Deconstruction of lignocellulosic plant cell walls to fermentable sugars by thermochemical and/or biological means is impeded by several poorly understood ultrastructural and chemical barriers. A promising thermochemical pretreatment called ammonia fiber expansion (AFEX) overcomes the native recalcitrance of cell walls through subtle morphological and physicochemical changes that enhance cellulase accessibility without extracting lignin and hemicelluloses into separate liquid streams. Multi-scale visualization and characterization of Zea mays (i.e., corn stover) cell walls were carried out by laser scanning confocal fluorescence microscopy (LSCM), Raman spectroscopy, atomic force microscopy (AFM), electron microscopy (SEM, TEM), nuclear magnetic resonance (NMR), and electron spectroscopy for chemical analysis (ESCA) to elucidate the mechanism of AFEX pretreatment. AFEX first dissolves, then extracts and, as the ammonia evaporates, redeposits cell wall decomposition products (e.g., amides, arabinoxylan oligomers, lignin-based phenolics) on outer cell wall surfaces. As a result, nanoporous tunnel-like networks, as visualized by 3D-electron tomography, are formed within the cell walls. We propose that this highly porous structure greatly enhances enzyme accessibility to embedded cellulosic microfibrils. The shape, size (10 to 1000 nm), and spatial distribution of the pores depended on their location within the cell wall and the pretreatment conditions used. Exposed pore surface area per unit AFEX pretreated cell wall volume, estimated via TEM-tomogram image analysis, ranged between 0.005 and 0.05 nm² per nm³. AFEX results in ultrastructural and physicochemical modifications within the cell wall that enhance enzymatic hydrolysis yield by 4–5 fold over that of untreated cell walls.

The promise of cellulose: Cellulosic bioenergy, obtained from the lignocellulose that makes up nearly half of plant biomass, has considerable potential as an environmentally friendly energy source, but it still requires substantial resources to produce. Robertson et al. review the trade-offs between the use of cellulosic biofuels and climate mitigation, biodiversity, reactive nitrogen loss, and water use to direct more effective policies for their production. Growing native species on unfarmed land is a promising way forward. Science , this issue p. eaal2324

Current technology using corn stover (CS) as feedstock, Ammonia Fiber Expansion (AFEX) as the pretreatment technology, and Saccharomyces cerevisiae 424A(LNH-ST) as the ethanologenic strain in Separate Hydrolysis and Fermentation was able to achieve 191.5 g EtOH/kg untreated CS, at an ethanol concentration of 40.0 g/L (5.1 vol/vol%) without washing of pretreated biomass, detoxification, or nutrient supplementation. Enzymatic hydrolysis at high solids loading was identified as the primary bottleneck affecting overall ethanol yield and titer. Degradation compounds in AFEX-pretreated biomass were shown to increase metabolic yield and specific ethanol production while decreasing the cell biomass generation. Nutrients inherently present in CS and those resulting from biomass processing are sufficient to support microbial growth during fermentation. This platform offers the potential to improve the economics of cellulosic ethanol production by reducing the costs associated with raw materials, process water, and capital equipment.

Background: When producing biofuels from dedicated feedstock, agronomic factors such as harvest time and location can impact the downstream production. Thus, this paper studies the effectiveness of ammonia fibre expansion (AFEX) pretreatment on two harvest times (July and October) and ecotypes/locations (Cave-in-Rock (CIR) harvested in Michigan and Alamo harvested in Alabama) for switchgrass (Panicum virgatum). Results: Both harvest date and ecotype/location determine the pretreatment conditions that produce maximum sugar yields. There was a high degree of correlation between glucose and xylose released regardless of the harvest, pretreatment conditions, or enzyme formulation. Enzyme formulation that produced maximum sugar yields was the same across all harvests except for the CIR October harvest. The least mature sample, the July harvest of CIR switchgrass, released the most sugars (520 g/kg biomass) during enzymatic hydrolysis while requiring the least severe pretreatment conditions. In contrast, the most mature harvest released the least amount of sugars (410 g/kg biomass). All hydrolysates were highly fermentable, although xylose utilisation in the July CIR hydrolysate was poor. Conclusions: Each harvest type and location responded differently to AFEX pretreatment, although all harvests successfully produced fermentable sugars. Thus, it is necessary to consider an integrated approach between agricultural production and biochemical processing in order to insure optimal productivity.