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Okra: A potential future bioenergy crop in Iran
Renewable and Sustainable Energy Reviews 93 (2018) 517–524
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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
Okra: A potential future bioenergy crop in Iran a,⁎
a
T b
Seyed Amir Moosavi , Majid Aghaalikhani , Barat Ghobadian , Ebrahim Fayyazi a b
b
Department of Agronomy, Faculty of Agriculture, Tarbiat Modares University, Tehran, Iran Department of Biosystems Engineering, Faculty of Agriculture, Tarbiat Modares University, Tehran, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Abelmoschus esculentus Biodiesel Biofuel Energy
The Iranian energy sector has recently discovered a great interest in the concept of renewable and clean energy. The interest is motivated primarily by concerns about greenhouse gas (GHG) emissions and global climate change, as well as the desire to find alternative and sustainable energy sources and create potential job opportunities related to these new technologies for future generations of Iranians. This study supports the search for alternative, sustainable energy sources by assessing okra's usability as a biofuel. Okra is an annual, warm season crop that provides a rich source of industrial oil and protein. According to our investigation, the seeds of Iranian okra ecotypes that have an oil content of 20% could produce up to 325 kg/ha oil yield. Our study on Iranian okra seed oil showed that, the most dominant fatty acids of are linoleic acid (C18:2) (38–40%), Palmitic acid (C16:0) (29–30%), and Oleic acid (C18:1) (19–22%). The biodiesel derived from okra via a transesterification reaction using an ultrasonic system could meet ASTM D6751 standards with satisfactory results in methyl ester content (more than 96%), viscosity (2.3–2.4 mm−2/S, and flash point (155–158 °C)). Because of its high oil yield, quality, and large ecological adaptation window, okra is a strong contender to provide a new source of non-edible oil for biodiesel production in bioenergy farms.
1. Introduction 1.1. Energy demands and challenges ahead Energy demand is increasing as the human population grows and urbanization expands [1]. Energy production and use increases each year to supply human activities, despite the fact that some energy sources are causing widespread environmental pollution. The latest report from the International Energy Agency and World Health Organizations reveals that 18,000 people die every day as the result of air pollution [2]. Furthermore, the use of fossil fuels has generated additional environmental concerns, particularly as concerns the effects of global climate change, and fossil fuel sources are limited and will not be able to continue to meet demand [1]. On the other hand, energy consumption has a direct relationship to economic activity, which brings better life quality and development opportunities to humans [3]. Countries with adequate land and other farming inputs, such as water, human resources, and good biodiversity, may be suitable producers of bioenergy [4]. Bioenergy is energy generated from the conversion of any organic matter available on a renewable basis, such as feedstock derived from animal and plants (i.e., animal fats or vegetable oils) [5]. It should be noted that land use for bioenergy expansion need
⁎
not compete with land use for food and other needs [6]. So, there is a need to determine how to gain all benefits of bioenergy and reduce the landscape-scale tradeoffs. Intensive research on bioenergy sources, including crops with less food importance that are capable of high biomass production, can help to fill this gap. The ideal bioenergy crop should be fast growing, water- and nutrient-efficient, have low invasive potential, and have high tolerance for stress [7]. 1.1.1. Why okra? Okra (Abelmoschus esculentus L.) is an annual, warm season food crop that is usually cultivated for its immature pods [8–10]. Okra prefers well-drained soils and can tolerate a pH range from 5.5 to 8 [11]. Flowers are located on axillary parts; they have five white to yellow petals with a dark, reddish violet center. The flowers rapidly turn to fruit within two or three days. Fruits contain rows of rounded, dark green to gray seeds. Depending on the cultivar, okra fruits mature within 60–180 days of sowing [12]. Generally, okra is an erect crop that becomes woody at maturity [13,14]. Recently, some reports suggest that okra seeds could provide a new source of oil and protein [12,15,16]. Studies of different varieties showed that okra seeds contain 21.72% crude oil, 31.4% crude fiber, and 27% crude protein (on average) [17–19]. Okra seed oil is very
Corresponding author. E-mail address: [email protected] (M. Aghaalikhani).
https://doi.org/10.1016/j.rser.2018.04.057 Received 4 February 2017; Received in revised form 30 September 2017; Accepted 14 April 2018 1364-0321/ © 2018 Elsevier Ltd. All rights reserved.
Renewable and Sustainable Energy Reviews 93 (2018) 517–524
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favorable environmental conditions for growing okra, particularly in the summer (Fig. 2). Despite the high production potential of okra in Iran, the difficulty in harvesting and bringing the crop to market are barriers to production for most farmers who would grow it as a food crop. However, if the crop was cultivated to produce seed oil and protein, rather than fresh pods, then farmers would find this crop to be a much more attractive option. Geographical distribution will affect okra plant characteristics. This study reviewed the oil, biomass, and biodiesel qualities of three Iranian okra ecotypes.
similar to cotton and peanut seed oil; it has a high content of palmitic, oleic, and linoleic acids [20,21]. Unfortunately, Okra is a forgotten crop in Iran's agriculture. Although it has considerable advantages, such as high drought tolerance, an indeterminate growth habit that decreases the risk of yield loss, and high seed oil content for a summer crop, its cultivation is very limited. In part, this is because farmers have a very limited window of three or four days to pick the crop before the pods become woody. This makes okra production for human consumption a high cost endeavor in terms of human labor, stress, and the costs of production. Recently, we found that okra seeds are a valuable source of oil that may be suitable for bioenergy purposes. The labor requirements and harvest stress would be far less for okra grown for biofuel than for okra grown for human consumption, because the desired product of cultivation would be pods with mature seeds, and it would not matter if the pods became woody before the seed is harvested. Although a large number of genetic resources of the same genus as okra are known to exist [22–24] and the varietal effects on the chemical composition of okra seeds have been documented [18,25], there is a lack of information on the range of variation within the seed oil content of each Iranian okra ecotype in terms of fatty acid composition, biomass characteristics, and the potential application of the seed for bioenergy purposes. This review will focus on okra as a potential bioenergy crop in Iran, based on research assessing the biofuel production possibilities from this crop.
1.2. Literature section (Bioenergy crops in action) 1.2.1. Vegetable oils and biofuels Since the invention of diesel engines, vegetable oils have been considered as potential liquid fuels. Rudolf Diesel used peanut oil as fuel in 1900 [28]. Researchers have investigated many vegetable oils and their derivatives, which are most commonly biodiesel or the monoalkyl esters of oils or fats, for their potential application as fuel, whether for transportation or heating purposes [29–34]. Although biodiesel production is encouraged globally as an environmentally-friendly fuel and a short-term substitute for conventional diesel fuel [35], it also negatively influences food production and could cause food shortages. As a result, some researchers recommend blending conventional fuel with biodiesel in order to help minimize the emission of greenhouse gases while ensuring food security [36]. There are large potential feed stocks with which biodiesel could be produced from non-edible vegetable oils. USDA reports indicate that total vegetable oil production increased by 18.08% from 2011 to 2015. The main sources of vegetable oils are rapeseed, corn, and soybean, which are also largely used for food purposes [37,38]. The main hindrance to adopting biodiesel from vegetable oils is dedicating enough arable land to produce a sufficient volume of oil seed crops without affecting the food supply and cost of food production [39]. Most of these concerns relate to crops that primarily used for food purposes. Based on FAO reports, nearly 41.88 million km2 of land is available for agriculture, but only 15.06 million km2 is being used, and only 0.14 million km2 is dedicated for biofuel production [40,41]. Crops that provide nonedible oil are under review to use as a feedstock for
1.1.2. Okra general statistics The total world production of okra in 2013 was estimated to be 8.947 million tons, grown from 1.126 million ha [26]. Okra is primarily cultivated in Asia and Africa. India is the largest okra producer in the world, followed by Nigeria (Fig. 1). However, its high tolerance for unfavorable environmental conditions such as drought and salinity (Fig. 2) make okra a highly likely prospect for cultivation in Iran. 1.1.3. Okra in Iran Okra cultivation in Iran was last officially recorded in 2002; the report showed that the total cultivation area of okra in Iran was 1038 ha [27]. The Khouzestan province was the largest site of okra production in Iran, with 993 ha of okra fields [27]. However, many provinces have
Fig. 1. Largest okra producers of the world (FAO, 2016). 518
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Fig. 2. Potential sites for okra cultivations in Iran.
The next section of the study will present the experimental procedure for producing biodiesel from the seed oil of three different Iranian okra ecotypes using the ultrasonic transesterification method. The third section of the paper evaluates the results of the okra biodiesel's FAME profile, viscosity, and flash point, as well as okra's oil yield potential.
bioenergy in order to avoid impacts to food pricing and supply. Studies revealed that, the correlation between fatty acids composition and transesterification optimization performance is significant [42–45]. Oils with more monounsaturated fatty acids (palmitoleic and oleic acids) are highly recommended oils for biodiesel production. C 18:1 and C 16:1 are the best acids in terms of oxidative stability and suitability of application within a cold weather climate [38,39,46].
2. Experiment exploring okra's potential as a bioenergy crop for Iran
1.2.2. Newly green source of non-edible vegetable oils to overcome energy crises In recent years, various researchers have aimed to introduce a new, non-edible source of vegetable oil for the purposes of biodiesel production. Most of these studies were focused on the production of biodiesel from Calophyllum inophyllum L., Jatropha curcas, Simmondsia chinensis, Ricinus communis, Madhuca indica, and Pongamia pinnata seed oil [46–51].
2.1. Iranian okra ecotype description Iranian okra ecotypes are categorized into two main groups, yellow and green, based on their fruit color. Ecotype descriptions were carefully recorded during two growing season of 2014 and 2015 at the research field of Tarbiat Modares University, Table 1 [53]. 2.1.1. Seed properties Seeds are relatively large and heavy, at approximately 65 g per 100 seeds. Un-ripened okra seeds are white and light yellow during seed development and become dark green or gray at full maturity [54] (Fig. 4). Because okra is an indeterminate plant, its seeds do not mature uniformly. However, our research on okra seed production showed that first seed harvest most commonly occurs 80 days after planting. At this time, the primary pods had dried and the seeds no longer required energy from the plant. Seed could be harvested three times from every crop, approximately 14 days apart.
1.3. Source identification of non-edible vegetable oil for biodiesel production in Iran This review projects the possible application in Iran of Abelmoschus esculentus as a new source of green vegetable oil for biodiesel production using an ultrasonic assisted transesterification reaction. Our focus was to discern whether this crop can easily be used as biodiesel feedstock due to its high adaptability to drought and less fertile soils, which are two main problems facing biofuel cultivators in the arid and semiarid regions of Iran. This study is different from previous studies in its method to produce biodiesel from okra seed oil, which uses an ultrasonic-assisted transesterification reaction. The study also is the first to report the fatty acid, oil yield, and seed oil content of three native Iranian okra ecotypes in order to consider what would be the best new summer energy crop to grown in Iran. In the process of vegetable oil conversion to liquid biofuel, transesterification is one of the main pathways toward biofuel production (Fig. 3).
2.1.2. Seed oil extraction The seeds of three Iranian okra ecotypes (Ahwaz, Isfahan, and Mashhad) were subjected to study to determine their seed oil content, oil fatty-acid profile, and biodiesel properties. Oil extraction was performed using an n-hexane solvent in Soxhlet. From each batch of okra, 100 g of okra seeds were crushed finely using a coffee grinder. The solvent was distilled off at 45 °C using a rotary evaporator [55]. 519
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Fig. 3. Pathways of biofuel production from different feedstocks [52].
Hielscher, Germany. The operating frequency of this ultrasound source was 24 kHz, and its vibration amplitude was 100% (Fig. 6). Ultrasonification provides a sufficient mixing and energy so that the transesterification can proceed at a faster rate [58,59]. This technique can destroy cell walls, increase the specific surface area, and reduce the degree of polymerization. As a result, it can provide a more degradability of biomass. This method has been garnering increasing interest because the lower energy input requirements to produce transesterification reactions has a smaller impact on the environment than other methods [60,61]. Furthermore, it provides shorter reaction times [48,49]. When the reaction time was up, we removed the biodiesel mixture from the reactor and put it into a cold ice bath to stop the reaction. The mixture was transferred to separation plate in order to remove glycerin from the biodiesel (Fig. 7). Glycerin is heavier than biodiesel, and it will settle to the bottom while the biodiesel will remain on top. After separating the biodiesel from the glycerin, we purified the biodiesel by subjecting to a water wash. Then we centrifuged it for five minute at 6000 rpm to remove any additional impurities. The biodiesel methylester content was calculated based on the following formula [50,51]:
Table 1 Iranian okra ecotypes descriptions. Ecotype
Height
Blossom time
Fruit color
Fruit character
Seedling establishment
Ahwaz
Tall
Yellow
Dwarf
Mashhad
Tall
Fat–moderate length Thin–long length Fat–moderate length
Good
Isfahan
Late season crop Short season crop Moderate season crop
Green Green
Good Good
The fatty acid profile of the okra seed oil was determined using Metcalfe procedure [56]. One microliter of sample was injected to Clarus 580 Perkin-elmer gas chromatography unit, which was equipped with an FTIR detector and BP-30 column that was maintained isothermally calibrated to meet BS-EN 14103 standard (Fig. 5). The total run time for each sample was 30 min. Fatty acid composition was determined by identifying and calculating relative peak areas. 2.2. Converting the oil to biodiesel using an ultra-sonic reactor Okra seed oil was transformed into biodiesel via transesterification using an ultra-sonic reactor. We selected this method due to its simplicity, low production cost, small equipment size, and high efficiency of biodiesel production [57]. To increase the purity of the fuel, the produced biodiesels were subjected to a water washing treatment. Methoxide was prepared by mixing methanol that had a molar ratio of 1:7, as an alcohol agent, with 1% potassium hydroxide (KOH), as a catalyst [55]. The applied ultrasound source was a UP400-S from
C=
∑ A − AIS MIS × ×100 AIS M
Where: ∑A was the total area under the peak of total fatty acids, AIS was the area under peak for internal standard (C19:0), MIS was the weight of internal standard, and M was the weight of the sample. We measured biodiesel viscosity, which is a measure of resistance to flow of a liquid due to the internal friction of one part of a fluid moving Fig. 4. Okra seeds at different maturity phases.
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Fig. 5. Perkin-Elmer gas chromatography equipped with auto sampler robotic arm.
Fig. 6. Ultra-sonic reactor UP400 for okra biodiesel production.
(Table 2). According to our results, genetic resource is a very important criteria determining seed oil quality of okra (Fig. 10). Mashhad and Isfahan had more similar chemical constituents to each other than they had to Ahwaz. Mashhad and Isfahan are both categorized as green pod okra, while Ahwaz, which is adapted to the warmer parts of the country, is categorized as a yellow pod. The Ahwaz ecotype has better oil quality, which could be attributed to the higher ratio of unsaturated fatty acids in the seed oil. The linoleic acid content of all the ecotypes studied is higher than that of rape seed (22%); corn (6%) [65], palm (10.1%), and castor bean (1.3%) [66].
over another, using an Anton Paar viscometer SVM 3000. 3. Results and discussion 3.1. Seed oil content The average oil content of the studied okra ecotypes was 19–20% (Fig. 8). Oil yield was more than 300 kg.ha−1. However, the higher seed production in Isfahan and Mashhad compensated for their lower seed oil percentage; thus, overall oil yield did not significantly differ among the three ecotypes (Fig. 9). Study of 238 okra varieties for seed oil content revealed that seed oil content is ranges between 17% and 22%; expected oil yield could be 289–612 kg ha−1 [62].
3.3. Okra biodiesel properties The biodiesel produced from okra seed oil contained more than 90% methyl ester and passed the EN 14214 standard for biodiesel (Table 3). Furthermore, it processed well in the ultra-sonic reactor. Higher viscosity biodiesels tend to have weaker vaporization and larger fuel droplets, which can cause undesirable combustion in the engine [68–70]. Among okra ecotypes, the Isfahan biodiesel exhibited the highest viscosity (2.463 mm2 s−1). Previous studies revealed that
3.2. Seed oil profile Okra seed oil consists of both saturated and unsaturated fatty acids. The seed oil of our ecotypes mostly contains C18:2 (Linoleic acid) (38–40%), which is also found in olive and peanut oils [63,64]. Palmitic acid (C16:0) (29–30%) is okra seed oil's second major constituent 521
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Fig. 7. Illustrations of okra biodiesel production. 21
the kinematic viscosity of different biodiesel derived from vegetable oils would fall within the narrow range of 4–5 mm2 s−1. Biodiesel from some vegetable oils has a slightly lower viscosity—coconut derived biodiesel showed a kinematic viscosity value of 2.75 mm2 s−1 [70]. The ASTM D6751 viscosity specification of 1.9–6.0 mm2 s−1 was satisfied with okra derived biodiesel, which had a kinematic viscosity range of 2.3–2.4 mm2 s−1.
Oil content (%)
20.5 20 19.5 19 18.5 Ahwaz
Isfahan
Mashhad
Okra Ecotype
Fig. 8. Iranian okra ecotypes seed oil content.
4. Conclusion 330
Different climatic conditions and vast areas of the country are pushing Iran toward becoming a highly populated country in the Middle East. As a result, it is a rapidly developing country with increasing energy demands. The environmental pollution and greenhouse gas emission concerns about fossil fuels, in addition to the limited number of fossil fuel reservoirs, have brought renewable and clean energy resources to the center of attention in the energy sector. Vegetable oils with suitable oil quality and environmental adaptability not only can help to provide clean and renewable energy, but also can create new job opportunities in both the agriculture and energy sectors of the country. Okra is a crop that has adapted well to Iran's climatic
Oil yield (kg/ha)
310
290
270
250 Isfahan
Ahwaz
Mashhad
Okra Ecotype
Fig. 9. Oil yield of Iranian okra ecotypes.
Table 2 Fatty acid composition of okra seed oil. Fatty acid composition
Myristic acid (C14:0) Palmitic acid (C16:0) Palmitoleic acid (C16:1) Margaric acid (C17:0) Stearic acid (C18:0) Oleic acid (C18:1) Linoleic acid (C18:2) Linolenic acid (C18:3) Arachidic acid (C20:0) Behenic acid (C22:0) Heneicosylic acid (C21:0)
Iranian okra ecotype
USA okra germplasm
Malaysian okra
Ahwaz
Isfahan
Mashhad
[67]
[9]
0.82 30.52 1.03 0.94 3.89 19.04 40.13 1.97 1.03 — 0.85
0.554 29.944 0.674 0.744 3.694 21.564 39.784 1.764 0.744 — 0.524
0.652 29.412 0.752 0.802 3.792 22.612 38.272 2.002 0.862 — 0.642
0.31 30.42 0.39 – 3.93 21.085 37.78 0.17 0.494 0.25 —
0.32 29.58 0.45 0.13 3.80 19.72 44.21 0.28 0.52 0.22 —
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Fig. 10. Ratio of saturated and unsaturated fatty acids in Iranian okra ecotypes’ seed oil. [16] Steyn NP, Mchiza Z, Hill J, Davids YD, Venter I, Hinrichsen E, et al. Nutritional contribution of street foods to the diet of people in developing countries: a systematic review. Public Health Nutr 2014;17:1363–74. [17] Oyelade OJ, Adeomi VF. Influence of variety on protein, fat contents and some physical characteristics of okra seeds. J Food Eng 2003;57:111–4. [18] Sami R, Lianzhou J, Yang L, Ma Y, Jing J. Evaluation of fatty acid and amino acid compositions in okra (Abelmoschus esculentus) grown in different geographical locations. Biomed Res Int 2013;2013. http://dx.doi.org/10.1155/2013/574283. [19] Balasubramanian T, Sadasivam S. Changes in starch, oil, protein and amino acids in developing seeds of okra (Abelmoschus esculentus L. Moench). Qual Plant Plant Foods Hum Nutr 1987;37:41–6. http://dx.doi.org/10.1007/BF01092299. [20] Chisholm MJ, Hopkins CY. An oxygenated fatty acid from the seed oil of Hibiscus esculentus. Can J Chem 1957;35:358–64. [21] Crossley A, Hilditch TP. The fatty acids and glycerides of okra seed oil. J Sci Food Agric 1951;2:251–5. [22] Hamon S, Sloten van DH. Okra. Smartt J, Simmonds NW, editors. Evolution of Crop Plants, 605. John Wiley Sons; 1995. p. 350–7. [23] Bisht IS, Bhat KV. Okra (Abelmoschus spp.) Genetic resources, chromosome engineering, and crop improvement. 3. CRC Press; 2006. p. 147–83. [24] Bisht IS, Patel DP, Mahajan RK. Classification of genetic diversity in Abelmoschus tuberculatus germplasm collection using morphometric data. Ann Appl Biol 1997;130:325–35. [25] Ndangui CB, Kimbonguila A, Nzikou JM, Matos L, Pambou-Tobi NPG, Abena AA. Nutritive composition and properties physico-chemical of gumbo ( Abelmoschus esculentus L.) seed and oil. Res J Environ Earth Sci 2010;2:49–54. [26] FAO. FAO; 2016. 〈http://faostat3.fao.org/〉. [27] 〈Www.Maj.ir〉. Year Book Statistics. [28] Knothe G, Dunn RO, Bagby MO. Biodiesel: the use of vegetable oils and their derivatives as alternative diesel fuels. [666 SV-:172]. Energy1997. http://dx.doi.org/ 10.1021/ja975677. [+]. [29] Shimada Y, Watanabe Y, Samukawa T, Sugihara A, Noda H, Fukuda H, et al. Conversion of vegetable oil to biodiesel using immobilized Candida antarctica lipase. J Am Oil Chem Soc 1999;76:789–93. [30] Kim H-J, Kang B-S, Kim M-J, Park YM, Kim D-K, Lee J-S, et al. Transesterification of vegetable oil to biodiesel using heterogeneous base catalyst. Catal Today 2004;93:315–20. [31] Demirbaş A. Biodiesel from vegetable oils via transesterification in supercritical methanol. Energy Convers Manag 2002;43:2349–56. [32] Liu X, He H, Wang Y, Zhu S, Piao X. Transesterification of soybean oil to biodiesel using CaO as a solid base catalyst. Fuel 2008;87:216–21. [33] Safieddin Ardebili M, Najafi G, Ghobadian B, Tavakkoli Hashjin T. Determination of some mechanical properties of castor seed (Ricinus communis L.) to design and fabricate an oil extraction machine. J Agric Sci Technol 2012;14:1219–27. [34] Yee KF, Tan KT, Abdullah AZ, Lee KT. Life cycle assessment of palm biodiesel: revealing facts and benefits for sustainability. Appl Energy 2009;86:S189–96. [35] Hajjari M, Tabatabaei M, Aghbashlo M, Ghanavati H. A review on the prospects of sustainable biodiesel production: a global scenario with an emphasis on waste-oil biodiesel utilization. Renew Sustain Energy Rev 2017;72:445–64. [36] Hasan MM, Rahman MM. Performance and emission characteristics of biodiesel–diesel blend and environmental and economic impacts of biodiesel production: a review. Renew Sustain Energy Rev 2017;74:938–48. [37] Asif S, Ahmad M, Zafar M, Ali N. Prospects and potential of fatty acid methyl esters of some non-edible seed oils for use as biodiesel in Pakistan. Renew Sustain Energy Rev 2017;74:687–702. [38] Singh SP, Singh D. Biodiesel production through the use of different sources and characterization of oils and their esters as the substitute of diesel: a review. Renew Sustain Energy Rev 2010;14:200–16. http://dx.doi.org/10.1016/j.rser.2009.07.
Table 3 Biodiesel properties produced from okra and other vegetable oils. Crop
Ecotype
Methyl ester content (%)
Viscosity (mm2 s−1)
Flash point (°C)
Okra
Ahwaz Isfahan Mashhad [71] [72]
93.48667224 90.16709033 93.1295075 96 96.9
2.304 2.463 2.379 5.241 4.2
155 158 156 164 171
Sunflower Soybean
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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
Okra: A potential future bioenergy crop in Iran a,⁎
a
T b
Seyed Amir Moosavi , Majid Aghaalikhani , Barat Ghobadian , Ebrahim Fayyazi a b
b
Department of Agronomy, Faculty of Agriculture, Tarbiat Modares University, Tehran, Iran Department of Biosystems Engineering, Faculty of Agriculture, Tarbiat Modares University, Tehran, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Abelmoschus esculentus Biodiesel Biofuel Energy
The Iranian energy sector has recently discovered a great interest in the concept of renewable and clean energy. The interest is motivated primarily by concerns about greenhouse gas (GHG) emissions and global climate change, as well as the desire to find alternative and sustainable energy sources and create potential job opportunities related to these new technologies for future generations of Iranians. This study supports the search for alternative, sustainable energy sources by assessing okra's usability as a biofuel. Okra is an annual, warm season crop that provides a rich source of industrial oil and protein. According to our investigation, the seeds of Iranian okra ecotypes that have an oil content of 20% could produce up to 325 kg/ha oil yield. Our study on Iranian okra seed oil showed that, the most dominant fatty acids of are linoleic acid (C18:2) (38–40%), Palmitic acid (C16:0) (29–30%), and Oleic acid (C18:1) (19–22%). The biodiesel derived from okra via a transesterification reaction using an ultrasonic system could meet ASTM D6751 standards with satisfactory results in methyl ester content (more than 96%), viscosity (2.3–2.4 mm−2/S, and flash point (155–158 °C)). Because of its high oil yield, quality, and large ecological adaptation window, okra is a strong contender to provide a new source of non-edible oil for biodiesel production in bioenergy farms.
1. Introduction 1.1. Energy demands and challenges ahead Energy demand is increasing as the human population grows and urbanization expands [1]. Energy production and use increases each year to supply human activities, despite the fact that some energy sources are causing widespread environmental pollution. The latest report from the International Energy Agency and World Health Organizations reveals that 18,000 people die every day as the result of air pollution [2]. Furthermore, the use of fossil fuels has generated additional environmental concerns, particularly as concerns the effects of global climate change, and fossil fuel sources are limited and will not be able to continue to meet demand [1]. On the other hand, energy consumption has a direct relationship to economic activity, which brings better life quality and development opportunities to humans [3]. Countries with adequate land and other farming inputs, such as water, human resources, and good biodiversity, may be suitable producers of bioenergy [4]. Bioenergy is energy generated from the conversion of any organic matter available on a renewable basis, such as feedstock derived from animal and plants (i.e., animal fats or vegetable oils) [5]. It should be noted that land use for bioenergy expansion need
⁎
not compete with land use for food and other needs [6]. So, there is a need to determine how to gain all benefits of bioenergy and reduce the landscape-scale tradeoffs. Intensive research on bioenergy sources, including crops with less food importance that are capable of high biomass production, can help to fill this gap. The ideal bioenergy crop should be fast growing, water- and nutrient-efficient, have low invasive potential, and have high tolerance for stress [7]. 1.1.1. Why okra? Okra (Abelmoschus esculentus L.) is an annual, warm season food crop that is usually cultivated for its immature pods [8–10]. Okra prefers well-drained soils and can tolerate a pH range from 5.5 to 8 [11]. Flowers are located on axillary parts; they have five white to yellow petals with a dark, reddish violet center. The flowers rapidly turn to fruit within two or three days. Fruits contain rows of rounded, dark green to gray seeds. Depending on the cultivar, okra fruits mature within 60–180 days of sowing [12]. Generally, okra is an erect crop that becomes woody at maturity [13,14]. Recently, some reports suggest that okra seeds could provide a new source of oil and protein [12,15,16]. Studies of different varieties showed that okra seeds contain 21.72% crude oil, 31.4% crude fiber, and 27% crude protein (on average) [17–19]. Okra seed oil is very
Corresponding author. E-mail address: [email protected] (M. Aghaalikhani).
https://doi.org/10.1016/j.rser.2018.04.057 Received 4 February 2017; Received in revised form 30 September 2017; Accepted 14 April 2018 1364-0321/ © 2018 Elsevier Ltd. All rights reserved.
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favorable environmental conditions for growing okra, particularly in the summer (Fig. 2). Despite the high production potential of okra in Iran, the difficulty in harvesting and bringing the crop to market are barriers to production for most farmers who would grow it as a food crop. However, if the crop was cultivated to produce seed oil and protein, rather than fresh pods, then farmers would find this crop to be a much more attractive option. Geographical distribution will affect okra plant characteristics. This study reviewed the oil, biomass, and biodiesel qualities of three Iranian okra ecotypes.
similar to cotton and peanut seed oil; it has a high content of palmitic, oleic, and linoleic acids [20,21]. Unfortunately, Okra is a forgotten crop in Iran's agriculture. Although it has considerable advantages, such as high drought tolerance, an indeterminate growth habit that decreases the risk of yield loss, and high seed oil content for a summer crop, its cultivation is very limited. In part, this is because farmers have a very limited window of three or four days to pick the crop before the pods become woody. This makes okra production for human consumption a high cost endeavor in terms of human labor, stress, and the costs of production. Recently, we found that okra seeds are a valuable source of oil that may be suitable for bioenergy purposes. The labor requirements and harvest stress would be far less for okra grown for biofuel than for okra grown for human consumption, because the desired product of cultivation would be pods with mature seeds, and it would not matter if the pods became woody before the seed is harvested. Although a large number of genetic resources of the same genus as okra are known to exist [22–24] and the varietal effects on the chemical composition of okra seeds have been documented [18,25], there is a lack of information on the range of variation within the seed oil content of each Iranian okra ecotype in terms of fatty acid composition, biomass characteristics, and the potential application of the seed for bioenergy purposes. This review will focus on okra as a potential bioenergy crop in Iran, based on research assessing the biofuel production possibilities from this crop.
1.2. Literature section (Bioenergy crops in action) 1.2.1. Vegetable oils and biofuels Since the invention of diesel engines, vegetable oils have been considered as potential liquid fuels. Rudolf Diesel used peanut oil as fuel in 1900 [28]. Researchers have investigated many vegetable oils and their derivatives, which are most commonly biodiesel or the monoalkyl esters of oils or fats, for their potential application as fuel, whether for transportation or heating purposes [29–34]. Although biodiesel production is encouraged globally as an environmentally-friendly fuel and a short-term substitute for conventional diesel fuel [35], it also negatively influences food production and could cause food shortages. As a result, some researchers recommend blending conventional fuel with biodiesel in order to help minimize the emission of greenhouse gases while ensuring food security [36]. There are large potential feed stocks with which biodiesel could be produced from non-edible vegetable oils. USDA reports indicate that total vegetable oil production increased by 18.08% from 2011 to 2015. The main sources of vegetable oils are rapeseed, corn, and soybean, which are also largely used for food purposes [37,38]. The main hindrance to adopting biodiesel from vegetable oils is dedicating enough arable land to produce a sufficient volume of oil seed crops without affecting the food supply and cost of food production [39]. Most of these concerns relate to crops that primarily used for food purposes. Based on FAO reports, nearly 41.88 million km2 of land is available for agriculture, but only 15.06 million km2 is being used, and only 0.14 million km2 is dedicated for biofuel production [40,41]. Crops that provide nonedible oil are under review to use as a feedstock for
1.1.2. Okra general statistics The total world production of okra in 2013 was estimated to be 8.947 million tons, grown from 1.126 million ha [26]. Okra is primarily cultivated in Asia and Africa. India is the largest okra producer in the world, followed by Nigeria (Fig. 1). However, its high tolerance for unfavorable environmental conditions such as drought and salinity (Fig. 2) make okra a highly likely prospect for cultivation in Iran. 1.1.3. Okra in Iran Okra cultivation in Iran was last officially recorded in 2002; the report showed that the total cultivation area of okra in Iran was 1038 ha [27]. The Khouzestan province was the largest site of okra production in Iran, with 993 ha of okra fields [27]. However, many provinces have
Fig. 1. Largest okra producers of the world (FAO, 2016). 518
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Fig. 2. Potential sites for okra cultivations in Iran.
The next section of the study will present the experimental procedure for producing biodiesel from the seed oil of three different Iranian okra ecotypes using the ultrasonic transesterification method. The third section of the paper evaluates the results of the okra biodiesel's FAME profile, viscosity, and flash point, as well as okra's oil yield potential.
bioenergy in order to avoid impacts to food pricing and supply. Studies revealed that, the correlation between fatty acids composition and transesterification optimization performance is significant [42–45]. Oils with more monounsaturated fatty acids (palmitoleic and oleic acids) are highly recommended oils for biodiesel production. C 18:1 and C 16:1 are the best acids in terms of oxidative stability and suitability of application within a cold weather climate [38,39,46].
2. Experiment exploring okra's potential as a bioenergy crop for Iran
1.2.2. Newly green source of non-edible vegetable oils to overcome energy crises In recent years, various researchers have aimed to introduce a new, non-edible source of vegetable oil for the purposes of biodiesel production. Most of these studies were focused on the production of biodiesel from Calophyllum inophyllum L., Jatropha curcas, Simmondsia chinensis, Ricinus communis, Madhuca indica, and Pongamia pinnata seed oil [46–51].
2.1. Iranian okra ecotype description Iranian okra ecotypes are categorized into two main groups, yellow and green, based on their fruit color. Ecotype descriptions were carefully recorded during two growing season of 2014 and 2015 at the research field of Tarbiat Modares University, Table 1 [53]. 2.1.1. Seed properties Seeds are relatively large and heavy, at approximately 65 g per 100 seeds. Un-ripened okra seeds are white and light yellow during seed development and become dark green or gray at full maturity [54] (Fig. 4). Because okra is an indeterminate plant, its seeds do not mature uniformly. However, our research on okra seed production showed that first seed harvest most commonly occurs 80 days after planting. At this time, the primary pods had dried and the seeds no longer required energy from the plant. Seed could be harvested three times from every crop, approximately 14 days apart.
1.3. Source identification of non-edible vegetable oil for biodiesel production in Iran This review projects the possible application in Iran of Abelmoschus esculentus as a new source of green vegetable oil for biodiesel production using an ultrasonic assisted transesterification reaction. Our focus was to discern whether this crop can easily be used as biodiesel feedstock due to its high adaptability to drought and less fertile soils, which are two main problems facing biofuel cultivators in the arid and semiarid regions of Iran. This study is different from previous studies in its method to produce biodiesel from okra seed oil, which uses an ultrasonic-assisted transesterification reaction. The study also is the first to report the fatty acid, oil yield, and seed oil content of three native Iranian okra ecotypes in order to consider what would be the best new summer energy crop to grown in Iran. In the process of vegetable oil conversion to liquid biofuel, transesterification is one of the main pathways toward biofuel production (Fig. 3).
2.1.2. Seed oil extraction The seeds of three Iranian okra ecotypes (Ahwaz, Isfahan, and Mashhad) were subjected to study to determine their seed oil content, oil fatty-acid profile, and biodiesel properties. Oil extraction was performed using an n-hexane solvent in Soxhlet. From each batch of okra, 100 g of okra seeds were crushed finely using a coffee grinder. The solvent was distilled off at 45 °C using a rotary evaporator [55]. 519
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Fig. 3. Pathways of biofuel production from different feedstocks [52].
Hielscher, Germany. The operating frequency of this ultrasound source was 24 kHz, and its vibration amplitude was 100% (Fig. 6). Ultrasonification provides a sufficient mixing and energy so that the transesterification can proceed at a faster rate [58,59]. This technique can destroy cell walls, increase the specific surface area, and reduce the degree of polymerization. As a result, it can provide a more degradability of biomass. This method has been garnering increasing interest because the lower energy input requirements to produce transesterification reactions has a smaller impact on the environment than other methods [60,61]. Furthermore, it provides shorter reaction times [48,49]. When the reaction time was up, we removed the biodiesel mixture from the reactor and put it into a cold ice bath to stop the reaction. The mixture was transferred to separation plate in order to remove glycerin from the biodiesel (Fig. 7). Glycerin is heavier than biodiesel, and it will settle to the bottom while the biodiesel will remain on top. After separating the biodiesel from the glycerin, we purified the biodiesel by subjecting to a water wash. Then we centrifuged it for five minute at 6000 rpm to remove any additional impurities. The biodiesel methylester content was calculated based on the following formula [50,51]:
Table 1 Iranian okra ecotypes descriptions. Ecotype
Height
Blossom time
Fruit color
Fruit character
Seedling establishment
Ahwaz
Tall
Yellow
Dwarf
Mashhad
Tall
Fat–moderate length Thin–long length Fat–moderate length
Good
Isfahan
Late season crop Short season crop Moderate season crop
Green Green
Good Good
The fatty acid profile of the okra seed oil was determined using Metcalfe procedure [56]. One microliter of sample was injected to Clarus 580 Perkin-elmer gas chromatography unit, which was equipped with an FTIR detector and BP-30 column that was maintained isothermally calibrated to meet BS-EN 14103 standard (Fig. 5). The total run time for each sample was 30 min. Fatty acid composition was determined by identifying and calculating relative peak areas. 2.2. Converting the oil to biodiesel using an ultra-sonic reactor Okra seed oil was transformed into biodiesel via transesterification using an ultra-sonic reactor. We selected this method due to its simplicity, low production cost, small equipment size, and high efficiency of biodiesel production [57]. To increase the purity of the fuel, the produced biodiesels were subjected to a water washing treatment. Methoxide was prepared by mixing methanol that had a molar ratio of 1:7, as an alcohol agent, with 1% potassium hydroxide (KOH), as a catalyst [55]. The applied ultrasound source was a UP400-S from
C=
∑ A − AIS MIS × ×100 AIS M
Where: ∑A was the total area under the peak of total fatty acids, AIS was the area under peak for internal standard (C19:0), MIS was the weight of internal standard, and M was the weight of the sample. We measured biodiesel viscosity, which is a measure of resistance to flow of a liquid due to the internal friction of one part of a fluid moving Fig. 4. Okra seeds at different maturity phases.
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Fig. 5. Perkin-Elmer gas chromatography equipped with auto sampler robotic arm.
Fig. 6. Ultra-sonic reactor UP400 for okra biodiesel production.
(Table 2). According to our results, genetic resource is a very important criteria determining seed oil quality of okra (Fig. 10). Mashhad and Isfahan had more similar chemical constituents to each other than they had to Ahwaz. Mashhad and Isfahan are both categorized as green pod okra, while Ahwaz, which is adapted to the warmer parts of the country, is categorized as a yellow pod. The Ahwaz ecotype has better oil quality, which could be attributed to the higher ratio of unsaturated fatty acids in the seed oil. The linoleic acid content of all the ecotypes studied is higher than that of rape seed (22%); corn (6%) [65], palm (10.1%), and castor bean (1.3%) [66].
over another, using an Anton Paar viscometer SVM 3000. 3. Results and discussion 3.1. Seed oil content The average oil content of the studied okra ecotypes was 19–20% (Fig. 8). Oil yield was more than 300 kg.ha−1. However, the higher seed production in Isfahan and Mashhad compensated for their lower seed oil percentage; thus, overall oil yield did not significantly differ among the three ecotypes (Fig. 9). Study of 238 okra varieties for seed oil content revealed that seed oil content is ranges between 17% and 22%; expected oil yield could be 289–612 kg ha−1 [62].
3.3. