Agricultural Materials as Renewable Resources. Nonfood and Industrial Applications

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Bioethanol is made biologically by fermentation of sugars derived from a variety of sources. Alcohols have been used as fuels since the inception of the automobile.

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The term alcohol often has been used to denote either ethanol or methanol as a fuel. With the oil crises of s, ethanol became established as an alternative fuel. Many countries started programs to study and develop fuels in an economic way from available raw materials. Countries including Brazil and the USA have long promoted domestic bioethanol production.

Bioethanol is widely recognized as a unique transportation fuel with powerful economic, environmental and strategic attributes. As bioethanol can be produced from biomass of crop plants, it offers opportunities to improve the income levels of smallholder farmers. At a community level, farmers can cultivate energy crops that fetch an income while also meeting their food needs. Ethanol derived from biomass is the only liquid transportation fuel that does not contribute to the green house gas effect.

Ethanol represents closed carbon dioxide cycle because after burning of ethanol, the released carbon dioxide is recycled back into plant material as plants use it to synthesize cellulose during photosynthesis. Ethanol production process only uses energy from renewable energy sources; no net carbon dioxide is added to the atmosphere, making ethanol an environmentally beneficial energy source. The toxicity of the exhaust emissions from ethanol is lower than that of petroleum sources Wyman and Hinman The largest bioethanol producers in the world are the US, Brazil, and China.

In , US produced China is nowadays investing heavily in ethanol production and is one of its largest producers Ivanova et al. In India, the interest in biofuels is growing so as to substitute oil for achieving energy security and promote agricultural growth. In addition, a national policy for biofuel has been framed including promotion of biofuel production, particularly on wastelands Ravindranath et al. The varied raw materials used in the manufacture of bioethanol are conveniently classified into three main types: sugars, starches, and cellulose materials. Sugars such as cane or sweet sorghum juice, molasses can be used directly for ethanol production via fermentation.

Starches from corn, cassava, potatoes, and root crops must first be hydrolyzed to fermentable sugars by the action of enzymes from malt or molds. Cellulose from wood, agricultural residues, waste sulfite liquor from pulp, and paper mills must likewise be converted into sugars, generally by the action of acids or cellulolytic enzymes Franks et al.

There are various forms of biomass resources in the world, which can be grouped into four categories, viz. These biomass resources seem to be the largest and most promising future resources for biofuels production. This is because of the ability to obtain numerous harvests from a single planting, which significantly reduces average annual costs for establishing and managing energy crops, particularly in comparison to conventional crops Franks et al.

For second-generation biofuel production, utilization of renewable biomass resources has received major focus in the world. Biomass to bioethanol process could help in mitigation of global climate change by reducing emissions mainly CO 2 as well as decreasing dependence upon fossil fuels.

Thus, deployment of biomass resources has been projected to play an important role in sustainable development. The second-generation biofuels include hydrogen, natural gas, bio-oils, producer gas, biogas, alcohols and biodiesel. In countries like India, agricultural production of various crops like cotton, mustard, chilli, sugarcane, sorghum, sweet sorghum, pulses, oilseeds, etc.

Hence, these could be used as good alternative resources to generate biofuels such as bioethanol, in an environmentally friendly manner. Use of agricultural residues helps in reduction of deforestation by decreasing our reliance on forest woody biomass. Moreover, crop residues have short harvest period that renders them more consistently available to bioethanol production Knauf and Moniruzzaman ; Kim and Dale ; Limayema and Ricke Maize, wheat, rice, and sugarcane are the four agricultural crops with maximum production as well as area under cultivation.

These four crops are responsible for generating majority of lignocellulosic biomass in agriculture sector and rest of the agrowastes constitute only a minor proportion of the total agrowaste production in the world. Corn stover is the left over residue after harvesting corn kernel and comprises stalks, leaves, cobs, and husks.

Rice straw is the leftover of rice production and includes stems, leaf blades, leaf sheaths, and the remains of the panicle after threshing. It is one of the most abundant lignocellulosic waste materials in the world.

Out of the annual global production of million tons of rice straw Asia alone produces Bagasse is produced in huge amounts during sugarcane processing. It is also a cheap renewable agricultural resource for ethanol production Bhatia and Paliwal Most of the agricultural residues have similar contents of cellulose, hemicelluloses, and lignin, but rice straw has more silica content while wheat straw contains significant amount of pectin and proteins Sarkar et al.

