Production Of First-generation Bioethanol Research Articles

Comparison of common carbon sources with unhydrolyzed, dilute acid and steam hydrolyzed distillers’ dried grains with solubles for lignocellulolytic enzyme productions by fungal strains

Distillers’ dried grains with solubles (DDGS) are the byproduct of first-generation bioethanol production. DDGS has a high fiber content, which can be used for hydrolytic enzyme production after pretreatment by physical or chemical methods such as dilute acid or semi-continuous steam explosion. This study compares the treated DDGS samples to common carbon sources such as glucose, Avicel (crystalline cellulose), and untreated DDGS along with other cellulosic materials during the production of cellulases and xylanases by selected four fungal strains. Acid treated DDGS without filtration produced up to 0.72 IU/ml of cellulases and 36.44 IU/ml of xylanases. The results showed that acid hydrolysis was a better treatment than untreated, and steam treated DDGS for both cellulase and xylanase productions (p < 0.05). The comparison of enzyme productions with acid hydrolyzed DDGS to common carbon sources also showed that acid hydrolyzed DDGS was a significantly better carbon source than others. Among four fungal strains, two Aspergillus niger strains (NRRL 567 and NRRL 330) were better than the others in enzyme activity levels. The results confirm the significant positive effect of dilute acid treatment in making DDGS a feasible feedstock for hydrolytic enzyme production.

Optimizing the balance between heterologous acetate- and CO2-reduction pathways in anaerobic cultures of Saccharomyces cerevisiae strains engineered for low-glycerol production.

In anaerobic Saccharomyces cerevisiae cultures, NADH (reduced form of nicotinamide adenine dinucleotide)-cofactor balancing by glycerol formation constrains ethanol yields. Introduction of an acetate-to-ethanol reduction pathway based on heterologous acetylating acetaldehyde dehydrogenase (A-ALD) can replace glycerol formation as 'redox-sink' and improve ethanol yields in acetate-containing media. Acetate concentrations in feedstock for first-generation bioethanol production are, however, insufficient to completely replace glycerol formation. An alternative glycerol-reduction strategy bypasses the oxidative reaction in glycolysis by introducing phosphoribulokinase (PRK) and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). For optimal performance in industrial settings, yeast strains should ideally first fully convert acetate and, subsequently, continue low-glycerol fermentation via the PRK-RuBisCO pathway. However, anaerobic batch cultures of a strain carrying both pathways showed inferior acetate reduction relative to a strain expressing only the A-ALD pathway. Complete A-ALD-mediated acetate reduction by a dual-pathway strain, grown anaerobically on 50g L-1 glucose and 5mmol L-1 acetate, was achieved upon reducing PRK abundance by a C-terminal extension of its amino acid sequence. Yields of glycerol and ethanol on glucose were 55% lower and 6% higher, respectively, than those of a nonengineered reference strain. The negative impact of the PRK-RuBisCO pathway on acetate reduction was attributed to sensitivity of the reversible A-ALD reaction to intracellular acetaldehyde concentrations.

Integrating 1G with 2G Bioethanol Production by Using Distillers’ Dried Grains with Solubles (DDGS) as the Feedstock for Lignocellulolytic Enzyme Production

