Plant glycans are used by humans for many purposes. Wood, which is composed predominantly of lignified secondary walls, is used as an energy source, as a building material, and for papermaking. Pectins isolated from the primary cell walls of fruits and polysaccharides isolated from seeds are used as thickeners, stabilizers, and gelling agents in many foods and beverages. Plant cell walls are the major component of forage used as animal feed. These walls, as dietary fiber, also contribute to human health. Recent concerns about unstable and diminishing oil supplies have led to renewed interest in using plant glycans as feedstocks for energy production, for the generation of polymers with improved or new functionalities, and for the generation of high-value chemical precursors. In this chapter, we briefly describe four broad categories—bioenergy, fine chemicals and feedstocks, polymeric materials, and nanomaterials—in which plant glycans have the potential to replace or to provide alternatives to petroleum-based products.

INTRODUCTION

Plant glycans are used by humans as an energy source, as a building material, and for making numerous products including paper. Cellulose from diverse plant sources is the primary component of many valuable materials, including textiles and plastics. Pectins are used as thickeners, stabilizers, and gelling agents in many foods and beverages. Plant cell walls are used as animal feed and, as dietary fiber, also contribute to human health. The potential adverse effects of burning fossil fuels on the Earths' climate have led to worldwide efforts to develop plant-derived glycans as a renewable raw material to displace or supplement fossil fuels for energy production, for the generation of polymers with improved or new functionalities, and for the generation of high-value chemical precursors.

GLYCANS AND BIOENERGY

The process of photosynthesis by terrestrial plants has been estimated to assimilate at least 100 billion metric tons of CO₂ annually. The chemical energy generated in this manner is stored predominantly in the form of carbohydrates. Some of these carbohydrates are used for plant growth and development or are converted to polysaccharides (starch and fructans) that provide a plant with a readily available form of stored energy. However, a considerable portion of the carbohydrate formed via photosynthesis is used to produce the polysaccharide-rich walls that surround plant cells (Chapter 24). Thus, plant cell walls account for a substantial amount of biological carbon sequestration and are a potentially sustainable and economical source of nonpetroleum-based energy and high-value chemicals.

First-generation bioethanol produced by fermenting the starch present in corn grains currently accounts for virtually all of the liquid transportation fuel generated from plant materials in the United States. The starch is first treated with enzymes that convert it to glucose. The glucose is then fermented to ethanol and carbon dioxide by adding yeast. Yeast can convert 1 kg of glucose to 0.33 gallons (1.25 L) of ethanol and an equivalent amount of carbon dioxide. The United States and Brazil together account for as much as 90% of world ethanol production. Approximately 13 billion gallons (49 billion liters) of ethanol are produced from corn annually in the United States. Brazil produces approximately 6 billion gallons (23 billion liters) of ethanol annually by fermenting the sucrose extracted from sugarcane. Corn and cane bioethanol is then blended with gasoline in varying amounts or used directly as a transportation fuel.

Concerns about the negative impacts of the large-scale production of corn-based ethanol on food production and the environment has led to renewed interest in generating ethanol and other liquid transportation fuels from sustainable plant lignocellulosic biomass. This biomass is comprised predominantly of lignified secondary walls (Chapter 24) that are composed of cellulose (40%–50% w/w), hemicellulose (25%–30% w/w), and lignin (15%–25% w/w). Several different plants, including poplar, switchgrass, miscanthus, eucalyptus, and sugarcane are being considered for use as bioenergy crops.

The biomass from energy crops can be converted to liquid fuel by fermentation or by gasification. In gasification, the biomass is combusted in a low-oxygen environment to generate syngas (hydrogen, carbon monoxide, and carbon dioxide) and heat. The syngas can then be reacted to produce diverse chemicals including alcohols and diesel. Most of the technical challenges to commercial biomass gasification are understood. However, the process has not been widely adopted because of the high capital costs involved.

The generation of liquid fuel from lignocellulose by fermentation currently involves using a cocktail of enzymes to convert the biomass to sugar, which is then fermented to give the desired product. This approach will likely be superseded by the development of bioprocessing technologies in which the microorganism that ferments the sugars also releases them from the biomass. The fermentation approach is simple in concept but there are many technical challenges that must be solved before it becomes commercially viable. One major obstacle is that the cellulose and hemicellulose in lignocellulosic biomass are not readily accessible to hydrolytic enzymes and thus are not efficiently converted to fermentable sugars. The biomass must be pretreated with dilute acid, ammonia, or steam to decrease this recalcitrance. Cost-effective and environmentally sound pretreatment technologies need to be developed if the commercial production of biofuels by fermentation is to become a reality. The efficiency of the enzymes used to convert the cellulose and hemicellulose to sugar must also be improved. To this end there is extensive ongoing research to engineer thermophilic microorganisms to more efficiently deconstruct biomass and to convert the released sugars to the desired fuel, avoiding the necessity of releasing sugars from the biomass with enzyme cocktails before fermentation. Increased understanding of cell wall structure together with knowledge of polysaccharide and lignin biosynthesis is expected to facilitate the engineering of plants to produce biomass that is more amenable to bioprocessing. However, the susceptibility of such plants to biotic and abiotic stress in the field will need to be addressed. Such concerns, together with those regarding the introduction of genetically modified plants into the environment, may be lessened by identifying natural plant variants that produce lignocellulosic biomass that is less recalcitrant to saccharification.