Maximum rice straw and wheat straw are generated in Asia and corn straw and sugarcane bagasse are mainly produced in America. In US alone, a total of 1, MT biomass are available for bioethanol production, out of which agrowastes with MT constitute major proportion, followed by forestry wastes, energy crops, grains and corn, municipal and industrial wastes and other wastes contributing , , 87, 58 and 48 MT, respectively Perlack et al.

Ref Kuhad et al. Worldwide availability of major agricultural wastes and their bioethanol production potential. Agricultural residues such as wheat straw, rice straw, bagasse, cotton stalk and wheat bran are rich in lignocellulose and primarily contain cellulose, lignin, hemicellulose, and extractives.

Cellulose forms a skeleton that is surrounded by hemicellulose and lignin functioning as matrix and encrusting materials, respectively Ingram and Doran Classically, cell wall polysaccharides have been grouped into three fractions: cellulose, hemicellulose and pectic polysaccharides, proteins and other miscellaneous compounds Chesson and Forsberg as discussed below. The highly crystalline regions of cellulose in the plant cell wall are separated by less ordered amorphous regions Chesson and Forsberg Hemicellulose is a short, highly branched polymer of pentoses e.

Its acetate groups were randomly attached with ester linkages to the hydroxyl groups of the sugar rings. The role of hemicellulose is to provide a linkage between lignin and cellulose Holtzapple Its other major components are rhamnose, arabinose, and galactose. Pectic substances are hydrophillic and therefore have certain adhesive properties. Proteins are a minor component of the plant cell wall which may be covalently cross-linked with lignin and polysaccharides Cassab and Varner Three types of phenolic compounds viz.

Lignin is a heterogeneous, amorphous, and cross-linked aromatic polymer where the main aromatic components are trans-coniferyl, trans-sinapyl and trans- p -coumaryl alcohols. Lignin is covalently bound to side groups on different hemicelluloses, forming a complex matrix that surrounds the cellulose micro-fibrils. The existence of strong carbon—carbon C—C and ether C—O—C linkages in the lignin gives the plant cell wall strength and protection from attack by cellulolytic microorganisms Mooney et al.

Tannins are high molecular weight —3, polyphenolic compounds, composed of either hydroxyflavans, leucoanthocyanidin flavan-3,4-diol and catechin flavanol or glucose. Phenolic acids are structural components of the lignin core in plant cell wall. The presence of carboxyl and phenolic groups in phenolic acids enable such compounds to link to lignin and carbohydrates by ether or ester bonds. Biomass to ethanol bioconversion process consists of several steps, including pretreatment of biomass, enzymatic hydrolysis, fermentation and product recovery.

Proper combination of each step is important for achieving higher bioethanol yield in a cost-effective and sustainable manner. The main processing challenge in the ethanol production from lignocellulosic biomass is the feedstock pretreatment. During pretreatment, the matrix of cellulose and lignin bound by hemicellulose should be broken to reduce the crystallinity of cellulose and increase the fraction of amorphous cellulose, the most suitable form for enzymatic attack.

For pretreatment of lignocellulosics, several physical, physico-chemical and biological processes have been developed that improve lignocellulose digestibility in very different ways Aden et al. Lignocellulosic biomass can be pulverized by chipping, grinding, shearing, or milling, which reduces the particle size and increases surface area, facilitating the access of cellulases to the biomass surface and increasing the conversion of cellulose. Primary size reduction employs hammer mills or Wiley mills to produce particles that can pass through 3- to 5-mm diameter sieve. Other useful physical treatment methods include pyrolysis, irradiation with gamma radiation, microwave, infrared, or sonication Brown ; Mosier et al.

Physico-chemical methods are considerably more effective than physical methods of pretreatment. Different chemical agents employed during these processes are ozone, acids, alkali, peroxide and organic solvents. The brown rot, white rot and soft-rot fungi such as Phanerochaete chrysosporium , Trametes versicolor , Ceriporiopsis subvermispora , and Pleurotus ostreatus are employed for biological pretreatment of lignocellulosic biomass.

Besides lignin peroxidases and manganese-dependent peroxidases, polyphenol oxidases, laccases and quinosine-reducing enzymes also degrade lignin by producing aromatic radicals. Biological treatment requires low energy and normal environmental conditions but the hydrolysis yield is low and requires long treatment times Brown Cellulose hydrolysis, also known as saccharification, is the process in which the cellulose is converted into glucose.

