Category Archives: BIOETHANOL

Single-step bioconversion

The idea of single-step bioconversion is to integrate all processes such as liquefaction, saccharafication and fermentation in one step and in one bioreactor. This alternative process will reduce contamination and the operation cost resulted from multistage processes of ethanol production. This also will reduce energy consumption of the overall process. The one-step bioconversion can be done by using recombinant clone or by co-culture or consortium of microorganisms that able to degrade or digest starch into intermediate product such as oligosaccharides and reducing sugar by starch fermenting microorganism(s). Then, the fermentation followed by fermenting the intermediate products into ethanol by microbe in the mixture. This process not only eliminates the use of enzymes to reduce the production cost but also yield added value by-products via co-culture of microbes. Besides, it also has a distinctive advantage as far as biorefinery is concerned. Unlike enzymes which normally required purification before recycled and added into the process, microbial growth can replace cells that have been removed. Even if cell separation and recycle are required, the processes are simpler compared to the more complex and sophisticated enzyme separation and purification process such as enzyme membrane reactor (Iorio et al., 1993) using ultrafiltration, extraction in aqueous two-phase system (ATPS) of water-soluble polymers and salts and/or two different water soluble polymers (Minami and Kilikian, 1998; Bezerra et al., 2006) and selective precipitation (Rao et al., 2007).

Benefits of sugarcane for ethanol

The reasons why we choose ethanol from sugarcane as the most promising biofuels are illustrated below. Firstly, the balance of GHG emissions of sugarcane ethanol is the best among all biofuels currently produced (Macedo et al., 2008; Cerri et al., 2009; Oliveira et al., 2005). As reviewed in several studies, bioethanol based on sugarcane can achieve greenhouse gas reductions of more than 80% compared to fossil fuel use (Macedo et al., 2008). Figure 2 (BNDES, 2008) showed correspond to the consumption of ethanol produced from maize (USA), from wheat (Canada and Europe) and from sugarcane (produced in Brazil and consumed in Brazil or in Europe). Sugarcane ethanol is much better than ethanol from maize and wheat (a maximum of 35%) in case of the avoided emissions.

Secondly, as we known, cropland is very limited for planting in China. So it is very important that the land use is keeping in a high efficient level. Ethanol from sugarcane is the most productive among different crops. The fortunate experience of ethanol use in Brazil may also be coupled with a superior sucrose yield and a higher potential of biomass production of sugarcane — an average of 87 tons per hectare in South Central Brazil — than observed in other crops. As shown in figure 3, only beets can be compared with sugarcane in terms of ethanol production per cultivated hectare. However, the industrial process of ethanol production from beets depends on an external power input (electricity and fuel) while sugarcane electricity is provided by bagasse burning at the mill. (BNDES, 2008). Ethanol produced from sugarcane is the biofuel with the best energy balance (see tablel). This can be illustrated as the ratio between renewable products and the energy input as fossil fuel for Brazilian sugarcane ethanol is 9.3 (compared with 1.2-1.4 in the case of ethanol produced from American maize, and approximately 2.0 in the case of ethanol produced from European wheat). Apart from these above, other environmental impacts of the sugarcane sector, such as water consumption, contamination of soils and water shields due

image021 Подпись: □ Ethanol produc ed from cellulosic

to the use of fertilizers and chemicals, and loss of biodiversity, are less important in comparison to other crops (Watanabe, 2009). Above in all, Sugarcane is by far the best alternative from the economical, energy and environmental point of view, for bio-fuel production.

Feedstock Energy ratio

Подпись: Sugarcane 9.3 Lignocellulosic residues 8.3~8.4 Cassava 1.6~1.7 Beet 1.2~1.8 Wheat 0.9~1.1 Corn 0.6~2.0
Table 1. Comparison of different feedstock for biofuel production. Source:BNDES(2008)

The production of bioethanol from lignocelluloses

1.2 Pretreatment

Lignocellulose containing biomass has to be pretreated prior to hydrolysis to improve the accessibility of the biomass. For this pretreatment, several processes are available: mechanical treatment for size reduction (e. g. chopping, milling, grinding), hydrothermal treatment (e. g. uncatalysed steam treatment with or without steam explosion, acid catalysed steam treatment, liquid hot water treatment) and chemical treatment (e. g. dilute acid, concentrated acid, lime, NH3, H2O2). Diverse advantages and drawbacks are associated with each pretreatment method (Mosier et al., 2005; Hendriks & Zeeman, 2009; Chen & Qui, 2010; Talebnia et al., 2010).

