining the behavior of the “lump” of the reacting material results in a simple expression which is capable of predicting the kinetics. Many features of actual enzyme systems can be included in the development of the hydrolysis model, such as the large size of the enzyme molecules, adsorption onto substrate, inhibition by different factors (solvent, glucose etc.), but, when the analysis is carried out to calculate the total sugar concentration, it is shown that the equations reduce to a simple expression. Analysis of this model is given with comparison to other models and experimental data available in the literature. In addition to predicting the long-term kinetics, it is shown that the model does a surprising job of predicting the initial hydrolysis rates as well.

Two-stage dilute-acid and organic-solvent lignocellulosic pretreatment for enhanced bioprocessing

A two stage pretreatment approach for biomass is developed in the current work in which dilute acid (DA) pretreatment is followed by a solvent based pretreatment (N-methyl morpholine N oxide - NMMO). When the combined pretreatment (DAWNT) is applied to sugarcane bagasse and corn stover, the rates of hydrolysis and overall yields (>90%) are seen to dramatically improve and under certain conditions 48h can be taken off the time of hydrolysis with the additional NMMO step to reach similar conversions. DAWNT shows a 2-fold increase in characteristic rates and also fractionates different components of biomass - DA treatment removes the hemicellulose while the remaining cellulose is broken down by enzymatic hydrolysis after NMMO treatment to simple sugars. The remaining residual solid is high purity lignin. Future work will focus on developing a full scale economic analysis of DAWNT for use in biomass fractionation.

Inhibitory effect of vanillin on cellulase activity in hydrolysis of cellulosic biomass

Pretreatment of lignocellulosic material produces a wide variety of inhibitory compounds, which strongly inhibit the following enzymatic hydrolysis of cellulosic biomass. Vanillin is a kind of phenolics derived from degradation of lignin. The effect of vanillin on cellulase activity for the hydrolysis of cellulose was investigated in detail. The results clearly showed that vanillin can reversibly and non-competitively inhibit the cellulase activity at appropriate concentrations and the value of IC50 was estimated to be 30 g/L. The inhibition kinetics of cellulase by vanillin was studied using HCH-1 model and inhibition constants were determined. Moreover, investigation of three compounds with similar structure of vanillin on cellulase activity demonstrated that aldehyde group and phenolic hydroxyl groups of vanillin had inhibitory effect on cellulase. These results provide valuable and detailed information for understanding the inhibition of lignin derived phenolics on cellulase.

Modeling the kinetics of complex systems: enzymatic hydrolysis of lignocellulosic substrates

Lignocellulosic biomass is mainly composed of cellulose, hemicellulose, and lignin. Fuzzy logic, in turn, is a branch of many-valued logic based on the paradigm of inference under vagueness. This paper presents a methodology, based on computational intelligence, for modeling the kinetics of a complex reactional system. The design of a fuzzy interpolator to model cellulose hydrolysis is reported, within the perspective of applying kinetic models in bioreactor engineering. Experimental data for various types of lignocellulosic materials were used to develop the interpolator. New experimental data from the enzymatic hydrolysis of a synthetic substrate, on the other hand, were used to validate the methodology. The accuracy of the results indicates that this is a promising approach to extend the application of models fitted for specific situations to different cases, thus enhancing their generality.

Enhanced glucose yield and structural characterization of corn stover by sodium carbonate pretreatment

Na2CO3 was employed as an efficient yet cheap alkaline catalyst for the pretreatment of corn stover. To systematically obtain an optimal condition, the effects of critical pretreatment parameters including Na2CO3 concentration (2-6%), temperature (120-160 °C), and reaction time (10-30 min) on glucose yield were evaluated in lab-scale using response surface methodology. The best conditions were found to be Na2CO3 of 4.1%, temperature of 142.6 °C, and reaction time of 18.0 min, under which glucose yield reached to 267.5 g/kg biomass. Physical properties, based on scanning electron microscopy (SEM) imagery, surface area, pore volume and size, and crystallinity of pretreated corn stover, were examined. The Na2CO3 pretreatment apparently damaged the surface and altered structural features of corn stover, which resulted in the enhancement of enzymatic of hydrolysis. These results evidently support that Na2CO3 is indeed a robust and feasible catalyst for pretreating lignocellulosic biomass.