Enzymatic hydrolysis is the key to cost-effective ethanol production from lignocellulosic substrates in the long run, as it is very mild process, gives potentially high yields, and the maintenance costs are low compared to acid or alkaline hydrolysis Kuhad et al. The process is compatible with many pretreatment methods, but materials poisonous to the enzymes need to be removed or detoxified when chemical pretreatment precedes enzymatic hydrolysis.

Factors affecting enzymatic saccharification process involve substrate concentration, enzyme loading, temperature and time of saccharification Tucker et al. The cellulose-degrading enzymes were discovered by Reese The term cellulase complex normally refers to a set of enzymes involved in complete cellulose hydrolysis. X class. Acting on soluble cellulose derivatives, their random cleavage causes rapid decrease in chain length and hence changes in viscosity relative to the release of reducing end groups. When acting on cellodextrins, the rate of hydrolysis increases with the degree of polymerization within the limits of substrate solubility, with cellobiose and cellotriose being the major final product.

Unlike exoglucosidases, the rate of hydrolysis of cellobiose decreases markedly as the degree of polymerization of the substrate increases. When the combination of two enzymes is more efficient than the sum of the enzymes acting alone, the two enzymes have synergy. This model for the synergy between endoglucanases and exoglucanases is called the endo-exo model Beguin and Aubert In nature there are many microorganisms that produce cellulolytic enzymes cellulases.

The cellulolytic organisms can be sorted into two different subcategories depending on their enzyme organization in the cell: a the microorganism with their cellulases organized into multi-enzyme complexes called cellulosomes, e. Clostridium thermocellum and Cellulomonas. Examples of fungi from this class are T. Trichoderma spp. Today several species of cellulase producing Penicillium spp. Moreover, many species of Aspergillus such as A. Cellulase production using fungal cultures is a complex system. Many factors affect cellulase production including nutrient availability, pH, temperature, dissolved oxygen concentration, agitation speed, etc.

No general medium composition can be given for growth and optimum cellulase production by all microbes, since the medium must be adapted to the organism in use.

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Basal medium after Mandels and Weber has been most frequently used for cellulase production by T. Among the cellulosic materials, sulfite pulp, printed papers, mixed waste paper, wheat straw, paddy straw, sugarcane bagasse, jute stick, carboxymethylcellulose corncobs, groundnut shells, cotton, ball milled barley straw, delignified ball milled oat spelt xylan, larch wood xylan, etc. Addition of Ammonium sulfate 0. Phosphorus is an essential requirement for fungal growth, metabolism and several intracellular processes. Temperature has a profound effect on lignocellulosic bioconversion.

When the environmental pH is over the operational pH pH 2. For cellulase production by T. In addition, the fungal cellulases have been found to have the highest catalytic capability between pH 3. Oxygen is required for cell growth of most eukaryotes; therefore, cell growth is affected by agitation tremendously. Cells die when oxygen is not enough and they stop growing afterwards. Although significant advances have been made, a considerable amount of work is still required to enhance the production efficiency of cellulase enzymes.

Improved enzyme production by coculturing of two or more microbial strain is being used increasingly for enhanced enzyme production Garcia-Kirchner et al. Enhanced cellulose hydrolytic activities have also been observed by the co-cultivation of A.

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Culture filtrates from fungal growth often contain a mixture of several extracellular enzymes besides cellulases and hemicellulases and present considerable purification problems. Therefore, multiple purification steps, including different chromatographic runs, are needed to purify cellulase components Stahlberg et al.

Various chromatographic techniques have been described in the literature for purification of cellulase enzyme such as molecular exclusion, affinity chromatography, ion-exchange chromatography, chromatofocusing, fast protein liquid chromatography FPLC and hydrophobic interaction chromatography HIC Tomaz and Queiroz Ref Rashid and Siddiqui The cost of cellulase enzymes is widely considered an important factor in the commercialization of lignocellulosic biomass-to-ethanol processes Wright Ref Nieves et al. To detect morphological and structural changes in polymers, some physico-chemical thermal analysis, X-ray diffraction, gel permeation chromatography , spectroscopic Infrared and Raman spectroscopy, nuclear magnetic resonance and mass spectroscopy and microscopic [scanning electron microscopy SEM , atomic force microscopy AFM , transmission electron microscopy TEM , and chemical force microscopy CFM ] Samir et al.

Standard methods generally employed to examine biodegradation of biopolymers are: visual observations, weight loss measurements through determination of residual polymer, changes in mechanical properties and molar mass and radio-labeling. A number of other techniques have also been used to assess the biodegradability of polymeric material. The biomass is hydrolyzed by cellulolytic enzymes into fermentable sugars pentoses or hexoses , which are fermented to ethanol by several microorganisms.