Steam explosion is a widely-employed process for this pretreatment. This process combines chemical effects due to hydrolysis (autohydrolysis) in high temperature water and acetic acid formed from acetyl groups, and mechanical forces of the sudden pressure discharge
(explosion). The steam explosion process offers several attractive features when compared to other technologies. These include less hazardous process chemicals and significantly lower environmental impact (Alvira et al., 2010). Typical operation conditions for steam explosion treatment of straw — temperature and duration of treatment — are summarised in Table 1.



in °C

Duration of pretreatment in minutes



Wheat straw




Tomas-Pejo et al., 2009

Wheat straw




Ballesteros et al., 2004

Wheat straw




Jurado et al., 2009

Wheat straw




Sun et al., 2005

Wheat straw




Chen et al., 2007

Barley straw




Garcia-Aparicio et al., 2006

Barley straw




Linde et al., 2007

Corn stover




Yang et al., 2010

Corn stover




Varga et al., 2004

Rice straw




Ibrahim et al., 2011

Table 1. Typical operation data for steam explosion of straw

image068 Подпись: (1)

According to Overend and Chornet (1987), the severity of the pretreatment can be quantified by the severity factor R0. The severity factor combines the temperature of the pretreatment (T in degree Celsius) and the duration of the pretreatment (t in minutes) thus:

The severity factor is based on the observation that it is possible to trade duration of treatment and the temperature of treatment so that equivalent final effects are obtained. However, it is not intended to give mechanistic insight into the process.

SSF Fermentation of Rape Straw and the Effects of Inhibitory Stress on Yeast

Anders Thygesen1, Lasse Vahlgren1, Jens Heller Frederiksen1, William Linnane2 and Mette H. Thomsen3

1Biosystems Division, Riso National Laboratory for Sustainable Energy,

Technical University of Denmark 2Roskilde University 3Chemical Engineering Program, Madsar Institute

1,2Denmark United Arab Emirates

1. Introduction

In 2003 R. E. Smalley (Smalley 2003) made a list of the top 10 problems mankind was going to face in the next 50 years.

1. Energy

2. Water

3. Food

4. Environment

5. Poverty

6. Terrorism and War

7. Disease

8. Education

9. Democracy

10. Population

With the declining amount of fossil fuels, and the increasing energy demand, it is not possible to satisfy our large energy consumption without alternative energy sources, especially sustainable ones that also take care of problem number 4, (Environment). One of the possible ways to produce a liquid sustainable energy source is to replace our large gasoline demand with fermented biomass (bioethanol). To ensure that the food availability does not decrease (problem 3), the biomass for bioethanol fermentation is only gathered from waste materials of the food production, such as the straw of cereals. The fermentation on waste products is commonly referred to as 2nd generation bioethanol.

Biomass of interest for cellulosic produced ethanol includes wheat straw, rape straw and macro algae. In 2009 the production of oilseed rape in EU was 21*106 ton together with an even larger amount of rape straw (Eurostat 2010). The most commonly used rape in Europe is a winter rape with a low erucic acid and glucosinolate content (Wittkop et. al. 2009). The rape straw is composed of 32% cellulose and 22% hemicelluloses. In this chapter the focus will be on the fermentation of sugars derived from cellulose.

The enzymatic hydrolysis can be described in three steps: Endoglucanase separates chains of cellulose by breaking down the bonds in amorphous regions of the crystalline cellulose, thereby creating more free ends in the cellulose. Exoglucanase breaks the cellulose down from the non-reducing end into cellubiose (the disaccharide derived from cellobiose) (Teeri and Koivula 1995). This explains the importance of endoglucanase as it creates more "attack points" for exoglucanase. в-glycosidase breaks down cellubiose into glucose. Cellulase enzymes are commonly produced by the fungus Trichoderma reesei (Busto et al. 1996). This process has two functions, first it produces the glucose needed for the fermentation, and second it turns the non-soluble cellulose into soluble sugars, which provides the liquid medium required for fermentation.