Product inhibition of cellulases studied with 14C-labeled cellulose substrates

BACKGROUND

As a green alternative for the production of transportation fuels, the enzymatic hydrolysis of lignocellulose and subsequent fermentation to ethanol are being intensively researched. To be economically feasible, the hydrolysis of lignocellulose must be conducted at a high concentration of solids, which results in high concentrations of hydrolysis end-products, cellobiose and glucose, making the relief of product inhibition of cellulases a major challenge in the process. However, little quantitative information on the product inhibition of individual cellulases acting on cellulose substrates is available because it is experimentally difficult to assess the hydrolysis of the heterogeneous polymeric substrate in the high background of added products.

RESULTS

The cellobiose and glucose inhibition of thermostable cellulases from Acremonium thermophilum, Thermoascus aurantiacus, and Chaetomium thermophilum acting on uniformly 14C-labeled bacterial cellulose and its derivatives, 14C-bacterial microcrystalline cellulose and 14C-amorphous cellulose, was studied. Cellulases from Trichoderma reesei were used for comparison. The enzymes most sensitive to cellobiose inhibition were glycoside hydrolase (GH) family 7 cellobiohydrolases (CBHs), followed by family 6 CBHs and endoglucanases (EGs). The strength of glucose inhibition followed the same order. The product inhibition of all enzymes was relieved at higher temperatures. The inhibition strength measured for GH7 CBHs with low molecular-weight model substrates did not correlate with that measured with 14C-cellulose substrates.

CONCLUSIONS

GH7 CBHs are the primary targets for product inhibition of the synergistic hydrolysis of cellulose. The inhibition must be studied on cellulose substrates instead of on low molecular-weight model substrates when selecting enzymes for lignocellulose hydrolysis. The advantages of using higher temperatures are an increase in the catalytic efficiency of enzymes and the relief of product inhibition.

The pattern of cell wall deterioration in lignocellulose fibers throughout enzymatic cellulose hydrolysis

Cell wall deterioration throughout enzymatic hydrolysis of cellulosic biomass is greatly affected by the chemical composition and the ultrastructure of the fiber cell wall. The resulting pattern of cell wall deterioration will reveal information on cellulose activity throughout enzymatic hydrolysis. This study investigates the progression and morphological changes in lignocellulose fibers throughout enzymatic hydrolysis, using (transmission electron microscopy) TEM and field emission scanning electron microscopy (FE-SEM). Softwood thermo-mechanical pulp (STMP) and softwood bleached kraft pulp (SBKP), lignocellulose substrates containing almost all the original fiber composition, and with lignin and some hemicellulose removed, respectively, was compared for morphology changes throughout hydrolysis. The difference of conversion between STMP and SBKP after 48 h of enzymatic hydrolysis is 11 and 88%, respectively. TEM images revealed an even fiber cell wall cross section density, with uneven middle lamella coverage in STMP fibers. SKBP fibers exhibited some spaces between cell wall and lamella layers due to the removal of lignin and some hemicellulose. After 1 h hydrolysis in SBKP fibers, there were more changes in the fiber cross-sectional area than after 10 h hydrolysis in STMP fibers. Cell wall degradation was uneven, and originated in accessible cellulose throughout the fiber cell wall. FE-SEM images illustrated more morphology changes in SBKP fibers than STMP fibers. Enzymatic action of STMP fiber resulted in a smoother fiber surface, along with fiber peeling and the formation of ribbon-disjunction layers. SBKP fibers exhibited structural changes such as fiber erosion, fiber cutting, and fiber splitting throughout enzymatic hydrolysis.

Investigation of enzyme formulation on pretreated switchgrass

This work studied the benefits of adding different enzyme cocktails (cellulase, xylanase, β-glucosidase) to pretreated switchgrass. Pretreatment methods included ammonia fiber expansion (AFEX), dilute-acid (DA), liquid hot water (LHW), lime, lime+ball-milling, soaking in aqueous ammonia (SAA), and sulfur dioxide (SO(2)). The compositions of the pretreated materials were analyzed and showed a strong correlation between initial xylan composition and the benefits of xylanase addition. Adding xylanase dramatically improved xylan yields for SAA (+8.4%) and AFEX (+6.3%), and showed negligible improvement (0-2%) for the pretreatments with low xylan content (dilute-acid, SO(2)). Xylanase addition also improved overall yields with lime+ball-milling and SO(2) achieving the highest overall yields from pretreated biomass (98.3% and 93.2%, respectively). Lime+ball-milling obtained an enzymatic yield of 92.3kg of sugar digested/kg of protein loaded.