For making ethanol production commercially viable, an ideal microorganism should utilize broad range of substrates, with high ethanol yield, titre and productivity, and should have high tolerance to ethanol, temperature and inhibitors present in hydrolysate. Ref Dien et al. As shown in Fig. First is separate or sequential hydrolysis and fermentation SHF , a two-stage process involving saccharification of the substrate, followed by the fermentation of saccharified fluid, separately.

Main features of SHF include optimal operating conditions for each step and minimal interactions between hydrolysis and fermentation steps. However, SHF process is limited by end-product inhibition and chances of contaminations, which may decreases ethanol yield Balat et al. Second process configuration is simultaneous saccharification and fermentation SSF , in which hydrolyzes of cellulose is consolidated with the direct fermentation of the produced glucose, avoiding the problem of product inhibition associated with enzymes.

Main advantages of simultaneous saccharification and fermentation process are comparatively lower costs, higher ethanol yields due to removal of feedback inhibition on enzymatic saccharification and reduction in the required number of vessels or reactors. Some of the disadvantages of SSF are different optimum conditions for enzyme hydrolysis and fermentation processes Bjerre et al. The most common and robust fermenting microorganisms employed in ethanol production are S.

Ethanol production from sugars derived from starch and sucrose has been commercially dominated by this yeast. However, S. The most promising yeasts that have the ability to use both pentoses and hexoses are Pichia stipitis , Candida shehatae and Pachysolan tannophilus. Thermotolerant yeasts, such as Kluyveromyces marixianus , could be more suitable for ethanol production at industrial level, because of their ability to ferment at higher temperatures. In high-temperature process energy savings can be achieved through a reduction in cooling costs.

Hence, thermotolerant yeasts are highly desirable in SSF process. Another strategy for ethanol fermentation is simultaneous saccharification and co-fermentation SSCF , in which co-fermentation of hexoses and pentoses is carried out. However, the ability to ferment pentoses along with hexoses is not widespread among microorganisms and lack of ideal co-fermenting microorganism is one of the greatest obstacles in industrial production of second-generation ethanol Talebnia et al.

Sometimes co-culture technique proves to be a useful technology whereby a combination of hexose and pentose fermenting microorganisms is utilized for complete utilization of biomass sugars. One more configuration for ethanol fermentation is consolidated bioprocessing CBP. In this process, ethanol and all required enzymes are produced by a single microorganism or microbial community, in the same reactor.

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The process is also known as direct microbial conversion DMC. The main advantage of CBP is that its application avoids the cost involved in purchase or production of enzymes Hamelinck et al. Approached pathways in the development of CBP organisms are described by Lynd et al. Bacteria such as Clostridium thermocellum and some fungi including Neurospora crassa , Fusarium oxysporum and Paecilomyces sp.

Ref Sarkar et al. Experimental design and statistical analysis for optimization of process conditions are some of the most critical stages in the development of an efficient and economic bioprocess. Classical such as one factor at a time and statistical methodologies are available for optimizing process conditions such as response surface methodology, RSM. RSM is an efficient statistical technique for optimization of multiple variables to predict the best performance conditions with a minimum number of experiments.

These designs are used to find improved or optimal process settings, troubleshoot process problems and weak points and make a product or process more robust against external and noncontrollable influences. Full factorial, partial factorial, Box-Behnken and central composite rotatable designs CCRD are the most common techniques used for process analysis and monitoring Sasikumar and Viruthagiri This method has been in use for hydrolysis of a wide variety of materials to find the optimum conditions for different lignocellulosic biomasses Talebnia et al.

To be competitive, and economically acceptable, the cost for bioconversion of biomass to liquid fuel must be lower than the current gasoline prices Subramanian et al. It seems, however, much more attainable because of increasing efforts of researchers working towards improvisation in the efficiency of biomass conversion technologies.

However, there is still huge scope to bring down the cost of biomass-to-ethanol conversion. The cost of feedstock and cellulolytic enzymes are the two important parameters for low-cost ethanol production. An analysis of the potential of bioethanol in short and long term in terms of performance, key technologies and economic aspects such as cost per kilometer driven has been conducted recently by Hamelinck et al.

The choice of feedstock for ethanol production depends upon its availability and the ongoing uses. Some dedicated energy sources like damaged rice, sorghum grains and sweet sorghum bagasse, sunflower stalks and hulls, Eicchornia crassipies , P. The use of integrated approach Process engineering, fermentation and enzyme and metabolic engineering could improve the ethanol production economics.