First-generation (1G) bioethanol is one of the most used liquid biofuels in the transport industry. It is generated by using sugar- or starch-based feedstocks, while second-generation (2G) bioethanol is generated by using lignocellulosic feedstocks. Distillers’ dried grains with solubles (DDGS) is a byproduct of first-generation bioethanol production with a current annual production of 22.6 million tons in the USA. DDGS is rich in fiber and valuable nutrients contents, which can be used to produce lignocellulolytic enzymes such as cellulases and hemicellulases for 2G bioethanol production. However, DDGS needs a pretreatment method such as dilute acid, ammonia soaking, or steam hydrolysis to release monosaccharides and short-length oligosaccharides as fermentable sugars for use in microbial media. These fermentable sugars can then induce microbial growth and enzyme production compared to only glucose or xylose in the media. In addition, selection of one or more suitable microbial strains, which work best with the DDGS for enzyme production, is also needed. Media optimization and fermentation process optimization strategies can then be applied to find the optimum conditions for the production of cellulases and hemicellulases needed for 2G bioethanol production. Therefore, in this review, a summary of all such techniques is compiled with a special focus on recent findings obtained in previous pieces of research conducted by the authors and by others in the literature. Furthermore, a comparison of such techniques applied to other feedstocks and process improvement strategies is also provided. Overall, dilute acid pretreatment is proven to be better than other pretreatment methods, and fermentation optimization strategies can enhance enzyme production by considerable folds with a suitable feedstock such as DDGS. Future studies can be further enhanced by the technoeconomic viability of DDGS as the on-site enzyme feedstock for the manufacture of second-generation bioethanol (2G) in first-generation (1G) ethanol plants, thus bridging the two processes for the efficient production of bioethanol using corn or other starch-based lignocellulosic plants.

Microbial oil and biodiesel production in an integrated sugarcane biorefinery: Techno-economic and life cycle assessment

Biodiesel and bioethanol play an important role as renewable liquid fuels. Bagasse, a by-product from the bioethanol industry, can generate a “sugarcane bagasse hemicellulose hydrolysate” (SCBH) that contains fermentable sugars, mainly xylose. Oleaginous yeasts (eg., Rhodotorula toruloides) can grow in SCBH, producing microbial oil (MO), a source of triacylglycerol for biodiesel production. The integration of bioethanol and biodiesel (from MO) production may be a promising approach in order to exploit synergies between bioethanol and biodiesel processes within a biorefinery. This integration may improve the economic and environmental performance of both processes. This work presents the techno-economic-environmental analysis of the integrated production of first-generation bioethanol, bioelectricity, and biodiesel in a Brazilian sugarcane biorefinery, where MO from the yeast R. toruloides feeds the biodiesel unit. The biorefinery, processing 4.0 million t of sugarcane per harvest, produces 71.7 m3/h of bioethanol, 2.55 m3/h of biodiesel (that can replace 75.6% of the diesel demand in the field), and 86.3 MW of surplus bioelectricity. A life cycle assessment shows that the integrated biorefinery had a lower environmental impact than the first-generation bioethanol plant. The integrated process exhibits a positive economic performance (net present value of approx. 110 million of dollars and internal rate of return of about 14.5% per year, higher than the minimum acceptable rate of return, assumed as 11% per year), indicating that this is a feasible industrial option. Sensitivity analysis shows that R&D should mainly focus on the MO bioreactor operation.

Environmental assessment of the valorization of glycerol for the production of hyperthermophilic β-glucosidase under a biorefinery approach

Bioethanol production technologies from lignocellulosic biomass are not yet optimized and do not compete economically with first-generation bioethanol production. Strategies have been investigated to produce more active, stable and temperature-tolerant enzymes to be used for biomass hydrolysis such as the hyperthermophilic β-glucosidase produced by Yarrowia lipolytica. The use of this strain offers an additional competitive advantage, as it can use glycerol stream from the biodiesel process as a carbon source. In this way, not only is a by-product of biofuel production used, but the enzyme could be applied in the production of lignocellulosic ethanol, increasing the value chain by closing the bioeconomy cycle. To this end, large-scale process modelling of β-glucosidase production has been developed to collect the inventory data needed for life cycle assessment methodology. The fermentation stage is the largest contributor to environmental impacts, with electricity being the main hotspot identified, contributing more than 50% in most impact categories. Residual glycerol has also been identified as a critical input, with a significant contribution in some categories. To improve the environmental profile, a sensitivity analysis has been carried out considering reductions in electricity and heat consumption, and other alternative oil-based resources for the production of biodiesel. This analysis identified that large environmental reductions could be achieved, which makes the valorization of the glycerol obtained as a side stream of biodiesel production more realistic.

