156 Journal homepage: www.fia.usv.ro/fiajournal Journal of Faculty of Food Engineering, Ştefan cel Mare University of Suceava, Romania Volume XIX, Issue 2- 2020, pag. 156 - 169 FEEDSTOCKS USED FOR PRODUCTION OF 2nd AND 3rd GENERATION BIOETHANOL - REVIEW *Vasile-Florin URSACHI 1, Gheorghe GUTT1 1Faculty of Food Engineering, ”Ștefan cel Mare” University, Suceava, Romania, florin.ursachi@fia.usv.ro *Corresponding author Received 23th Aprilie 2020, accepted 27th June 2020 Abstract: Global biofuel production has increased significantly over the last decade, but first- generation biofuels have been identified as a major concern, especially their sustainability as they are produced from food crops (such as cereals, sugar cane and vegetable oils). Depending on the feedstocks and cultivation technique, the production of second and third generation biofuels has the potential to provide benefits, such as the recovery of residues and unusable land. Therefore, second and third generation biofuels are indicated to meet the increasing demand for energy and contribute considerably to the development of rural areas and the increasing of bioeconomy. This short review shows that there may be different types of feedstocks (agricultural residues, forest residues, energy crops and algae) which can be used for the production of 2nd and 3rd generation bioethanol without affecting food security. Keywords: celullose, hemicelullose, starch, lignin, bioethanol 1. Introduction The feedstocks used to obtain biofuels are plants and cereals that are intended for human consumption. Lignocellulosic biomass (LCB) is easily accessible worldwide and is found in the form of residues and agricultural biomass such as corn straw, wheat straw and rice straw. The production of biofuels aims to protect the environment, to meet new energy requirements, to reduce the import and production of conventional fuels, thus stimulating the development of agriculture [1]. Second-generation biofuels are largely derived from LCB, which includes most plant-based, non-food materials that are inexpensive and found in huge quantities. Currently, the production of second- generation biofuels is not cost-effective because it requires overcoming technical barriers to obtaining them. In terms of bioethanol production, LCB is one of the most abundant and least used resources. LCB is usually burned for the production of heat and electricity, although it could be used to produce liquid biofuels. However, the production of biofuels from agricultural by-products could only meet a certain proportion of the growing demand for liquid biofuels, crops dedicated to the production of LCB are an important solution for the production of biofuels [2]. Compared to first generation biofuels which are mostly obtained from corn or sugar cane, biofuels obtaining from LCB is more expensive because lignocellulosic materials have a complex structure and require a specific technological process [3, 4]. Microalgae production is the key to the development of the third generation of bioethanol, as they could provide an alternative in terms of biomass production [5, 6]. Algae are also the fastest growing plants on Earth. Depending on how they are recovered, algae can be used to produce biofuels such as biodiesel, http://www.fia.usv.ro/fiajournal mailto:mariap@fia.usv.ro Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIX, Issue 2 – 2020 Vasile-Florin URSACHI, Gheorghe GUTT, Feedstocks used for production of 2nd and 3rd generation bioethanol - Review, Food and Environment Safety, Volume XIX, Issue 2 – 2020, pag. 156 – 169 157 bioethanol, but also other valuable substances [7]. 