The fiber structure consists of cellulose microfibrils, bound to each other with hemicellulose and lignin. A model of a plant fibre is shown in Fig. 1.


Fig. 1. Structure of cellulosic fibers in e. g. rape straw (adapted from Bjerre & Schmidt 1997)

The hemicellulose content consists of branched and acetylated carbohydrates. These molecules consist of 90% xylan and 10% arabinan in wheat straw (Puls and Schuseil 1993). The lignin content of the straw consists of polymerized molecules with a phenolic structure. The ethanol production process was conducted by simultaneous saccharification of cellulose with cellulase enzymes, and fermentation of the produced glucose with Turbo yeast (Saccharomyces cerevisiae, Brewer’s yeast) (SSF) (Thomsen et al. 2009).

The production efficiency of the fermentation is strongly dependant on the wellbeing of the yeast. To visualize the health of the yeast, microscopic tests using blue staining were conducted. The bioethanol is produced from wet oxidized rape straw and the effect of the important fermentation inhibitor furfuryl alcohol is tested.

Photocatalytic hydrogen production

In addition to biological process, photocatalytic oxidation of ethanol provides alternative interesting approach to generate hydrogen. Similar to photo-fermentation where enzyme is used to catalyze the conversion, solar energy is again utilized to offer sufficient power to produce hydrogen from ethanol under the facilitation of inorganic catalyst. Among many catalysts documented in the literature, TiO2 [8-10] is the most commonly used catalyst base due to its excellent photoreactivity which has a suitable band gap for efficient light photon absorption. Upon radiation, the electron contained in a semiconductor such as TiO2 will be excited and transferred from valence band to conduction band, resulting in the creation of an electron-hole pair and in turn providing an active site for redox reaction. As shown in Figure 1, reaction (1) is a typical redox reaction where H2O serves as the oxidant to oxidize ethanol while itself being reduced to H2. The adsorbed ethanol and water species will react with each other on the surface of the active sites of the synthesized photocatalyst to produce H2. Usually, certain amount of active metal (noble metal or transition metal) will be loaded to the TiO2 support to promote its photoactivity. According to the publications, Cu, Ni, V, Pt, Pd, Rh, Au, Ir, and Ru have been tested [11-14], among which Pt doped TiO2 exhibits the highest photoactivity toward hydrogen production from bioethanol. Various synthesis methods have been successfully demonstrated to get TiO2 supported catalyst with desirable particle size and morphology for hydrogen generation maximization. Besides TiO2 supported catalyst, there are multiple other novel semiconductors being developed recently for effective hydrogen production including CdS [15], VO2 [16], WO3 [17], and ZnSn(OH)6 [18]. Nevertheless, the hydrogen production efficiency from catalytic ethanol oxidation still remains at very low level probably due to two facts: the fast recombination rate of the created electron-hole pairs and the low photon absorption efficiency at visible light range. Although hydrogen evolution rate of 21 mmol/gcat/h has been reported and is the fastest rate claimed so far in the literature [19], it is still significantly lower than that obtained from thermochemical ethanol conversion. Therefore, the technical breakthrough is required in the field of photocatalysis before the commercialization of this technique can be seriously considered.


Fig. 1. The schematic diagram of photocatalytic ethanol reforming



Nine billion. This is the estimated number of people who will inhabit our planet in 2050. In a few decades, we will have nearly a quarter more than humans circling the globe in search of food, shelter, clothing and other manufactured products. Among the new individuals, the United Nations (UN) estimates that 98% will live in developing countries, with the highest level of economic growth, which in turn will result in a considerable expansion in per capita consumption worldwide.

On the one hand we have a significant increase in energy demand and consumption, resulting from population and income growth, and on the other, there is a considerable uncertainty about the world’s available supply of natural resources to support this development. Intergovernmental Panel on Climate Change (IPCC) recent reports have shown strong evidence of the impact of human activity on the climate of the planet. Estimates of the entity warn about a potential increase in global average temperature by up to 5 or 6oC by the end of this century. The raise in temperature itself would cause drastic changes in many ecosystems, but the reports also mention the intensification of extreme weather events such as hurricanes.

This apparently catastrophic scenario for the maintenance of the human species on Earth, opens up several possibilities for what is now called "green" or low carbon economy. We are talking about creating new businesses and industries geared to develop products and services with low consumption of natural resources and reduced emission of greenhouse gases. Within this category of business, biofuels is a highlight and the central theme of this book.

