Optimization of AFEX™ pretreatment conditions and enzyme mixtures to maximize sugar release from upland and lowland switchgrass

Multifaced application of AFEX-pretreated biomass in producing second-generation biofuels, ruminant animal feed, and value-added bioproducts

Lignocellulosic biomass holds a crucial position in the prospective bio-based economy, serving as a sustainable and renewable source for a variety of bio-based products. These products play a vital role in displacing fossil fuels and contributing to environmental well-being. However, the inherent recalcitrance of biomass poses a significant obstacle to the efficient access of sugar polymers. Consequently, the bioconversion of lignocellulosic biomass into fermentable sugars remains a prominent challenge in biorefinery processes to produce biofuels and biochemicals. In addressing these challenges, extensive efforts have been dedicated to mitigating biomass recalcitrance through diverse pretreatment methods. One noteworthy process is Ammonia Fiber Expansion (AFEX) pretreatment, characterized by its dry-to-dry nature and minimal water usage. The volatile ammonia, acting as a catalyst in the process, is recyclable. AFEX contributes to cleaning biomass ester linkages and facilitating the opening of cell wall structures, enhancing enzyme accessibility and leading to a fivefold increase in sugar conversion compared to untreated biomass. Over the last decade, AFEX has demonstrated substantial success in augmenting the efficiency of biomass conversion processes. This success has unlocked the potential for sustainable and economically viable biorefineries. This paper offers a comprehensive review of studies focusing on the utilization of AFEX-pretreated biomass in the production of second-generation biofuels, ruminant feed, and additional value-added bioproducts like enzymes, lipids, proteins, and mushrooms. It delves into the details of the AFEX pretreatment process at both laboratory and pilot scales, elucidates the mechanism of action, and underscores the role of AFEX in the biorefinery for developing biofuels and bioproducts, and nutritious ruminant animal feed production. While highlighting the strides made, the paper also addresses current challenges in the commercialization of AFEX pretreatment within biorefineries. Furthermore, it outlines critical considerations that must be addressed to overcome these challenges, ensuring the continued progress and widespread adoption of AFEX in advancing sustainable and economically viable bio-based industries.

Integrated Renewable Production of Sorbitol and Xylitol from Switchgrass

This work deals with the design of integrated facilities for the production of xylitol and sorbitol from lignocellulosic biomass. Xylitol can be obtained from xylose via fermentation or catalytic hydrogenation. Sorbitol is obtained from glucose, but preferably from fructose, and also via fermentation or catalytic hydrogenation. Fructose can be obtained from glucose via isomerization. Thus, a superstructure of alternatives is formulated to process switchgrass, corn stover, miscanthus, and other agricultural and forestry residues. Different pretreatments, such as dilute acid or ammonia fiber explosion (AFEX), for the fractionation of the biomass are evaluated. Next, after hydrolysis, the C5 and C6 sugars are processed separately for which a catalytic or a fermentation stage are considered. Glucose has to be isomerized before it can be processed. Finally, crystallization in a multistage evaporator system is used for purification. The optimization of the system is done by the use of dilute acid and the catalytic system. A system of 3 crystallizers is selected. For a facility that produces 145 kt/yr of xylitol and 157.6 kt/yr of sorbitol, the investment adds up to 120.74 M€ for a production cost of 0.28 €/kg products. The inverse engineering of biomass was also performed resulting in a composition of 15% water, 20% cellulose, 40% hemicellulose, 15% lignin, and 5% ash. The closest biomass corresponds to Sargassum (brown algae), which is capable of producing 230.5 kt/yr of xylitol and 116 kt/yr of sorbitol with investment and production costs of 120.5 M€ and 0.25 €/kg products, respectively.

