Abstract
Major challenges still exist in the cost-competitive production of cellulosic ethanol. Lignocellulose deconstruction using physiochemical pretreatments continues to be the most efficient way to remove lignin and expose cellulose for enzyme-mediated sugar reduction. Recent interest in the development of “value-added” lignin co-products and the desire to reduce the use of hazardous materials has prompted a resurgence of interest in alternative lignocellulosic feedstock and research into environment-friendly biological pretreatments. This article provides an overview of bioethanol processes and economics, standard and alternative feedstocks, physiochemical and biological pretreatments, and lignin chemistry. The chemistry and mode of action of ligninolytic and cellulolytic enzymes naturally expressed by white-rot and brown-rot fungi are described. This comprehensive review offers a renewed perspective on alternative high-lignin content cellulosic feedstock - specifically, bamboo - and the potential for microbial-based pretreatments that release cellulose for enzymatic breakdown and the subsequent fermentation of reduced sugars, while leaving lignin structures intact for conversion to valuable co-products.
Highlights
Bioethanol has long been acknowledged as an environmentfriendly, low-cost alternative to fossil fuel
A comprehensive review of each of these topics is beyond the scope of any single paper, we have attempted to present information that lies at the intersection of these topics to provide an interdisciplinary perspective on the state of biofuels production worldwide
Further justification for continued research into novel feedstock and biological pretreatment regimens is supported by new interest in marketable modified lignin co-products as well as the ongoing characterization of novel microorganisms that may form the basis for innovative biotechnological breakthroughs
Summary
Bioethanol has long been acknowledged as an environmentfriendly, low-cost alternative to fossil fuel. Ammonia Fiber Expansion Lignin (AXL): Addition of ammonia to a high-temperature (90-100°C) and high-pressure biomass deconstruction process that includes a timed, sudden pressure release will disrupt lignin-polysaccharide bonds in lignocellulose [64] This will swell the matrix and reduce cellulose crystallinity increasing surface area for subsequent enzyme-mediated hydrolysis. Since ammonia fiber expansion (AFEX) is a very common lignocellulose biomass pretreatment process in cellulosic bioethanol production, it is reasonable to explore the nature of the resulting lignin byproducts and their potential commercial value. Braun’s Lignin has a low molecular weight, high phenolic hydroxyl content, high syringyl content, and a high number of ester groups [68] Another extraction method that leaves native lignin intact is the Bjorkman lignin extraction method, which was developed in 1956 and uses a combination of mechanical pretreatment (i.e., vibratory or rotary ball milling), organic solvent extraction, and enzymes to produce a close to natural lignin product [68].
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