Biofuels are now the main alternative to automotive fossil fuels due to the fact that they are produced from renewable sources such as sugar cane, corn, cassava, oil seeds, agricultural waste, algae, etc.. Ethanol from sugar cane produced in large scale in Brazil, for example, illustrates the benefits of these products. Its production costs are low, which makes it competitive with oil derivatives. Each unit of fossil energy used in ethanol production is reversed in eight to nine units stored in the fuel. Finally, one of the most important qualities, each cubic meter of sugarcane bioethanol used as fuel reduces from 1.7 to 1.8 tonnes of CO2 (equivalent) emitted into the atmosphere. Due to the flex-fuel engine technology, more than 90% of light vehicles produced in Brazil are now able to run on 100% fuel ethanol.

The successful history of Brazilian ethanol is undoubtedly the first successful case of production and use of a biofuel in large scale, but is far from being the last. From the beginning of the century, two main factors have made the world turn its attention to research on biofuels. The first, already mentioned, is the increasing debate on climate issues. The second is the raise in the price of the oil barrel. In 1970, before the first shock in the price of fossil fuel, a barrel costs about $ 3. In 2008 the price was above $ 120. These facts stimulated scientists around the world to focus their research on themes that could result in the diversification of the energy matrix in many countries.

The globalization of the research on biofuels may bring a number of advancements to the industry and has already awakened a wish the market: to also convert cellulose into ethanol. Materials not used in the production of biofuels, such as sugarcane bagasse, corn stover and forest residues can be a significant source of additional ethanol, provided that appropriate industrial technologies are developed. In the case of sugarcane ethanol for example, data from the Brazilian Bioethanol Science and Technology Laboratory (CTBE) indicate that the conversion of bagasse and straw would increase the current production of bioethanol in Brazil in about 50%.

However, the challenges to make this technological potential an industrial reality are numerous and need investment in research and development (R&D). There are technological barriers with respect to the initial treatment of the raw material, production of microorganisms that break down cellulose into fermentable sugars, the fermentation of five-carbon sugars (pentoses), among others.

The new global market of bioenergy that has been structured in recent years has yet another relevant route for exploration: the biorefinery. Similar to the oil industry, it uses different types of processes to transform the same raw material in different products used by many industrial sectors, such as food, pharmaceutical, chemical, etc.. Companies and research institutes have studied and developed processes that convert biomass into raw materials for their production chain, potentially replacing substances that were produced from petroleum. Thus, in most cases, the environmental benefits and the reduction of dependence on fossil fuels are evident. Some studies even indicate that the use of biomass within the biorefinery concept may improve the profitability of cellulosic ethanol technology (second generation) and favor the integration of this new technology with the current first generation process.

The first section of this book presents some results for first generation ethanol production, i. e., from starch and sugar raw materials, which include cassava, sorghum, and sugarcane. In the second section, the chapters present results on some of the efforts being made around the world in order to develop an efficient technology for producing second-generation ethanol from different types of lignocellulosic materials. While efficient ethanol production technologies are being developed, one can also start thinking about different uses for it. In addition to the more straightforward use as fuel, it is worth to study other applications. The chapter in the third section points to the use of hydrogen in fuel cells, where this hydrogen could be produced from ethanol.


The editors would like to acknowledge Luiz Paulo Juttel for contributing with the information in the preface and the board of referees that made the technical revision of the chapters:

Antonio Bonomi Arnaldo Cesar da Silva Walter Carlos Eduardo Vaz Rossell George Jackson de Moraes Rocha Manoel Regis Lima Verde Leal Marcos Silveira Buckeridge Oscar Antonio Braunbeck

Marco Aurelio Pinheiro Lima and Alexandra Pardo Policastro Natalense

Brazilian Bioethanol Science and Technology Laboratory (CTBE),


Ethanol fuel from sorghum grain

4.1 Conventional dry grind

The five basic steps in the conventional dry-grind ethanol process are milling, liquefaction, saccharification, fermentation and ethanol distillation/ dehydration (Fig. 2). Mashing goes throughout the entire process beginning with mixing the grain meal with water (and possibly backset stillage) to obtain a mash ready for fermentation. Mashing is a wet-cooking process to turn the gelatinized starch into fermentable sugars first with the use of thermostable alpha- amylase and then with amyloglucosidase (Zhao et al., 2008; Solomon et al., 2007; Wu et al., 2007). Starch is the substrate for grain fuel ethanol. Unlike maize, the starch content of sorghum is not the best indicator of ethanol yield obtained by the dry-grind process because this carbohydrate greatly differs in availability or susceptibility to amylases.

