On a novel mechanistic model for simultaneous enzymatic hydrolysis of cellulose and hemicellulose considering morphology

Hydrolysis and fermentation steps of a pretreated sawmill mixed feedstock for bioethanol production in a wood biorefinery

The aim of this work was to demonstrate the feasibility of second-generation bioethanol production using for the first time a sawmill mixed feedstock comprising four softwood species, representative of biomass resource in Auvergne-Rhône-Alpes region (France). The feedstock was subjected to a microwave-assisted water/ethanol Organosolv pretreatment. The investigation focused on enzymatic hydrolysis of this pretreated sawmill feedstock (PSF) using Cellic® Ctec2 as the enzyme, followed by fermentation of the resulting sugar solution using Saccharomyces cerevisiae strain. The cellulose-rich PSF with 71% w/w cellulose content presented high saccharification yields (up to 80%), which made it perfect for subsequent fermentation; this yield was predicted vs. time up to 5.2% w/v PSF loading using a mathematical model fitted only on data at 1.5%. Finally, high PSF loading (7.5%) and scaleup were shown to impair the saccharification yield, but alcoholic fermentation could still be carried out up to 80% of the theoretical glucose-to-ethanol conversion yield.

A two-phase substrate model for enzymatic hydrolysis of lignocellulose: application to batch and continuous reactors

BACKGROUND

Enzymatic hydrolysis continues to have a significant projected production cost for the biological conversion of biomass to fuels and chemicals, motivating research into improved enzyme and reactor technologies in order to reduce enzyme usage and equipment costs. However, technology development is stymied by a lack of accurate and computationally accessible enzymatic-hydrolysis reaction models. Enzymatic deconstruction of cellulosic materials is an exceedingly complex physico-chemical process. Models which elucidate specific mechanisms of deconstruction are often too computationally intensive to be accessible in process or multi-physics simulations, and empirical models are often too inflexible to be effectively applied outside of their batch contexts. In this paper, we employ a phenomenological modeling approach to represent rate slowdown due to substrate structure (implemented as two substrate phases) and feedback inhibition, and apply the model to a continuous reactor system.

RESULTS

A phenomenological model was developed in order to predict glucose and solids concentrations in batch and continuous enzymatic-hydrolysis reactors from which liquor is continuously removed by ultrafiltration. A series of batch experiments were performed, varying initial conditions (solids, enzyme, and sugar concentrations), and best-fit model parameters were determined using constrained nonlinear least-squares methods. The model achieved a good fit for overall sugar yield and insoluble solids concentration, as well as for the reduced rate of sugar production over time. Additionally, without refitting model coefficients, good quantitative agreement was observed between results from continuous enzymatic-hydrolysis experiments and model predictions. Finally, the sensitivity of the model to its parameters is explored and discussed.

CONCLUSIONS

Although the phenomena represented by the model correspond to behaviors that emerge from clusters of mechanisms, and hence a set of model coefficients are unique to the substrate and the enzyme system, the model is efficient to solve and may be applied to novel reactor schema and implemented in computational fluid dynamics (CFD) simulations. Hence, this modeling approach finds the right balance between model complexity and computational efficiency. These capabilities have broad application to reactor design, scale-up, and process optimization.

Enzyme kinetics of cellulose hydrolysis of Miscanthus and oat hulls

Experiments were done to model enzymatic hydrolysis of Miscanthus and oat hulls treated with dilute solutions of nitric acid and sodium hydroxide in direct and reverse sequences. The enzymatic hydrolysis kinetics of the substrates was studied at an initial solid loading from 30 to 120 g/L. The effects of feedstock type and its pretreatment method on the initial hydrolysis rate and reducing sugar yield were evaluated. The fitting results by the developed models showed good agreement with the experimental data. These models designed for developing the production technology of concentrated glucose solutions can also be applied for glucose fermentation into ethanol. The initial solid loading of 60-90 g/L provides the reducing sugar concentration of 40-80 g/L necessary for ethanol synthesis. The kinetic model can also be applied to investigate enzymatic hydrolysis of other substrates (feedstock type, pretreatment method) under the similar conditions used herein, with adjusted empirical coefficient values.

A discretized model for enzymatic hydrolysis of cellulose in a fed-batch process

In the enzymatic hydrolysis of cellulose, several phenomena have been proposed to cause a decrease in the reaction rate with increasing conversion. The importance of each phenomenon is difficult to distinguish from batch hydrolysis data. Thus, kinetic models for the enzymatic hydrolysis of cellulose often suffer from poor parameter identifiability. This work presents a model that is applicable to fed-batch hydrolysis by discretizing the substrate based on the feeding time. Different scenarios are tested to explain the observed decrease in reaction rate with increasing conversion, and comprehensive assessment of the parameter sensitivities is carried out. The proposed model performed well in the broad range of experimental conditions used in this study and when compared to literature data. Furthermore, the use of data from fed-batch experiments and discretization of the model substrate to populations was found to be very informative when assessing the importance of the rate-decreasing phenomena in the model.

Fungal Enzymes for Bio-Products from Sustainable and Waste Biomass

Lignocellulose, the most abundant renewable carbon source on earth, is the logical candidate to replace fossil carbon as the major biofuel raw material. Nevertheless, the technologies needed to convert lignocellulose into soluble products that can then be utilized by the chemical or fuel industries face several challenges. Enzymatic hydrolysis is of major importance, and we review the progress made in fungal enzyme technology over the past few years with major emphasis on (i) the enzymes needed for the conversion of polysaccharides (cellulose and hemicellulose) into soluble products, (ii) the potential uses of lignin degradation products, and (iii) current progress and bottlenecks for the use of the soluble lignocellulose derivatives in emerging biorefineries.

Lignin enrichment and enzyme deactivation as the root cause of enzymatic hydrolysis slowdown of steam pretreated sugarcane bagasse

The enzymatic hydrolysis (EH) rate normally decreases during the hydrolysis, leaving unhydrolyzed material as residue. This phenomenon occurs during the hydrolysis of both cellulose (avicel) and lignocellulosic material, in nature or even pretreated. The progression of EH of steam pretreated sugarcane bagasse was associated with an initial (fast), intermediate (slower) and recalcitrant (slowest) phases, at glucan to glucose conversion yields of 61.7, 81.6 and 86%, respectively. Even though the EH of avicel as a simpler material than steam pretreated sugarcane bagasse, EH slowdown was present. The less thermo-stable endo-xylanase lost 58% of initial enzyme activity, followed by β-glucosidase that lost 16%, culminating in FPase activity loss of 30% in the first 24hours. After 72hours of EH the total loss of FPase activity was 40% compared to the initial activity. Analysis of the solid residue from EH showed that lignin content, phenolic compounds and ash increased while glucan decreased as hydrolysis progressed. During the initial fast phase of EH, the total solid residue surface area consisted predominantly of internal surface area. Thereafter, in the intermediate and recalcitrant phases of EH, the ratio of external:internal surface area increased. The proposed fiber damage and decrease in internal surface area, probably by EH action, was visualized by scanning electron microscopy imagery. The higher lignin/glucan ratio as EH progressed and enzyme deactivation by thermo instability were the main effects observed, respectively to substrate and enzyme.