Lignin will be a major by-product of commercial biorefineries, as it is not converted to a liquid fuel during fermentation. Early concepts of biorefineries envisioned burning the lignin to generate power. However, there is now a greater emphasis on using the recovered lignin to produce value-added compounds for the chemical industry. Considerable resources are being allocated to research and development worldwide to create a viable and sustainable lignocellulosic advanced biofuels and bioproducts industry. Nevertheless, many technical, environmental, and societal challenges must be solved if this industry is to develop and contribute to a bio-based economy and to reduce the demand for fossil fuels.

FINE CHEMICALS AND FEEDSTOCKS

Several of the sugars released from lignocellulosic biomass, including glucose and xylose, are being investigated for use in the production of functional chemical precursors that are then used to make industrially relevant chemicals and engineering polymers. Some examples of functional chemical precursors are alcohols (ethanol, propanol, butanol, xylitol, and sorbitol), furans (furfural, hydroxymethylfurfural), biohydrocarbons (isoprene and long-chain hydrocarbons), and organic acids (lactic acid, succinic acid, and levulinic acid). Current research is focused on optimizing the bioconversion of polysaccharides (yield, rate, separation, titer, and product specificity) by identifying and engineering improved fermentation organisms and fermentation processes and developing improved chemical catalysts.

POLYMERIC MATERIALS

Plant-derived polysaccharides including cellulose, xyloglucan, and xylan have been used to make a variety of polymeric materials used by industry. Cellulose has been extensively modified to develop synthetic cellulose-based polymers. Cellulose films (cellophane) and fibers (rayon) are produced using regenerated cellulose that is itself formed by dissolving natural cellulose in alkali and carbon disulfide and then precipitating the polymer in a process that has been used for at least 125 years.

With society's ongoing need for polymers with new properties and functions, there is increased effort to develop reaction pathways to modify the structure of a polysaccharide backbone or side chains to enable the production of polysaccharide derivatives with enhanced or new properties. Cellulose is one example of a plant polysaccharide that has been extensively modified to develop new synthetic cellulose-based polymers. Reaction pathways have been developed to generate specific cellulose derivatives by substituting accessible hydroxyl groups with other chemical groups. Such derivatives include cellulose acetate, cellulose acetate propionate, cellulose acetate butyrates, carboxymethyl cellulose, cellulose butyrate succinate, and cellulose acetate propionate. These products are used in many industrial applications as coatings, inks, binders, and thickening/gelling agents. They are also used in the pharmaceutical industries to produce controlled-release drug tablets and in the cosmetics and food industries as thickening and gelling agents.

With the complexity and variability of polysaccharide structures, there is still considerable potential for the development of new synthetic polysaccharides with new or enhanced functionality. To this end, current research aims to further understand and use new reaction pathways that target chemical modifications to specific locations on the polysaccharide and thereby generate regioselective functionalization.

NANOMATERIALS

Linear polysaccharides that have little or no branching of their backbone can self-assemble to form ordered structures in which the individual polymer chains stack in parallel with each other along the chain axis, thus forming a crystalline structure. Cellulose is one example of a plant polysaccharide that has this type of crystalline structure. During cellulose biosynthesis individual glucan chains assemble to form microfibril structures that contain both crystalline and disordered arrangements (see Figure 59.1A). The high mechanical stiffness and tensile strengths along the length of the microfibrils allows cellulose to provide high mechanical strength, high strength-to-weight ratio, and toughness for plant tissues and organs.

FIGURE 59.1.

(A) The stacking of cellulose chains showing that there are regions of “order” and “disorder.” (B) During one type of cellulose nanomaterial extraction process that uses acid hydrolysis, the amorphous regions are preferentially (more...)

The cellulose fibril structures and the crystalline regions can be isolated using specialized chemical-mechanical extraction methods. The resulting particles, which have dimensions on the nanoscale, are typically referred to as cellulose nanomaterials (CNs; Chapter 58). The CNs' morphology, properties, and surface chemistry vary depending on the plant source and the conditions used to extract the cellulose. Plant CNs are typically classified as cellulose nanocrystals or cellulose nanofibrils. Cellulose nanocrystals are rod-like particles, which are between 3 and 20 nm wide and between 50 and 500 nm in length (Figure 59.1B). Cellulose nanofibrils are fibril-like particles, which are between 5 and 100 nm wide and between 500 nm to several microns in length (Figure 59.2). Both types of CNs have high stiffness and tensile strength, low thermal expansion, and low density and are thermally stable up to between 200°C and 300°C. The surfaces of the CNs can be chemically functionalized and have the potential to be processed at industrial scale at low cost. Preliminary studies suggest that CNs have minimal environmental, health, and safety risks. Thus, CNs are being used to develop new composites that take advantage of the CNs' enhanced mechanical properties, higher surface-area-to-volume ratio, and engineered surface chemistries. For example, CNs have been added to diverse natural and synthetic plastics to enhance the mechanical, optical, thermal, and barrier properties of the resulting composite. Cellulose nanocrystals have also been used in various composite films, fibers, aerogels, and hydrogels, which have potential applications that include barrier films, separation membranes, antimicrobial films, and transparent films; flexible electronics, cements, and biomedical applications; and fibers and textiles, as well as template scaffolds for catalytic supports, batteries, supercapacitors, and body armor.