2. Feedstocks used in the production of second generation bioethanol (2ndG) In the food industry, significant quantities of non-food lignocellulosic biomass can be used to produce 2ndG bioethanol, thus making it a promising alternative for fossil fuels. Lignocellulosic biomass is one of the most abundant renewable resources on Earth and has a relatively low price [8]. Lignocellulosic feedstocks are renewable and cheap sources. These can come from the forestry, agricultural fields (grains, wheat straw, rice straw and sugar cane), agro-industrial as well as significant quantities of food residues [9]. LCMs are made of cellulose, hemicellulose and lignin which form a complex structure and are resistant to physico-chemical and biological treatments. One of the best strategies is to convert sugars from LCB by enzymatic hydrolysis because it does not require high energy consumption and is a clean process. However, it should be noted that the enzymatic step in the technological process of obtaining bioethanol also has a disadvantage, namely that related to the rigid structure between cellulose and lignin. Therefore, in order to facilitate enzymatic hydrolysis and implicitly to obtain high concentrations of cellulose, a pretreatment step is required, being considered a key step in obtaining an increased bioethanol yield from LCMs [10]. In literature several methods of pretreatment are described and are known as [10]: - physical pretreatment (milling and milling, microwave oven and extrusion); - chemical pretreatment (alkaline, acid, organosolvent, ozonolysis and ionic liquid); - physico-chemical pretreatment (steam explosion, hot water, AFEX ammonia fiber explosion, wet oxidation and CO2 explosion); - biological pretreatment. Cellulose Cellulose (C6H10O5)n is a carbohydrate found in agricultural and woody biomass [10]. It is a linear polymer composed of glucose (D-glucose) molecules that have β- (1,4) -glycoside bonds [11, 12]. Cellulose is insoluble in water therefore, a hydrolysis process must be applied to convert this polysaccharide into glucose molecules [13]. General hydrolysis of cellulose produces only glucose, which can be transformed into different forms of biochemical and chemical substances. Various biochemical and chemical substances such as bioethanol, organic acids, glycerol, sorbitol, mannitol, fructose, enzymes and biopolymers can be obtained through biological processes [14, 15]. Figure 1 shows the enzymatic hydrolysis of cellulose and the enzymes involved in this process. Hemicellulose Hemicellulose (C5H8O4)n is a short polymer that has a branched structure, comprising sugars such as pentoses (D- xylose and L-arabinose) and hexoses (D- glucose, D-mannose and D-galactose) [16]. Hemicelluloses are found in plants in the form of xyloglucans or xylans Hemicelluloses are present in woody biomass, softwood and hardwood [17]. Due to its branched structure, hemicellulose is easier to hydrolyzate as opposed to cellulose. Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIX, Issue 2 – 2020 Vasile-Florin URSACHI, Gheorghe GUTT, Feedstocks used for production of 2nd and 3rd generation bioethanol - Review, Food and Environment Safety, Volume XIX, Issue 2 – 2020, pag. 156 – 169 158 Fig 1. Enzymatic hydrolysis of cellulose [18] Xylose can be used to obtain xylitol. Xylitol is a non-carcinogenic sweetener, with the same sweetening power of sucrose. Xylose can be transformed with the help of microorganisms into proteins, fuels and solvents. There are certain yeast strains that can ferment xylose and transform it into bioethanol (Pichia stipitis, Candida sheratae) [15]. Lignin Lignin [C9H10O3 (OCH3)0,9–1.7]n is an organic compound and has a branched structure consisting of 3 different monomers (coniferyl alcohol, synapyl alcohol and p- coumaryl alcohol) [19]. Lignin is a barrier in the fermentation process of LCB and is resistant to chemical and biological degradation. Also, its presence affects the yield of bioethanol [20]. By utilizing lignin, carbon fibers, emulsifiers, dispersants, sequestrants, surfactants, binders and other chemicals can be obtained [21]. The chemical composition (%) and the main constituents of LCMs are shown in figure 2. 