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Wooley et al. The distillation cost of per unit amount of ethanol produced is substantially higher at low ethanol concentrations; the researchers have dealt with the idea of concentrating sugar solutions prior to fermentation. Ethanol distillation cost can be further improved using membrane distillation process. It has the lowest operational cost, simple to use and is easy to maintain and is the most efficient and cost-effective option among the available distillation processes Camacho et al.

Lignocellulosic biomass has long been advocated as a key feedstock for cost-effective bioethanol production in an environment-friendly and sustainable manner. Till now research on utilization of agricultural residues for second-generation bioethanol production has shown very promising results worldwide.

Several lab and pilot scale as well as demonstration studies for cellulosic ethanol production from agrowastes have been reported successful but still there exists a huge gap between the projected and actual bioethanol production at industrial level. Therefore, to make full use of these cheap, abundant and renewable resources for economically feasible bioethanol production, several difficulties have yet to be overcome. Considering the huge availability of feedstocks from agriculture and other sources and tremendous efforts being carried out to make second-generation biofuel production more cost-effective, there seems huge scope for the large-scale production of second-generation biofuels in near future.

This will certainly involve elimination of the current technology hurdles of lignocellulose to bioethanol conversion process by making microbial processes more efficient. Lignocellulosic biomass-derived second-generation biofuels are promising alternatives to petroleum-based fossil fuels. The utilization of agricultural residues and wastes for bioethanol production is a cost-effective and environmental-friendly approach for sustainable development.

Considering the recent research progress in the fields of enzyme production, pretreatment, as well as metabolic engineering of yeasts, production of bioethanol from lignocellulosic agricultural wastes will certainly prove to be a feasible technology to achieve energy security in very near future. All the authors declare that they have no conflict of interest in the publication. National Center for Biotechnology Information , U. Journal List 3 Biotech v. Published online Aug Reetu Saini Department of Microbiology, M. Garg P. Author information Article notes Copyright and License information Disclaimer.

Department of Microbiology, M. College, Laksar, Haridwar, India. Corresponding author. Received May 19; Accepted Aug 5. Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author s and the source are credited. This article has been cited by other articles in PMC.

Abstract Production of liquid biofuels, such as bioethanol, has been advocated as a sustainable option to tackle the problems associated with rising crude oil prices, global warming and diminishing petroleum reserves. Introduction One of the greatest challenges of twenty-first century is to meet the growing demand of energy for transportation, heating and industrial processes, and to provide raw materials for chemical industries in sustainable ways.

Bioethanol: an eco-friendly biofuel Bioethanol is made biologically by fermentation of sugars derived from a variety of sources. Feedstocks for bioethanol: agricultural residues The varied raw materials used in the manufacture of bioethanol are conveniently classified into three main types: sugars, starches, and cellulose materials. Open in a separate window. Agricultural wastes Availability a million tons Estimated bioethanol potential a Gl Wheat straw Structural organization of lignocellulosic feedstocks Agricultural residues such as wheat straw, rice straw, bagasse, cotton stalk and wheat bran are rich in lignocellulose and primarily contain cellulose, lignin, hemicellulose, and extractives.

Cell wall polysaccharides Classically, cell wall polysaccharides have been grouped into three fractions: cellulose, hemicellulose and pectic polysaccharides, proteins and other miscellaneous compounds Chesson and Forsberg as discussed below. Hemicellulose Hemicellulose is a short, highly branched polymer of pentoses e.

Phenolic compounds Three types of phenolic compounds viz. Bioconversion of lignocellulosic biomass to bioethanol Biomass to ethanol bioconversion process consists of several steps, including pretreatment of biomass, enzymatic hydrolysis, fermentation and product recovery.

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  • In the medium term, industry will have to expand its raw materials base, and in the long term it may have to renew it completely. Industrial biotechnology is one of the key technologies in the transition from an economy based on fossil fuels to one based on renewable resources. The microbes that produce renewable resources need access to a sufficient supply of biomass. But what kind of biomass will they use and do we have enough of it? How coveted biomass is depends a great deal on the type of biomass we are talking about. Although a battle is already raging about wood, we still have a rather relaxed attitude when it comes to biological waste.

    However, it is realistic to assume that the competition for renewable raw materials will intensify in the future. This creates a dilemma: grazing areas and agricultural and forest areas are limited, and the demand for biomass is enormous if it is to be used to partially replace fossil raw materials in industry.