FIGURE 59.2.

Transmission electron microscopy images showing two types of cellulose nanomaterials. (A) Cellulose nanocrystals produced by acid hydrolysis. (B) Cellulose nanofibrils produced by mechanical defibrillation of wood pulp.

There are many technical challenges that must be overcome before CNs can be used in commercial products. Research in CNs is accelerating and is covering an ever-broadening scope of topics, including, but not limited to, improved CN production technologies, CN suspensions, CN surface functionalization, improved and new CN characterization techniques, CN composite processing, CN composite properties, predictive modeling, life-cycle analysis, environmental health and safety, and exploration into new applications of CNs.

PERSPECTIVES AND FUTURE CHALLENGES

Considerable resources are being allocated to research and development to create economically viable and sustainable biofuels and bioproducts industries that use plant glycans as feedstocks. In the future, there is likely to be a greater emphasis on using both the glycan and the lignin components of plant biomass to produce liquid transportation fuels and value-added chemicals. Such developments will require the development of new chemical catalysts and robust enzymes in addition to plants engineered to produce biomass with enhanced processing characteristics. Many technical, economic, environmental, and societal challenges must be met if these industries are to develop and contribute to a bio-based economy and to reduce the demand for fossil fuels.

ACKNOWLEDGMENTS

The authors acknowledge helpful comments and suggestions from Andrew Kononov, Vivek Kumar, Anne Q. Phan, and Michael Vaill.

FURTHER READING

• Klemm D, Heublein B, Fink H-P, Bohn A. 2005. Cellulose: Fascinating biopolymer and sustainable raw material. Agnew Chem Int Ed 44: 3358–3393. [PubMed: 15861454]
• Ragauskas AJ, Williams CK, Davison BH, Britovsek G, Cairney J, Eckert CA, Frederick WJ, Hallett JP, Leak DJ, Liotta CL, et al. 2006. The path forward for biofuels and biomaterials. Science 311: 484–489. [PubMed: 16439654]
• Hansen N, Plackett D. 2008. Sustainable films and coatings from hemicelluloses: A review. Biomacromolecules 9: 1493–1505. [PubMed: 18457452]
• Carroll A, Somerville C. 2009. Cellulosic biofuels. Ann Rev Plant Biol 60: 165–182. [PubMed: 19014348]
• Mishra A, Malhotra AV. 2009. Tamarind xyloglucan: A polysaccharide with versatile application potential. J Mater Chem 19: 8528–8536.
• Tilman D, Socolow R, Foley JA, Hill J, Larson E, Lynd L, Pacala S, Reilly J, Searchinger T, Somerville C, et al. 2009. Beneficial biofuels—The food, energy, and environment trilemma. Science 325: 270. [PubMed: 19608900]
• Chung D, Cha M, Guss AM, Westpheling J. 2014. Direct conversion of plant biomass to ethanol by engineered Caldicellulosiruptor bescii. Proc Natl Acad Sci 111: 8931–8936. [PMC free article: PMC4066518] [PubMed: 24889625]
• Doblin MS, Johnson KL, Humphries J, Newbigin EJ, Bacic A. 2014. Are designer plant cell walls a realistic aspiration or will the plasticity of the plant's metabolism win out? Curr Opin Biotechnol 26: 108–114. [PubMed: 24679266]
• Habibi Y. 2014. Key advances in the chemical modification of nanocelluloses. Chem Soc Rev 43: 1519–1542. [PubMed: 24316693]
• Ragauskas AJ, Beckham GT, Biddy MJ, Chandra R, Chen F, Davis MF, Davison BH, Dixon RA, Gilna P, Keller M, et al. 2014. Lignin valorization: Improving lignin processing in the biorefinery. Science 344: 1246843. [PubMed: 24833396]
• Zhao Q, Dixon RA. 2014. Altering the cell wall and its impact on plant disease: From forage to bioenergy. Annu Rev Phytopathol 52: 69–91. [PubMed: 24821183]
• Liao C, Seo S-O, Celik V, Liu H, Kong W, Wang Y, Blaschek H, Jin Y-S, Lu T. 2015. Integrated, systems metabolic picture of acetone-butanol-ethanol fermentation by Clostridium acetobutylicum. Proc Natl Acad Sci 112: 8505–8510. [PMC free article: PMC4500237] [PubMed: 26100881]