The comparatively higher protein content of sorghum compared to maize should be advantageous because the protein is partially degraded into free amino nitrogen compounds during biocatalysis. These compounds are a source of nitrogen for yeast nutrition. However, the relatively lower protein digestibility and nature of the endosperm proteins associated to sorghum counteracts its higher protein concentration. As a result, sorghum mashes almost always contain less free amino nitrogen compared to maize mashes. The use of proteases during or after liquefaction is a good alternative to increase free amino nitrogen in sorghum mashes (Perez-Carrillo & Serna-Saldivar, 2007). Protein digestibility in wet-cooked sorghum is relatively lower compared to other cereals, mainly because of the cross-linking of prolamins. This phenomenon reduces the availability of nitrogenous compound in sorghum mashes needed to support yeast metabolism during fermentation.

Yeast cannot use proteins as source of nitrogen, instead it utilizes amino acids and short peptides (di or tri), indicating the importance of protein fragmentation altogether with starch hydrolysis in mashing. Beyond yeast nutrimental quandary, there are also issues related to starch digestibility that affects the performance of amylolytic enzymes during liquefaction and saccharification. This trend is also related to proteins because of the interaction between protein and starch that in sorghum reduces the susceptibility of this polysaccharide in both native and gelatinized conditions. Sorghum starch has higher gelatinization temperature compared to maize and more prolamin containing bodies within the endosperm, differences that can restrict gelatinization of starch granules (Zhao et al., 2008).

It has been reported that ethanol yields from sorghum decreases as protein content increases; however, at the same protein level, ethanol fermentation efficiency can vary as much as 8%. The difference is higher than typical experimental variations which indicate that additional factors to protein affects starch-conversion rate. In a work reported by Wang et al. (2008), nine sorghum genotypes were selected and used to study the effect of protein availability on efficiency of ethanol fermentation. The results showed a strong positive linear relationship between protein digestibility and fermentation efficiency, indicating the influence, and at the same time, the usefulness of this sorghum grain features as predictor of ethanol yield (Rooney et al., 2007; Wang et al., 2008; Wu et al., 2007; Wu et al., 2010a).

In Fig. 2 a typical process of dry-grind ethanol production is depicted. An average yield of 390 L of ethanol from 1 ton of sorghum can be obtained, but yields as high as 400 L/ ton with fermentation efficiencies of more than 90% has been achieved and reported (Chuck-Hernandez et al., 2009; Perez-Carrillo & Serna-Saldivar, 2007). The Dried Distillers Grains with Solubles (DDGS) obtained in these processes contribute to the economics of biorefineries. The wet distillers grains can be dried to 12% moisture with the aim to produce a shelf-stable byproduct.

Its nutritional composition (39 and 49% of protein and carbohydrates respectively) makes it an excellent option for livestock feed, especially for ruminants.

Products from lignocellulosic biomass

Lignocellulosic biomass is a potential source of several bio-based products according to the biorefinery approach. Currently, the products made from bioresources represent only a minor fraction of the chemical industry production. However, the interest in the bio-based products has increased because of the rapidly rising barrel costs and an increasing concern about the depletion of the fossil resources in the near future (Hatti-Kaul et al., 2007). The goal of the biorefinery approach is the generation of energy and chemicals from different biomass feedstocks, through the combination of different technologies (FitzPatrick et al., 2010).

The biorefinery scheme involves a multi-step biomass processing. The first step concerns the feedstock pretreatment through physical, biological, and chemical methods. The outputs from this step are platform (macro) molecules or streams that can be used for further processing (Cherubini & Ulgiati, 2010). Recently, a detailed report has been published by

DOE describing the value added chemicals that can be produced from biomass (Werpy, 2004). Figure 2 displays a general biorefinery scheme for the production of specialty polymers, fuel, or composite materials (FitzPatrick et al., 2010).