Effects of harvest time on bioethanol production and bagasse characteristics for combustion and forage in sweet sorghum

Determining the optimal harvest time is very crucial to achieve sustainable production of first-generation bioethanol as well as efficient use of its bagasse for combustion or feed in sweet sorghum. The aim of the current research was to understand the effects of different harvest times, which comprised flowering (FLW), soft dough (SD), hard dough (HD), and physiological maturity (PM) on the bioethanol production and the combustion and forage characteristics of its bagasse in sweet sorghum. In one study, averaged over a period of 2 years, the mean theoretical total ethanol yield (TEY) was between 4802 and 5316 L ha–1. The PM harvest produced the highest TEY, which was followed by the HD and SD harvests, with sligh differences. These results suggested that sweet sorghum should be harvested at any phenological stage after FLW to achieve sustainable total (juice + lignocellulosic) bioethanol production in a semi-arid Mediterranean environment. Averaged over a period of 2 years, the mean dry bagasse yield (DBY) achieved from the harvest times herein ranged between 12.0 and 14.1 t ha–1. As with TEY, the PM harvest exhibited the highest DBY and desirable bagasse combustion caharacteristics due to exhibiting the lowest moisture and ash contents, which indicated that it seemed be the most suitable harvest time if obtaining solid biofuel production from the bagasse is the main target after the production of juice ethanol. On the other hand, the HD harvest provided the highest bagasse forage quality among the harvest times due to exhibiting the significantly lowest neutral detergent fiber and acid detergent fiber contents, but the significantly highest relative feed value. These results suggested that the HD harvest time was the most suitable for sweet sorghum cultivation when producing juice ethanol that is complemented by the production of forage utilizing the bagasse.

Benchmarking of Separation Methods for Bioethanol (&lt;5 wt %) Recovery

Distillation and dehydration account for around 20 to 40% of the total energy consumption for first-generation bioethanol production. Due to the more dilute alcohol concentrations encountered in the second-generation processes, the conventional separation methods are not economically feasible. The main obstacles are the high-energy input demand and low thermodynamic efficiency. In this work, six alternative advanced layouts for alcohol recovery are simulated and benchmarked based on energy consumptions and thermodynamic efficiencies. Among these is the internally heat integrated distillation column, which, despite the research efforts of the last 40 years, is not yet established in the industry. Feed composition has been varied in the range of 0.75–5.0 wt % ethanol. The analysis results show that intensified distillation schemes can improve second law efficiency of about 65 to 80% and reduce energy consumptions by 67% compared to conventional distillation. The most optimal separation technologies investigated requires about 9.8 to 10.4 MJ/kg-EtOH for 0.75 wt % ethanol feed and 1.9–2.0 MJ/kg-EtOH for 5.0 wt % ethanol feed. These are the externally heat-integrated column, the internally heat-integrated column, and a four-effect distillation sequence. The results obtained in this study can allow for more optimal biofuel production, giving rise to potential improved business for sustainable green technology in the future.

Optimal fed-batch bioreactor operating strategies for the microbial production of lignocellulosic bioethanol and exploration of their economic implications: A step forward towards sustainability and commercialization

The enduring adoption of greener technologies from the biofuel manufacturing industries requires the balanced integration of multiple inter-disciplinary concerns (i.e., engineering, business, sustainability, societal) to make production processes more attractive to the industrial sector. From an engineering perspective, the commercial establishment of lignocellulosic bioethanol plants is still hindered by increased production cost. High product concentrations with improved yield and productivity at the fermentation stage seem to be inviolable condition to straightly compete with fossil-based and first-generation bioethanol facilities. This study proposes a model-based multi-objective dynamic optimization approach to systematically derive optimal operating policies for the fermentation bioreactor as a step forward towards enhancing the economic performance of lignocellulosic bioethanol plants. The economic benefit of the derived optimal bioreactor strategies is evaluated via a scalable economic index, linking for the first time the performance of the fermentation bioreactor to the economics of the entire production process. It is demonstrated that high values in productivity and substantial compromises in yield and final bioethanol concentration are conditions which favor economic feasibility with the achieved fermentation performance (3.32 g L−1 h−1, 0.399 g g−1, 106.0 g L−1) being comparable to first-generation bioethanol production. Finally, a global system analysis framework is employed to investigate the robustness of the optimal value of the economic index to the variability of key model parameters, illustrating the impact of model uncertainty on the process economic performance.

Understanding the resource-use and environmental impacts of bioethanol production in China based on a MRIO-based hybrid LCA model

To understand the resource and environmental costs of bioethanol production, this study estimates the water and farmland use and CO2 emissions in China due to first- and second-generation bioethanol production technologies using a multi-regional input-output-based hybrid life cycle assessment model. Sensitivity analysis and structural path analysis are combined to investigate the key pathways for bioethanol production impact mitigation. Results show that the first-generation technology has higher resource-use and environmental impacts compared to the second-generation technology. The gasoline-to-bioethanol transition enables CO2 emission reductions, but at the cost of increased water and farmland use. Key impact mitigation pathways are then investigated for the first four production layers (PL0→3) in bioethanol industries because these layers contribute to a significant environmental footprint. For PL0, water-saving technologies must be developed to reduce direct water use in first-generation bioethanol production. For PL1, the primary production material suppliers of the bioethanol industry, such as biomass feedstock, food and tobacco, and water supply industries were ascertained with high potential for impact mitigation. For PL2→3, key pathways are investigated by combining their sensitivity and impact values. The results highlight the significance and necessity of cross-sectoral collaboration for resource and environmental impact reduction.

Values and Drawbacks of Biofuel Production From Microalgae

Increased energy consumption leads to a reduction in limited nonrenewable resources called fossil fuels. Due to this fact, researchers look for alternative energy sources to satisfy the need of the current generation without compromising the ability of future generations to meet their needs. As a result, nowadays the production of energy from biological sources is highly applicable and takes advantage of fossil fuel due to the limited impact on the environment. On the other hand, food cost, land use for some other purposes, and carbon emissions have risen due to the increased production of first-generation bioethanol. Even though the second-generation bioethanol from lignocellulose agricultural waste solved this problem, it again faced difficulties-to-overcome technological barriers. This, in turn, pushed researchers to come up with another alternative called the third-generation biofuel production. The renewed promise is held in microalgae biomass as an alternative feedstock. This review deals with the important aspects of biofuel production from algae as a renewable resource. The production processes and their merits and demerits of algae capacity in producing biofuel are also discussed.

Sustainable biomass supply chain network design with biomass switching incentives for first-generation bioethanol producers

Sustainable energy requires renewable energy sources to reduce reliance on fossil fuels. As a renewable energy source, first-generation bioethanol has been produced from corn. However, the production of such a biofuel increases corn-based food prices resulting in serious food versus fuel debates. Financial incentives would motivate first-generation bioethanol producers switching to second-generation bioethanol production. This study investigates the effects of two financial incentives (incentive payments and emissions penalties) motivating first-generation bioethanol producers to use second-generation biomass. These financial incentives are integrated into linear programming models to maximize the profit of the bioethanol supply chains in the state of North Dakota. Numerical results indicate that first-generation bioethanol production is more efficient than the second-generation bioethanol. Hence, the social value of using corn as a source for food instead of fuel must be at least $2.38/bushel. Furthermore, to switch from the first to the second-generation biofuel production, bioethanol producers must either receive at least an incentive payment of $0.8495 per gallon of second-generation bioethanol or pay at least a penalty of $3.2573 per Kg CO2e emitted due to first-generation bioethanol production. The results of this study support policymaker decisions in developing incentive programs to promote sustainable second-generation bioethanol in the US.