Accessibility of Enzymatically Delignified Bambusa bambos for Efficient Hydrolysis at Minimum Cellulase Loading: An Optimization Study

In the present investigation, Bambusa bambos was used for optimization of enzymatic pretreatment and saccharification. Maximum enzymatic delignification achieved was 84%, after 8 h of incubation time. Highest reducing sugar yield from enzyme-pretreated Bambusa bambos was 818.01 mg/g dry substrate after 8 h of incubation time at a low cellulase loading (endoglucanase, β-glucosidase, exoglucanase, and xylanase were 1.63 IU/mL, 1.28 IU/mL, 0.08 IU/mL, and 47.93 IU/mL, respectively). Enzyme-treated substrate of Bambusa bambos was characterized by analytical techniques such as Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The FTIR spectrum showed that the absorption peaks of several functional groups were decreased after enzymatic pretreatment. XRD analysis indicated that cellulose crystallinity of enzyme-treated samples was increased due to the removal of amorphous lignin and hemicelluloses. SEM image showed that surface structure of Bambusa bambos was distorted after enzymatic pretreatment.

Optimization of microwave-assisted FeCl3 pretreatment conditions of rice straw and utilization of Trichoderma viride and Bacillus pumilus for production of reducing sugars

In this study, Box-Behnken design (BBD) and response surface methodology (RSM) were used to optimize microwave-assisted FeCl(3) pretreatment conditions of rice straw with respect to FeCl(3) concentration, microwave intensity, irradiation time and substrate concentration. When rice straw was pretreated at the optimal conditions of FeCl(3) concentration, 0.14 mol/L; microwave intensity, 160°C; irradiation time, 19 min; substrate concentration, 109 g/L; and inoculated with Trichoderma viride and Bacillus pumilus, the production of reducing sugars was 6.62 g/L. This yield was 2.9 times higher than that obtained with untreated rice straw. The microorganisms degraded 37.8% of pretreated rice straw after 72 h. The structural characteristic analyses suggest that microwave-assisted FeCl(3) pretreatment damaged the silicified waxy surface of rice straw, disrupted almost all the ether linkages between lignin and carbohydrates, and removed lignin.

Oxidative lime pretreatment of Alamo switchgrass

Previous studies have shown that oxidative lime pretreatment is an effective delignification method that improves the enzymatic digestibility of many biomass feedstocks. The purpose of this work is to determine the recommended oxidative lime pretreatment conditions (reaction temperature, time, pressure, and lime loading) for Alamo switchgrass (Panicum virgatum). Enzymatic hydrolysis of glucan and xylan was used to determine the performance of the 52 studied pretreatment conditions. The recommended condition (110°C, 6.89 bar O(2), 240 min, 0.248 g Ca(OH)(2)/g biomass) achieved glucan and xylan overall yields (grams of sugar hydrolyzed/100 g sugar in raw biomass, 15 filter paper units (FPU)/g raw glucan) of 85.9 and 52.2, respectively. In addition, some glucan oligomers (2.6 g glucan recovered/100 g glucan in raw biomass) and significant levels of xylan oligomers (26.0 g xylan recovered/100 g xylan in raw biomass) were recovered from the pretreatment liquor. Combining a decrystallization technique (ball milling) with oxidative lime pretreatment further improved the overall glucan yield to 90.0 (7 FPU/g raw glucan).

Comparison of mechanistic models in the initial rate enzymatic hydrolysis of AFEX-treated wheat straw

BACKGROUND

Different mechanistic models have been used in the literature to describe the enzymatic hydrolysis of pretreated biomass. Although these different models have been applied to different substrates, most of these mechanistic models fit into two- and three-parameter mechanistic models. The purpose of this study is to compare the models and determine the activation energy and the enthalpy of adsorption of Trichoderma reesei enzymes on ammonia fibre explosion (AFEX)-treated wheat straw. Experimental enzymatic hydrolysis data from AFEX-treated wheat straw were modelled with two- and three-parameter mechanistic models from the literature. In order to discriminate between the models, initial rate data at 49 degrees C were subjected to statistical analysis (analysis of variance and scatter plots).