Besides ethanol, several other products can be obtained following the hydrolysis of the carbohydrates in the lignocellulosic materials. For instance, xylan/xylose contained in hemicelluloses can be thermally transformed into furans (2-furfuraldeyde, hydroxymethil furfural), short chain organic acids (formic, acetic, and propionic acids), and cheto compounds (hydroxy-1-propanone, hydroxy-1-butanone) (Gullu, 2010; Bozell & Petersen, 2010).


Fig. 2. Scheme of a lignocellulosic biorefinery. The shape of each step describes the type of process used, chemical, biological, and physical (legend) (FitzPatrick et al., 2010)

Furfural can be further processed to form some building blocks of innovative polymeric materials (i. e. 2, 5-furandicarboxylic acid). In addition, levulinic acid could be formed by the degradation of hydroxymethil furfural (Demirabas, 2008). Another product prepared either by fermentation or by catalytic hydrogenation of xylose is xylitol (Bozell & Petersen, 2010). Furthermore, through the chemical reduction of glucose it is possible to obtain several products, such as sorbitol (Bozell & Petersen, 2010). The residual lignin can be an intermediate product to be used for the synthesis of phenol, benzene, toluene, xylene, and other aromatics. Similarly to furfural, lignin could react to form some polymeric materials (i. e. polyurethanes) (Demirabas, 2008).

Towards Increasing the Productivity of Lignocellulosic Bioethanol: Rational Strategies Fueled by Modeling

Hyun-Seob Song, John A. Morgan and Doraiswami Ramkrishna

School of Chemical Engineering, Purdue University, West Lafayette, IN


1. Introduction

Bioethanol is not only currently the most widely used biofuel, but also potentially the most promising alternative to fossil fuels. The majority of bioethanol in today’s use is made from sucrose-containing (e. g., sugarcane, sugar beet, and sweet sorghum) or starch-based feedstocks (e. g., corn, wheat, rice, barley, and potatoes). The excessive production of such crop-based (first generation) bioethanol, however, imposes an adverse effect on global food supply. A sustainable alternative feedstock which can be used for non-crop (second generation) bioethanol is lignocellulosic biomass such as rice straw (Binod et al., 2010), wheat straw (Talebnia et al., 2010), corn stover (Kadam & McMillan, 2003), switchgrass (Keshwani & Cheng, 2009), sugarcane bagasse (Cardona et al., 2010), and various other agriculture and forest residues.

Lignocellulose primarily consists of cellulose, hemicellulose and lignin. Cellulose is a homopolymer of glucose, while hemicellulose is a heteropolymer of pentoses (i. e., xylose and arabinose) and hexoses (i. e., glucose, mannose, and galactose) sugars. Lignin is a rich source of aromatic carbon compounds but extremely recalcitrant. Lignocellulose is decomposed via pretreatment and hydrolysis into a spectrum of sugars in which glucose and xylose are the first and second most dominant. These cellulosic sugars are finally converted to bioethanol by fermentation. The lignocellulosic bioethanol has not yet been produced on a commercial scale due to lack of cost-effectiveness. For ensuring its economical viability, comprehensive efforts are required to reduce cost (and maximize the profit) throughout the entire process from biomass to bioethanol.

In the current discussion, we limit ourselves to the fermentation step only and examine various issues with increasing bioethanol productivity. Cost-benefit analysis of the fermentation process shows that the processing cost is more dominant (two-thirds of the total cost) than the feed cost (Lange, 2007; Wingren et al., 2003). It is thus important to improve the processing efficiency, not just the sugar conversion alone. In this regard, increasing the productivity should be a preferred target over increasing the yield, not only in the reactor optimization, but also in strain improvement.

The yeast Saccharomyces cerevisiae has typically been used for the production of crop-based bioethanol. This wild-type strain is, however, not suitable for converting cellulosic sugars as it can efficiently ferment glucose but hardly xylose. Considerable effort has been made to

endow S. cerevisiae with the ability to utilize xylose (Hahn-Hagerdal et al., 2007). Basic approaches to this end are to "push" and "pull" xylose into the central metabolism of S. cerevisiae. Push strategies introduce the transport and initial metabolic routes of xylose by expressing exogenous (i. e., foreign) genes. In pull strategies, reactions in the central metabolism are selectively overexpressed. Introduction of foreign plasmids imposes a "metabolic burden" or "metabolic load" on the host cell by consuming a significant amount of internal resources, hurting the normal metabolic functioning of the host cell (Glick, 1995). The most common observation is the decrease of cell growth rate (Bentley et al., 1990; Ricci & Hernandez, 2000). It is often (while not always) that as the product yield is increased, the production rate is reciprocally low (Chu & Lee, 2007).