ЭВОЛЮЦИЯ МЕХАНИЗМОВ ГОСУДАРСТВЕННОЙ ПОДДЕРЖКИ ОТРАСЛИ ПРОИЗВОДСТВА ТРАНСПОРТНОГО БИОТОПЛИВА В США И ПЕРСПЕКТИВЫ ДЛЯ РОССИИ

Over the past 30 years the United States has made significant progress in the development of the biofuel industry. At present US holds a leading position in the global production, consumption and export of first-generation bioethanol, and steadily increasing the production and consumption of first and second generation biodiesel. These successes are based on a state policy aimed at creating domestic demand for transport biofuels and stimulating its production by enterprises located in the United States. The main prerequisites for the implementation of this policy are based on the positive economic and environmental effects that accompany the production and consumption of bioethanol and biodiesel. These include, first of all, the intensification of economic activity in agriculture, the reduction of greenhouse gas emissions and emissions of pollutants by road, and the diversification of energy consumption patterns. In the Russian Federation, the production and consumption of transport biofuel is currently not carried out on a significant scale. However, the production of first-generation bioethanol can have a positive impact on individual agricultural markets that require the formation of new sources of demand.

Life Cycle Assessment of Bioethanol Production from Sweet Potato (Ipomoea batatas L.) in an Experimental Plant

First-generation bioethanol production from fermentation is a widely applied technology, with the United States of America and Brazil being the global leaders. However, several concerns regarding the widespread production of biofuels have arisen, particularly its environmental impacts. To address this issue, this study details the environmental impacts of the production of fist-generation bioethanol from sweet potato (Ipomoea batatas L.) in a pilot scale facility, using the Life Cycle Assessment (LCA) methodology. Cultivation and transformation of sweet potato into bioethanol are considered in the system boundaries. The background data for the life cycle inventory were mainly obtained from the Ecoinvent 3.1 database and considered both literature and field data. The SimaPro software was used to perform the impact assessment considering the CML IA baseline 3.02 method. Three different agricultural waste scenarios are also considered and compared to evaluate the best-case scenarios. In addition, sensitivity analysis and comparison with other similar studies are thoroughly explored. Overall, the analyzed production system has lower impacts than indicated in similar studies, but considering its pilot scale, improvement of its environmental impacts is necessary, which might be addressed with larger scale and centralized support systems regarding transportation and other aspects. Based on the results, several recommendations to decrease environmental impacts are also listed.

Exergetic Analysis between a First-Generation Bioethanol Production Plant and a Second-Generation Plant Coupled to Conventional Process

Large consumption of non-renewable fuels in the world has strongly encouraged the search for biofuels, due to inevitable shortage of fossil energy sources and catastrophic problems generated by their use. Under these circumstances the so called second generation biofuel production has been highlighted, where lignocellulosic materials such as sugarcane bagasse are used to produce bioethanol, unlike the traditional first-generation process, whose feedstock is sugarcane. Therefore, this work aimed to perform a comparative exergetic analysis between first and second-generation bioethanol production processes, in order to evaluate from the exergetic point of view, which of these processes is the most feasible. It was found an overall exergetic efficiency of 60.89 % for the exclusively first-generation plant, whereas for the integrated plant, it was obtained 41.58 %. In this way, insertion of new stages in production process, required by the second generation, has caused consequently an increase of irreversibilities and losses.

Water-Soluble Carbohydrate Extraction from Sweet Pearl Millet and Sweet Sorghum Biomass as Affected by Bagasse Impregnation