Recent advances on ammonia-based pretreatments of lignocellulosic biomass

Ammonia-based pretreatments have been extensively studied in the last decade as one of the leading pretreatment technologies for lignocellulose biorefining. Here, we discuss the key features and compare performances of several leading ammonia-based pretreatments (e.g., soaking in aqueous ammonia or SAA, ammonia recycled percolation or ARP, ammonia fiber expansion or AFEX, and extractive ammonia or EA). We provide detailed insight into the distinct physicochemical mechanisms employed during ammonia-based pretreatments and its impact on downstream bioprocesses (e.g., enzymatic saccharification); such as modification of cellulose crystallinity, lignin/hemicellulose structure, and other ultrastructural changes such as cell wall porosity. Lastly, a brief overview of process technoeconomics and environmental impacts are discussed, along with recommendations for future areas of research on ammonia-based pretreatments.

Development of minimal enzyme cocktails for hydrolysis of sulfite-pulped lignocellulosic biomass

Despite recent progress, saccharification of lignocellulosic biomass is still a major cost driver in biorefining. In this study, we present the development of minimal enzyme cocktails for hydrolysis of Norway spruce and sugarcane bagasse, which were pretreated using the so-called BALI™ process, which is based on sulfite pulping technology. Minimal enzyme cocktails were composed using several glycoside hydrolases purified from the industrially relevant filamentous fungus Trichoderma reesei and a purified commercial β-glucosidase from Aspergillus niger. The contribution of in-house expressed lytic polysaccharide monooxygenases (LPMOs) was also tested, since oxidative cleavage of cellulose by such LPMOs is known to be beneficial for conversion efficiency. We show that the optimized cocktails permit efficient saccharification at reasonable enzyme loadings and that the effect of the LPMOs is substrate-dependent. Using a cocktail comprising only four enzymes, glucan conversion for Norway spruce reached >80% at enzyme loadings of 8mg/g glucan, whereas almost 100% conversion was achieved at 16mg/g.

Plant-based biofuels

This review is a short synopsis of some of the latest breakthroughs in the areas of lignocellulosic conversion to fuels and utilization of oils for biodiesel. Although four lignocellulosic ethanol factories have opened in the USA and hundreds of biodiesel installations are active worldwide, technological improvements are being discovered that will rapidly evolve the biofuels industry into a new paradigm. These discoveries involve the feedstocks as well as the technologies to process them.

Controlling microbial contamination during hydrolysis of AFEX-pretreated corn stover and switchgrass: effects on hydrolysate composition, microbial response and fermentation

BACKGROUND

Microbial conversion of lignocellulosic feedstocks into biofuels remains an attractive means to produce sustainable energy. It is essential to produce lignocellulosic hydrolysates in a consistent manner in order to study microbial performance in different feedstock hydrolysates. Because of the potential to introduce microbial contamination from the untreated biomass or at various points during the process, it can be difficult to control sterility during hydrolysate production. In this study, we compared hydrolysates produced from AFEX-pretreated corn stover and switchgrass using two different methods to control contamination: either by autoclaving the pretreated feedstocks prior to enzymatic hydrolysis, or by introducing antibiotics during the hydrolysis of non-autoclaved feedstocks. We then performed extensive chemical analysis, chemical genomics, and comparative fermentations to evaluate any differences between these two different methods used for producing corn stover and switchgrass hydrolysates.

RESULTS

Autoclaving the pretreated feedstocks could eliminate the contamination for a variety of feedstocks, whereas the antibiotic gentamicin was unable to control contamination consistently during hydrolysis. Compared to the addition of gentamicin, autoclaving of biomass before hydrolysis had a minimal effect on mineral concentrations, and showed no significant effect on the two major sugars (glucose and xylose) found in these hydrolysates. However, autoclaving elevated the concentration of some furanic and phenolic compounds. Chemical genomics analyses using Saccharomyces cerevisiae strains indicated a high correlation between the AFEX-pretreated hydrolysates produced using these two methods within the same feedstock, indicating minimal differences between the autoclaving and antibiotic methods. Comparative fermentations with S. cerevisiae and Zymomonas mobilis also showed that autoclaving the AFEX-pretreated feedstocks had no significant effects on microbial performance in these hydrolysates.