2.1. Agricultural residues, municipal solid residues and different types of grass Agricultural residues (corn cobs, corn stover, sugarcane bagasse, rice straw, and wheat straw) are important sources for 2ndG bioethanol production. The grain harvesting period is relatively short and so these residues are available throughout the year. Each year, between 350 and 450 million tonnes of crops are harvested, resulting in huge quantities of agricultural residues [18]. For example, up to 1 - 3 tons of straw can grow from 1 acre area grown with wheat. From the cost point of view, the price of sugar cane and maize rises to 60.9 USD / tonne respectively, 185.9 USD / tonne, while those for the sugarcane bagasse and corn stover the price is much lower, of only 36.4 USD / tonne and 58,5 USD / tonne respectively [22]. One should know that almost 70% of the cost of bioethanol production is represented by the cost of obtaining the feedstocks [23]. Therefore, for half the costs it would be preferable to use agricultural residues and not to use energy crops. By capitalizing these residues, forestry and arable land held by herbaceous plants (switchgrass, miscanthus) would be reduced. Municipal solid residues and residues from food industry have been studied for ethanol production [23, 25], because it has an important carbohydrate content, and the protein and mineral content can support the Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIX, Issue 2 – 2020 Vasile-Florin URSACHI, Gheorghe GUTT, Feedstocks used for production of 2nd and 3rd generation bioethanol - Review, Food and Environment Safety, Volume XIX, Issue 2 – 2020, pag. 156 – 169 159 fermentation process. The study by Matsakas et al. (2014) showed that food residues can be transformed successively into bioethanol after a double fermentation. After the enzymatic hydrolysis and also the fermentation phase completed, an ethanol content of 43 g / L was obtained. Then, a microwave-assisted hydrothermal pretreatment was applied to the remaining solid residue and again subjected to fermentation.After the second fermentation was completed, an alcohol content of 59 g / L of ethanol was obtained [26]. Fig.2. Chemical composition (%) and main constituents of LCM [24] Switchgrass is a feedstock that has high glucose content, is highly resistant to disease and has high biomass productivity. Miscanthus giganteus is another type of grass that can be used for biofuel production, especially as it has a fast growth rate. It is native to Asia, but it is also cultivated in Europe. This grass represents 50 - 70% of the total biomass feedstocks (including forest wood biomass and agricultural residues) that are used for the production of cellulosic biofuels [18]. It was estimated that approximately 133 × 109 L of ethanol could be produced if 9.3% of US agricultural land were cultivated with this plant, thus 1/5 of the country's gasoline consumption could be replaced [27]. Scagline-Mellor et al. (2018) argue that bioethanol yield is higher for miscantus compared to Switchgrass [28]. In the Mediterranean area, lignocellulosic materials are found that can be used for the production of 2ndG bioethanol. These raw materials include: cereal crops, olives, tomatoes, grapes and residues resulting from the processing of grapes, solid residues of olives, "date" palm trunks, perennial lignocellulosic herbs (Arundodonax, Saccharum spont. Aegyptiacum and Miscanthusus giganteus) or the cactus species Luffa cylindrica and Luffa prickly pear. Stipa tenacissima or Esparto grass belongs to the family poaceae; it is a perennial plant that has a fast growth rate. The leaves of this plant have high fiber content [29] and can reach up to 1 m in height. The Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIX, Issue 2 – 2020 Vasile-Florin URSACHI, Gheorghe GUTT, Feedstocks used for