<abstract> <b><sc>Abstract.</sc></b> The extraction of the sugary juice from sweet sorghum and sweet pearl millet biomass is a major step for first-generation bioethanol production. However, an important amount of water-soluble carbohydrates (WSC) remains in the bagasse following a single pressing of the biomass in-field. Therefore, bagasse impregnation is required to collect residual carbohydrates. This study investigated the effects of bagasse impregnation liquid (water or juice from the first pressing of the biomass) and three impregnation durations (0, 30, or 60 min) for a second pressing on the WSC concentrations of the biomass, the bagasse, and the juice of sweet sorghum and sweet pearl millet. Our results showed that the impregnation duration had no effect on WSC concentration in the biomass, bagasse, and juice. However, water was more efficient than the first-pressing juice in extracting the residual WSC remaining in the biomass after the first pressing. The use of water for bagasse impregnation allowed extraction of a total of about 36% and 47.5% of WSC from sweet sorghum and sweet pearl millet biomass, respectively, compared to 28% and 41% when the first-pressing juice was used. Sweet sorghum and sweet pearl millet juice extracted from the biomass impregnated with water contained 21% and 28.5% more WSC, respectively, than the juice extracted from the biomass impregnated with the first-pressing juice. When pressing is carried out in-field, impregnation of the bagasse with water for a second pressing is highly recommended for more residual WSC extraction. However, the required quantity of water needs to be optimized.

Bioprospecting for native microalgae as an alternative source of sugars for the production of bioethanol

While the production of biofuels holds potential to contribute to energy security, concerns on food prices, land use, and carbon emissions have arisen from increased production of first-generation bioethanol. While second-generation bioethanol from lignocellulosic agricultural waste faces difficult-to-overcome technological barriers, renewed promise is held in microalgae biomass as an alternative feedstock. In this work we show the results of bioprospecting for microalgae native of South America for accumulation of carbohydrates under conditions of nitrogen deficiency, and constant light and temperature. After a preliminary analysis of seventeen strains, we selected strain SP2-3, because its biomass could be enriched in carbohydrates over 70% (w/w) on a dry biomass basis, and Desmodesmus sp. strain FG for its fermentable sugars productivity. After optimization of microalgae culture conditions, biomass hydrolysis and fermentation with baker's yeast Saccharomyces cerevisiae, we demonstrated ethanol yields of up to 0.24gethanol·gofbiomass−1, and an ethanol concentration in the fermentation broth of 24gethanol·Loffermentationbroth−1, for up to 87.4% of the maximum theoretical value. These results contribute to support the potential of microalgae biomass as an alternative feedstock for bioethanol and the value of bioprospecting programs to identified candidate strains among natural biodiversity.

Looking beyond Saccharomyces: the potential of non-conventional yeast species for desirable traits in bioethanol fermentation.

Saccharomyces cerevisiae has been used for millennia in the production of food and beverages and is by far the most studied yeast species. Currently, it is also the most used microorganism in the production of first-generation bioethanol from sugar or starch crops. Second-generation bioethanol, on the other hand, is produced from lignocellulosic feedstocks that are pretreated and hydrolyzed to obtain monomeric sugars, mainly D-glucose, D-xylose and L-arabinose. Recently, S. cerevisiae recombinant strains capable of fermenting pentose sugars have been generated. However, the pretreatment of the biomass results in hydrolysates with high osmolarity and high concentrations of inhibitors. These compounds negatively influence the fermentation process. Therefore, robust strains with high stress tolerance are required. Up to now, more than 2000 yeast species have been described and some of these could provide a solution to these limitations because of their high tolerance to the most predominant stress conditions present in a second-generation bioethanol reactor. In this review, we will summarize what is known about the non-conventional yeast species showing unusual tolerance to these stresses, namely Zygosaccharomyces rouxii (osmotolerance), Kluyveromyces marxianus and Ogataea (Hansenula) polymorpha (thermotolerance), Dekkera bruxellensis (ethanol tolerance), Pichia kudriavzevii (furan derivatives tolerance) and Z. bailii (acetic acid tolerance).