    As land is limited and there is a large number of competitive applications biomass can be used for, it is important to set priorities. Experts have defined a priority list for the different applications of biomass OECD, , p. Fertilizers and soil conditioners come at the end of the list. However, all this is still a vision of the future. The idea behind the 5F Cascade is that the material use of renewable resources biomass before they are used to produce energy saves fossil fuels, cuts greenhouse gas emissions because C0 2 is bound for longer, and keeps biomass in a longer recycling chain, which therefore delivers greater added value.

    Biomass is currently predominantly used to make bioethanol and wood pellets, which means that it is combusted very early on in the recycling process. The cascading resource-efficient and sustainable use of biomass is currently not yet standard. At present, it is basically only used for energy production. Material flows i. Around million tons of crude oil were used in Germany in for industrial processes and the production of energy.

    Crude oil is used for the production of fuels, lubricants, chemicals and plastics — modern life is inconceivable without crude oil. Biomass is currently the only renewable carbon source. Maybe one day it will also be possible to use carbon dioxide or carbon monoxide from industrial processes, i. However, this pathway is far from sustainable because the carbon in this case would still derive from fossil resources such as crude oil, gas or coal.

    The first generation of biobased chemicals employs resources that are also used as food and feed. The second generation of biobased chemicals uses non-food biomass that mainly contains cellulose, hemicellulose or lignocellulose. Having said that, edible biological resources are still the principal resources used for the production of chemicals. However, the ongoing debate on sustainability is bringing biogenic residues and industry waste and its elusive material flows more into the spotlight Raschka and Carus, In future, it is highly likely that a thing like biological waste will no longer exist.

    It can safely be assumed that the barriers to the material use of biomass will be removed and biomass will be turned into products and become the basis of value-added processes. Biomass should be available, competitive and sustainable. In Europe, this applies for example to residues from the harvesting of round timber or material from landscape management i.

    Nevertheless, round timber is a special case: the demand from the traditional wood industries is so high that there is very little wood available to produce anything else. Moreover, the potential for using wood residues from the forestry sector is already fully exploited. So what about algae? Will they become the major suppliers of biomass? They take up CO 2 and convert it into hydrocarbons. Whether they will become an alternative to land plants is difficult to say because methods that enable large-scale cultivation of algae are still in their infancy.

    However, they are certainly excellent candidates, due in particular to their CO 2 -based metabolism and to the fact that they can be cultivated on areas that cannot be used for forestry or agriculture. The sustainability debate clearly affects first-generation 1G biofuels which are produced from starch contained in foods like corn, sugar crops, wheat or oil from oilseeds such as rapeseed.

    A transition to using other renewable resources that are not starch-based and hence do not compete with food and feed is immanent. However, this transition to non-food renewables is a difficult one. Bacteria and yeasts that are traditionally used in biotechnological applications are unable to metabolise hemicellulose, which is a major component of lignocellulose.

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    This is due to the composition of the molecule. Hemicellulose is a polymer of sugars containing five carbon atoms each C5 sugars. However, the microorganisms used in the field of biotechnology prefer glucose, a sugar with six carbon atoms that is contained in cellulose and starch. Nevertheless, some microorganisms are able to metabolise C5 sugars. Lignin is a relatively complex biopolymer that is associated with even greater problems. This is another case where the microorganisms that are traditionally used in biotechnological applications struggle to break it down.

    Some fungi are able to break down lignin, but they are unsuitable for mass culture in bioreactors. Researchers are working intensively on the development of enzymes that can break down lignocellulose, and lignin in particular. Once it is broken down, lignin can be used by microorganisms that are able to metabolise the C5 sugars of hemicellulose and transform them into products or product precursors. The goal is not only to find ways that enable the chemical transformation of the molecules, but also to find enzymes and organisms that are suitable for large-scale production. The latter is a huge challenge that will be difficult to achieve.

    Some second-generation pilot- and demonstration-scale bioethanol plants 2G ethanol plants that are able to use non-food substrates already exist and further large-scale plants are under construction. The only commercial-scale plant, which has an annual production capacity of 75 million litres, is located outside the city of Crescentino Italy and is able to produce bioethanol from rice straw and energy crops through enzymatic conversion.

    The specialty chemicals company Clariant operates a demonstration-scale 1. Some 2G plants that are able to produce biobased fuels are operated — some on a trial basis - overseas. Proof of their cost-effectiveness is not yet available Krieger, The microbial production of biodiesel ed.