CONCLUSIONS

Our results showed that autoclaving the pretreated feedstocks offered advantages over the addition of antibiotics for hydrolysate production. The autoclaving method produced a more consistent quality of hydrolysate, and also showed negligible effects on microbial performance. Although the levels of some of the lignocellulose degradation inhibitors were elevated by autoclaving the feedstocks prior to enzymatic hydrolysis, no significant effects on cell growth, sugar utilization, or ethanol production were seen during bacterial or yeast fermentations in hydrolysates produced using the two different methods.

Mixtures of thermostable enzymes show high performance in biomass saccharification

Optimal enzyme mixtures of six Trichoderma reesei enzymes and five thermostable enzyme components were developed for the hydrolysis of hydrothermally pretreated wheat straw, alkaline oxidised sugar cane bagasse and steam-exploded bagasse by statistically designed experiments. Preliminary studies to narrow down the optimization parameters showed that a cellobiohydrolase/endoglucanase (CBH/EG) ratio of 4:1 or higher of thermostable enzymes gave the maximal CBH-EG synergy in the hydrolysis of hydrothermally pretreated wheat straw. The composition of optimal enzyme mixtures depended clearly on the substrate and on the enzyme system studied. The optimal enzyme mixture of thermostable enzymes was dominated by Cel7A and required a relatively high amount of xylanase, whereas with T. reesei enzymes, the high proportion of Cel7B appeared to provide the required xylanase activity. The main effect of the pretreatment method was that the required proportion of xylanase was higher and the proportion of Cel7A lower in the optimized mixture for hydrolysis of alkaline oxidised bagasse than steam-exploded bagasse. In prolonged hydrolyses, less Cel7A was generally required in the optimal mixture. Five-component mixtures of thermostable enzymes showed comparable hydrolysis yields to those of commercial enzyme mixtures.

Hydrogen peroxide presoaking of bamboo prior to AFEX pretreatment and impact on enzymatic conversion to fermentable sugars

Bamboo is a fast growing plant found worldwide that has high potential as an energy crop. This project evaluated the effectiveness of AFEX pretreatment for converting moso bamboo (Phyllostachys heterocycla var. pubescens) to fermentable sugars, both with and without pre-soaking in hydrogen peroxide. Pretreatment conditions including temperature, water loading, residence time, ammonia loading, and hydrogen peroxide loadings were varied to maximize hydrolysis yields. The optimal conditions for AFEX were 150°C, 0.8 or 2.0 (w/w) water loading, 10-30 min residence time, and 2.0-5.0 (w/w) ammonia loading. The optimal conditions for H-AFEX were same AFEX conditions with 0.7-1.9 (w/w) 30% (wt) hydrogen peroxide solutions loading. Using 15 FPU/g glucan cellulase and under optimal conditions, AFEX pretreatment achieved a theoretical sugars yield of 64.8-72.7% and addition of hydrogen peroxide presoaking increased the yield to 83.4-92.1%. It is about 5-fold and 7-fold increase in sugars yield for AFEX-treated and H-AFEX-treated bamboo respectively.

Can dispersed biomass processing protect the environment and cover the bottom line for biofuel?

This paper compares environmental and profitability outcomes for a centralized biorefinery for cellulosic ethanol that does all processing versus a biorefinery linked to a decentralized array of local depots that pretreat biomass into concentrated briquettes. The analysis uses a spatial bioeconomic model that maximizes profit from crop and energy products, subject to the requirement that the biorefinery must be operated at full capacity. The model draws upon biophysical crop input-output coefficients simulated with the Environmental Policy Integrated Climate (EPIC) model as well as market input and output prices, spatial transportation costs, ethanol yields from biomass, and biorefinery capital and operational costs. The model was applied to 82 cropping systems simulated across 37 subwatersheds in a 9-county region of southern Michigan in response to ethanol prices simulated to rise from $1.78 to $3.36 per gallon. Results show that the decentralized local biomass processing depots lead to lower profitability but better environmental performance, due to more reliance on perennial grasses than the centralized biorefinery. Simulated technological improvement that reduces the processing cost and increases the ethanol yield of switchgrass by 17% could cause a shift to more processing of switchgrass, with increased profitability and environmental benefits.