RESULTS

For three-parameter models, the HCH-1 model best fitted the experimental data; for two-parameter models Michaelis-Menten (M-M) best fitted the experimental data. All the three-parameter models fitted the data better than the two-parameter models. The best three models at 49 degrees C (HCH-1, Huang and M-M) were compared using initial rate data at three temperatures (35 degrees , 42 degrees and 49 degrees C). The HCH-1 model provided the best fit based on the F values, the scatter plot and the residual sum of squares. Also, its kinetic parameters were linear in Arrhenius/van't Hoff's plots, unlike the other models. The activation energy (Ea) is 47.6 kJ/mol and the enthalpy change of adsorption (DeltaH) is -118 kJ/mol for T. reesei enzymes on AFEX-treated wheat straw.

CONCLUSION

Among the two-parameter models, Michaelis-Menten model provided the best fit compared to models proposed by Humphrey and Wald. For the three-parameter models, HCH-1 provided the best fit because the model includes a fractional coverage parameter (varphi) which accounts for the number of reactive sites covered by the enzymes.

Enhanced enzymatic hydrolysis and structural features of corn stover by FeCl3 pretreatment

Corn stover was pretreated with FeCl(3) to remove almost all of the hemicellulose present and then hydrolyzed with cellulase and beta-glucosidase to produce glucose. Enzymatic hydrolysis of corn stover that had been pretreated with FeCl(3) at 160 degrees C for 20 min resulted in an optimum yield of 98.0%. This yield was significantly higher than that of untreated corn stover (22.8%). FeCl(3) pretreatment apparently damaged the surface of corn stover and significantly increased the enzymatic digestibility, as evidenced by SEM and XRD analysis data. FTIR analysis indicated that FeCl(3) pretreatment could disrupt almost all the ether linkages and some ester linkages between lignin and carbohydrates but had no effect on delignification. The FeCl(3) pretreatment technique, as a novel pretreatment method, enhances enzymatic hydrolysis of lignocellulosic biomass by destructing chemical composition and altering structural features.

Lime pretreatment

Lime pretreatment has proven to be a useful method for selectively reducing the lignin content of lignocellulosic biomass without significant loss in carbohydrates, thus realizing an important increase in biodigestibility. In lime pretreatment, the biomass is pretreated with calcium hydroxide and water under different conditions of temperature and pressure. It can be accomplished in one of three fashions: (1) short-term pretreatment that lasts up to 6 h, requires temperatures of 100-160 degrees C, and can be applied with or without oxygen (pressure approximately 200 psig); (2) long-term pretreatment taking up to 8 weeks, requiring only 55-65 degrees C, and capable of running with or without air (atmospheric pressure); and (3) simple pretreatment requiring 1 h in boiling water, without air or oxygen. Nonoxidative conditions are effective at low lignin contents (below approximately 18% lignin), whereas oxidative conditions are required for high lignin contents (above approximately 18% lignin).

Anaerobic organic acid production of food waste in once-a-day feeding and drawing-off bioreactor

Acidogenesis of food waste was studied in a 2-L reactor with semi-continuous mode operation (once-a-day feeding and draw-off) for maximum 65 days to examine optimal volatile acid compositions for biological nitrogen removal (BNR) and enhanced biological phosphorus removal (ENPR). Various operational parameters of hydraulic retention time (HRT), organic loading rate (ORL), pH and temperature were investigated for soluble chemical oxygen demand (SCOD), volatile fatty acid composition, nitrogen and phosphate. The yields (gTVFA/g VS) and the volumetric productivity (gTVFA/d L) increased with HRT from 0.26-0.32, 1.25-1.50 (at 4 days) to 0.36-0.39, 1.71-1.83 (at 12 days). However, the acetate fraction (%) decreased with HRT from 35.7-37.5 at 4 days to 23.5-25 at 12 days. The yields decreased with increase of organic loading from 0.34-0.37 at 5 g/L d to 0.29-0.30 at 13 g/L d and the productivity increased from 1.63-1.65 to 3.61-3.75. The yield and productivity were highest at 35 degrees C among 25, 35 and 45 degrees C. The yield and productivity at pH 5.5 and 6.0 were best and very similar to each other. The condition of 35 degrees C, pH 6.0, HRT 8 days, ORL 9 g/L d resulted in TVFA, SCOD, acetate and butyrate of 25, 39.5, 12 and 5.25 g/L, respectively.