Most of the recombinant yeast strains currently available show a sequential pattern in their consumption of mixed sugars (i. e., glucose and xylose). They preferably consume glucose with xylose on standby as denoted by the vertical line in Fig. 1.1(a). Then, simultaneous consumption take places along the tilted line only when the preferred substrate is depleted to a very low level (say, one tenth or one fifth of xylose level). Obviously, the productivity can be increased if simultaneous consumption occurs earlier (i. e. at higher concentrations of glucose). To achieve this, two different strategies can be considered. First, we may develop a more efficient fermenting organism through further pathway modifications of existing recombinant yeast. The goal of this attempt at the genetic level corresponds to making the slope of the tilted line steeper (Fig. 1.1(b)). Alternatively, we may design a more efficient fermentation process through optimization of operating conditions or reconfiguration of reactors. For example, if we change initial sugar composition in batch culture by increasing relative portion of xylose in the culture medium, this also leads to earlier start of the simultaneous consumption (Fig. 1.1(c)).


Fig. 1.1. (a) Sequential consumption of mixed sugars by existing recombinant yeast. Two possible ways to promote the simultaneous consumption: (b) metabolic pathway modification of fermenting organisms and (c) adjustment of sugar composition in the culture medium. Adapted from Song and Ramkrishna (2010) with minor modification.

In this chapter, we present model-based strategies for increasing the bioethanol productivity both at the genetic and reactor levels. Metabolic models help not only reduce trial and error, but also discover fresh strategies (Bailey, 1998). In view of the issues discussed above, there are two essential aspects of metabolic models required for the application to reactor and
metabolic engineering. First, the mathematical models should be able to address productivity as well as yield. Second, it should be possible to account for metabolic burden. While diverse modeling approaches have been suggested as a tool, the cybernetic framework (Ramkrishna, 1983) is unique in this regard (Maertens & Vanrolleghem, 2010). The cybernetic modeling approach describes cellular metabolism from the viewpoint that a microorganism is an optimal strategist making frugal use of limited internal resources to maximize its survival (Ramkrishna, 1983). Metabolic regulation of enzyme synthesis and their activities is made as the outcome of such optimal allocation of resources. This unique feature of accounting for metabolic regulation endows cybernetic models with the capability to accurately predict peculiar metabolic behaviors such as sequential or simultaneous consumption of multiple substrates. Further, in view of the constraint placed on resources, the cybernetic model provides a mechanism to account for metabolic burden imposed on the organism as a result of genetic changes.

After a brief sketch of the model structure (Section 2), we will see how metabolic models are used to establish rational strategies for increasing the productivity. In Section 3, basic guidelines for genetic modification of fermenting organisms are provided by identifying the potential target pathway and reactions. Diverse reactor-level strategies are also discussed in Section 4.

Competing Plant Cell Wall Digestion Recalcitrance by Using Fungal Substrate — Adapted Enzyme Cocktails

Vincent Phalip, Philippe Debeire and Jean-Marc Jeltsch

University of Strasbourg France

1. Introduction

1.1 General needs for energy

General needs for energy are still increasing. In 2000, the energy provided worldwide was 10 Gt of oil equivalent (Gtoe) and the demand is forecasted to be around 15 Gtoe for 2020 (source: Energy Information Administration [EIA], 2002, as cited in Scragg, 2005). During the 20th century, coal proportion in energy supply decreased whereas oil and gas increased drastically. First after the 1973 oil crisis and afterwards periodically depending on oil prices, developments for producing energy by new ways were considered. In the last decade, the depletion of fossil energy sources appeared as a reality although exhaustion time remains highly controversial. Currently, it is clear that considerable efforts to promote alternative sources of energy are driven by both environmental concern (limiting fuel by-products emissions) and economic necessity linked to the fossil fuel depletion.