A comparison of sweet sorghum and maize as first-generation bioethanol feedstocks in Greece

SUMMARYAccording to the EU 28/2009 directive, member states are mandated to substitute 10% of fossil fuels used in transportation with biofuels by the year 2020. Bioethanol production is expected to contribute significantly towards fulfilling Greece's obligations. First-generation bioethanol, produced from amylaceous and sugar crops, is the most important biofuel globally. Maize (Zea mays L.) is the main feedstock for production worldwide, while sweet sorghum (Sorghum bicolor L. Moench), although a promising raw material source, has not yet enjoyed substantial commercial exploitation due to the high seasonality of the crop. Sustainability criteria set by the EU constitute a key factor in the characterization and future use of biofuels. A 3-year study including 20 maize and 4 sweet sorghum varieties was conducted in order to compare these two crops in terms of emitted greenhouse gases (GHG) during the cultivation phase as well as regarding emission savings by substituting bioethanol for petrol/gasoline. Both crops demonstrated promising bioethanol yields reaching 5235·7 and 6443·7 l/ha/yr for maize and sweet sorghum, respectively, and showed that they could be employed towards first-generation bioethanol production in Greece. Sweet sorghum varieties produced higher bioethanol yields per hectare coupled with lower emissions during the cultivation phase and better overall GHG savings compared to maize.

ベトナムにおけるバイオエタノール導入政策の経済・環境影響

Bioethanol can reduce CO2 emissions and improve domestic production by replacing gasoline consumption. However, bioethanol competes with food consumption, if it is produced from the edible parts of agricultural products. This study aims to evaluate the pros and cons of bioethanol in the Vietnamese economy by the computable general equilibrium （CGE） model. The simulation results demonstrate the following points. First, a policy that replaces the fixed rate of bioethanol in gasoline brings about reductions in long-run electricity production in addition to direct reduction of petroleum consumption. Moreover, the import of petroleum products, highly dependent on imports at present, can be reduced, and a decrease in imports transfers to domestic production and consequently the self-sufficient rate of energy is raised. Hence, this policy can cut CO2 emissions by 0.7-3.0% more than without the policy.Second, the introduction of bioethanol, produced by first generation technology or by second-generation technology with technological progress, increases total income and raises GDP by 0.19% at most. The effects of second-generation bio-ethanol production are higher than for first generation technology because of the high added value of second generation technology. However, second-generation production with the present technology may decrease gross domestic products, because production costs are too high to produce a profit. A government subsidy for bio-ethanol production may cause domestic savings to decline and decrease private investment through crowding-out effects. Although first-generation bio-ethanol production increases competition with food consumption, second-generation bio-ethanol production can avoid competition with food consumption. Hence, policy makers should consider different pathways to affect the national economy and should support technological improvements.Third, the environmental effects of the bio-ethanol introduction policy are lower than the direct effects of CO2 emissions from gasoline consumption reduced by this policy, because this policy stimulates domestic production. In addition to the economic effects, situations in which the economic growth accelerates CO2 emissions can be avoided, as long as a policy that replaces the fixed rate of gasoline consumption is continued., The CGE model is a useful policy tool to show the above complicated effects.JEL Classification: C68, D58, Q21, Q28, Q48

Structure of starch synthase I from barley: insight into regulatory mechanisms of starch synthase activity

Starch, a polymer of glucose, is the major source of calories in the human diet. It has numerous industrial uses, including as a raw material for the production of first-generation bioethanol. Several classes of enzymes take part in starch biosynthesis, of which starch synthases (SSs) carry out chain elongation of both amylose and amylopectin. Plants have five classes of SS, each with different roles. The products of the reaction of SS are well known, but details of the reaction mechanism remain obscure and even less is known of how different SSs select different substrates for elongation, how they compete with each other and how their activities are regulated. Here, the first crystal structure of a soluble starch synthase is presented: that of starch synthase I (SSI) from barley refined to 2.7 Å resolution. The structure captures an open conformation of the enzyme with a surface-bound maltooligosaccharide and a disulfide bridge that precludes formation of the active site. The maltooligosaccharide-binding site is involved in substrate recognition, while the disulfide bridge is reflective of redox regulation of SSI. Activity measurements on several SSI mutants supporting these roles are also presented.