production of 2nd and 3rd generation bioethanol - Review, Food and Environment Safety, Volume XIX, Issue 2 – 2020, pag. 156 – 169 160 Stipa tenacissima bushes have a circular and homogeneous shape when they are young, but as they age they dry out and in the center empty spots form. The leaves are thin, ribbon-shaped, smooth, shiny, and solid and at the base they are covered by a hairy sheath. Esparto leaves reach maturity between the fourth and eleventh months after flowering, depending on the geographical area and climatic conditions [30]. Stipa tenacissima is spread over an area of about 3 million hectares in Algeria [31] and over 400 thousand hectares in Tunisia, located mainly in the Ksserine, Sidi-Bouzid, Gafsa and Kairouan regions. For about four decades, the Alfa plant has been considered of great importance for the production of fibers intended for the manufacture of paper. For example, every year Tunisia produces an amount of Alpha pulp in excess of 30,000 tonnes [30, 32]. Given that Stipa tenacissima is a plant that has adapted to the semi-arid climate and does not require large quantities of water to grow, it is an important source for bioethanol production. The central-western part of Tunisia faces water shortages, and by cultivating energy plants that require significant quantities of water would put huge pressure on food crops. Specifically, various authors argue that in terms of adaptation, but also environmental sustainability, it would be advisable to grow energy-tolerant drought plants, such as sweet sorghum [33]. Table 1 shows the quantities of some LCMs and their potential for bioethanol production. Table 2 presents a series of LCMs with their main constituents. Table 1 The worldwide available quantity of the main agricultural residues and their potential for bioethanol production [34] Feedstock for 2 nd G bioethanol Worldwide quantities of agricultural residues (Tg) Potential bioethanol production (gallons) Total bioethanol (gallons) Gasoline equivalent (gallons) Corn grain residue 20.7 14.38 72.98 52.4 Corn stover 203.62 58.6 Barley grain residue 3.66 2.46 20.56 14.8 Barley straw 58.45 17.1 Oat grain residue 0.55 0.39 3.17 2.27 Oat straw 10.62 2.78 Rice grain residue 25.44 16.8 221.4 159 Rice straw 731.34 204.6 Wheat grain residue 17.2 11.33 115.13 82.71 Wheat straw 354.35 103.8 Sorghum grain residue 3.12 2.14 4.93 3.54 Sorghum straw 10.32 2.79 Sugarcane residue 3.2 1.59 52.89 38 Sugarcane bagasse 180.73 51.3 2.2. Forest wood biomass Forest wood biomass is known as one of the most promising renewable feedstocks for the production of 2ndG bioethanol. Wood biomass can be obtained from maintenance or forestry exploitation. This has a high energy value and the acquisition costs are low, therefore it could be used for bioethanol production [41]. In Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIX, Issue 2 – 2020 Vasile-Florin URSACHI, Gheorghe GUTT, Feedstocks used for production of 2nd and 3rd generation bioethanol - Review, Food and Environment Safety, Volume XIX, Issue 2 – 2020, pag. 156 – 169 161 the US, woody biomass accounts for about 30% of total biomass used annually to generate bioenergy [42]. The wood forest materials used in the USA generally come from 3 species of resinous Pinus contorta, Pseudotsuga menziesii and Pinus ponderosa. These conifer species have a high content of hemicellulose (18 - 33%) and cellulose (39 - 55%) [42]. Nearly 90% of the dry weight of forest wood biomass is composed of lignin, hemicellulose, cellulose and pectin [43]. Specifically, woody biomass comprises 30 - 60% cellulose, 15 - 40% hemicellulose and 10 - 25% lignin [17, 22, 37]. Table 2 Chemical composition for different LCMs [35, 36] Feedstock Carbohydrate compositions (%) References Cellulose Hemicellulose Lignin Sugarcane tops 35 32 14 [37] Sugarcane bagasse 32 - 48 19 - 25 23 - 32 [36, 38] Corn stover 38 - 40 26 - 28 7 - 21 Corn cob 45 35 15 [38] Sorghum stalks 27 25 11 [39] Sorghum straw 32 24 13 Sweet sorghum Bagasse 34 - 45 18 - 28 14 - 22 [36, 39] Barley straw 31 - 45 27 - 38 14 - 19 [39] Rice straw 28 - 38 23 - 32 12 - 14 [36, 39] Rice husk 37 29 24 [40] Wheat straw 33 - 41 20 - 32 13-20 [36, 40] Cotton, flax, etc. 80 - 95 5 - 20 - [36, 39] Coir 36 - 43 0.15 - 0.25 41 - 45 [39] Switchgrass 40 - 45 30 - 35 12 [17] Leaves 15 - 20 80 - 85 0 [38] Grasses 25- 43 8 - 50 8 - 30 [17, 23] Agriculture residues 37 - 50 25 - 50 5 - 15 [17] Industrial residue from chemical pulp 50 - 80 20 - 30 2 - 10 [17, 36] Newspaper 40-55 25 - 40 18 - 30 [39] Paper residues 65 13 1 [40] There are 35 species of the genus Populus that have a fast growth rate and can produce large quantities of woody material that can be used to obtain bioethanol 2ndG. The harvest time of forest wood biomass is more flexible as compared to agricultural residues. Forest residues, such as dry trees, wood chips and sawdust, could be an important feedstock that can be converted into bioethanol [17]. The carbohydrate content as well as of other wood extractable substances and the bark of the various trees are presented in the tables 3, 4, 5 and 6. 3. Feedstocks used in the production of third generation bioethanol (3rdG) Currently, the use of algae biomass for 3rdG biofuel production is of high interest, as well as investments in the biofuel, PETROLEUM and agri-food industries. It has been proven that major biofuel producing countries, such as the US, Europe and Asia, cannot produce sufficient quantities of corn, soy or rapeseed for their biofuel targets. Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIX, Issue 2 – 2020 Vasile-Florin URSACHI, Gheorghe GUTT, Feedstocks used for production of 2nd and 3rd generation bioethanol - Review, Food and Environment Safety, Volume XIX, Issue 2 – 2020, pag. 156 – 169 162 Table 3 Proportion of major main constituents of wood [47] Sample of wood Holocellulose (%) Cellulose (%) Hemicellulose (%) Lignin (%) Beech 82.5 46.7 35.8 20.7 Birch 84.2 45.4 38.8 17.7 Alder 77.2 44.1 33.1 22.0 Maple 80.1 44.6 35.5 24.9 Spruce 77.8 50.0 27.8 26.5 Pine 73.1 47.3 25.8 25.6 Oak 69.4 39.1 30.3 22.8 Table 4 Content of carbohydrates and other extractable substances (%) of some trees in North America [44, 45] Scientific Name Common Name X A G Ga M Ua Ac Lg Ash Hardwoods Acer rubrum Red maple 19 0.5 46 0.6 2.4 3.5 3.8 24 0.2 Acer saccharum Sugar maple 15 0.8 52 <0.1 2.3 4.4 2.9 23 0.3 Betula alleghaniensis Yellow birch 20 0.6 47 0.9 3.6 4.2 3.3 21 0.3 Betula papyrifera White birch 26 0.5 43 0.6 1.8 4.6 4.4 19 0.2 Fagus grandifolia Beech 19 0.5 46 1.2 2.1 4.8 3.9 22 0.4 Platanus occidentalis Sycamore 15 0.6 43 2.2 2.0 5.1 5.5 23 0.7 Populus deltoides Eastern cottonwood 15 0.6 47 1.4 2.9 4.8 3.1 24 0.8 Populus tremuloides Quaking aspen 17 0.5 49 2.0 2.1 4.3 3.7 21 0.4 Quercus falcata Southern red oak 19 0.4 41 1.2 2.0 4.5 3.3 24 0.8 Softwoods Abies balsamea Balsam fir 6.4 0.5 46 1.0 12 3.4 1.5 29 0.2 Gingo biloba Ginko 4.9 1.6 40 3.5 10 4.6 1.3 33 1.1 Larix laricina Tamarack 4.3 1.0 46 2.3 13 2.9 1.5 29 0.2 Picea abies Norway spruce 7.4 1.4 43 2.3 9.5 5.3 1.2 29 0.5 Picea glauca White spruce 9.1 1.5 45 1.2 11 3.6 1.3 27 0.3 Picea mariana Black spruce 6.0 1.5 44 2.0 9.4 5.1 1.3 30 0.3 Picea rubens Red spruce 6.2 1.4 44 2.2 12 4.7 1.4 28 0.3 Pinus resinosa Red pine 9.3 2.4 42 1.8 7.4 6.0 1.2 29 0.4 Pinus rigida Pitch pine 6.6 1.3 47 1.4 9.8 4.0 1.2 28 0.4 Pinus sylvestris Scots pine 7.6 1.6 44 3.1 10 5.6 1.3 27 0.4 Pinus taeda Loblolly pine 6.8 1.7 45 2.3 11 3.8 1.1 28 0.3 Pseudotsuga menziesii Douglas-fir 2.8 2.7 44 4.7 11 2.8 0.8 32 0.4 Thuja occidentalis Northern white cedar 10.0 1.2 43 1.4 8.0 4.2 1.1 31 0.2 Tsuga canadensis Eastern hemlock 5.3 0.6 44 1.2 11 3.3 1.7 33 0.2 X- Xylose, A- Arabinose, G- Glucose, Ga- Galactose, M- Mannose, Ua- Uronic acids, Ac- Acetyl, Lg- Lignin 3.1. Macroalgae and microalgae Carbohydrate percentages for seaweed depend on the species and hydrolytic treatment used. These sugars can be fermented by microorganisms and converted into bioethanol and / or biobutanol [49]. Researches on brown algae have shown that from 50 g / L sugar the ethanol yield is 7.0 - 9.8 g / L, and the fermentation process lasted for 40 hours (acidic medium) [50]. In the case of acid hydrolysis of green macroalgae (Ulva), a Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIX, Issue 2 – 2020 Vasile-Florin URSACHI, Gheorghe GUTT, Feedstocks used for production of 2nd and 3rd generation bioethanol - Review, Food and Environment Safety, Volume XIX, Issue 2 – 2020, pag. 156 – 169 163 content of 15.2 g sugars/L was obtained and the average yield of butanol was 4 g/L [50]. Table 5 Content of carbohydrates and other extractable substances in the bark of some trees [44, 46] Species X A G Ga M Rh Ua Ac Abies amabilis 3.2 3.2 37.4 1.6 8.0 - 5.6 0.8 Picea abies 4.8 1.8 36.6 1.3 6.5 0.3 - - Picea engelmannii 3.8 3.3 35.7 2.4 2.9 - 8.0 0.5 Pinus contoria Inner bark 3.7 10.6 40.9 4.3 2.5 - 9.9 0.2 Outer bark 3.4 5.5 26.8 4.2 2.5 - 7.7 0.8 Pinus sylvestris 5.8 2.1 30.2 2.4 5.4 0.3 - - Pinus taeda Inner bark 2.1 5.6 21.3 3.1 2.5 0.3 4.6 - Outer bark 3.8 1.8 15.8 2.5 2.6 0.1 2.1 - Betula papyrifera Inner bark 21.0 2.7 28.0 1.0 0.2 - 2.2 - Fagus sylvatica 20.1 3.1 29.7 3.1 0.2 1.2 - Quercus robur 16.4 2.0 32.3 1.3 0.5 0.5 - - X- Xylose, A- Arabinose, G- Glucose, Ga- Galactose, M- Mannose, Rh- Rhamnose, Ua- Uronic acids, Ac- Acetyl Table 6 The content of fermentable sugars from hydrolyzed biomass and the ethanol content resulting from fermentation [48] Mixed Sugar content Ethanol content Sample Amount (2/1) Percentage (%) Amount (g/l) Percentage (%) Sawdust 18.20 36.40 8.51 17.02 Corn residues 19.24 38.48 8.99 17.98 In recent years it has been found that microalgae are a promising starting material for bioenergy production, because they have a high content of carbohydrates that can be used for the purpose of obtaining bioethanol and biobutanol, respectively a lipid content that could be used to obtain biodiesel. Also, a series of gaseous biofuels, such as biomethane and biohydrogen, can be produced from microalgae or their residues (after obtaining bioethanol and biodiesel) [51]. Unlike plants that do not grow in the aquatic environment, microalgae do not have biopolymers like lignin and hemicelluloses in the chemical structure [52 - 54]. Under specific conditions, the biomass formed from the microalgae can undergo a hydrolysis step and the carbohydrates can be fermented by the yeasts in bioethanol [55]. In the case of fermentation of microalgae biomass for the release of fermentable sugars it is possible not to use chemical and enzymatic pretreatments. It is known that in the case of cellulose feedstocks by applying these pretreatments, significant amounts of energy are consumed. However, mechanical pretreatments are still needed to disintegrate algal cells by various techniques [56]. Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIX, Issue 2 – 2020 Vasile-Florin URSACHI, Gheorghe GUTT, Feedstocks used for production of 2nd and 3rd generation bioethanol - Review, Food and Environment Safety, Volume XIX, Issue 2 – 2020, pag. 156 – 169 164 Different microalgae species, such as Chlamydomonas sp., Chlorella sp., Spirulina sp., Spirogyra sp. Also Dunaliella sp. can be used to obtain 3rdG bioethanol because they have a starch content of about 64% [57]. Another important aspect is that they have a fast development rate, high photosynthesis activity and high CO2 absorption capacity [58]. Liyamen and Ricke (2012) concluded that microalgae can produce about 10 times more bioethanol than maize on the cultivation surface. In recent years, with the help of genetic engineering, species of microalgae have been created that have a higher carbohydrate content, resulting in higher yields of 3rdG bioethanol. For example, Chlamydomonas reinhardtii and Chlorella vulgaris JSC-6 cultivated under controlled conditions had a carbohydrate content of 71% and 54%, respectively [59 - 60]. Table 7 Carbohydrate content of different algae and microalgae species [61] Algal species Carbohydrate content (%) Reference C. vulgaris 20.99 - 55.0 [62, 63] Chlorella sorokiniana 35.67 [64] Chlorella minutissima 61 [65] Chlorella homosphaera 54 [66] Chlamydomonas reinhardtii UTEX 90 60.0 [67] Spirulina platensis 30.21 30.21 [64] Spirulina platensis LEB 52 65 [66] Scenedesmus dimorphus 21 - 52 [69, 70] Scenedesmus obliquus 46 [71] Scenedesmus ecosystem 42 - 53 [72] Nannochloropsis oceanica 22.70 [73] Spirogyra sp. 33 - 64 [74] Porphyridium cruentum 40 - 57 Ulva lactuca 55-60 [75] Dunaliella salina 32 [70] Dunaliella tertiolecta 21.69 [76] Tetraselmis sp. 24 [77] Porphyra 40-76 [78] Palmaria 38-74 [79] Also, several researchers claim that from microalgae a bioethanol yield can be obtained with values between 0.240 and 0.888 g ethanol / g substrate, at 25 - 30 ° C [56, 80, 81]. Laboratory research has shown that bioethanol yield from biomass formed from microalgae under optimal conditions is about 65% [56]. Chlorella vulgaris biomass was enzymatically hydrolyzed, and the resulting carbohydrates were fermented by Saccharomyces cerevisiae and converted to ethanol. The yields obtained for sugars following hydrolysis and ethanol were 0.55 and 0.17 g / g biomass respectively [82]. After extracting from Schizochytrium sp. lipids and proteins, the remaining carbohydrates (D- glucose and L- galactose), were transformed by Escherichia coli KO11 into bioethanol. Following the fermentation process of the concentration of 25.7 g / L glucose, a yield of 11.8 g ethanol / L was obtained [83]. Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIX, Issue 2 – 2020 Vasile-Florin URSACHI, Gheorghe GUTT, Feedstocks used for production of 2nd and 3rd generation bioethanol - Review, Food and Environment Safety, Volume XIX, Issue 2 – 2020, pag. 156 – 169 165 It is estimated that in a year, between 5,000 - 15,000 gallons of ethanol / acre (46,760 - 140,290 L / ha) can be produced from microalgae [84]. Table 7 shows the carbohydrate content of algae by species. Table 8 shows the bioethanol yield for different algae and microalgae. Table 8 Bioethanol yield for different species of algae and microalgae [84] Feedstock Bioethanol Reference Chlorococcum infusionum 260 g ethanol/Kg algae [85] Spirogyra 80 g ethanol/kg algae [86] Chlorococcum humicola 520g ethanol/kg microalgae [87] Chlamydomonas reinhardtii UTEX 90 11.73 ethanol g/1 [88] Chlamydomonas reinhardtii 29.2 % [89] Chlamydomonas fasciata 19.4 [90] Chlorella vulgaris 11.66 % ethanol g/1 [91] Arthrospira platensis 16% g EtOH per g of dry biomass. [91] 4. Conclusion By reviewing the current state of research regarding biofuel production, the following conclusions can be drawn: - Every year, huge quantities of lignocellulosic materials (LCM) are generated from agriculture, which instead of being wasted can be converted into second-generation 2ndG bioethanol. - The forestry sector generates a huge amount of wood biomass which is relatively cheap and can be used for bioethanol production. - The efficiency of lignocellulosic feedstocks depends mainly on their availability and composition (cellulose, hemicellulose, lignin, ash). - In order to obtain high yields of carbohydrates, respectively bioethanol 2ndG, it is indicated that different pretreatments described in the literature should be applied on lignocellulosic biomass (LCB). - Recent research has shown that algae / microalgae are a source of biomass from which significant quantities of third- generation bioethanol (3rdG) and biodiesel could be produced. 5. References [1]. 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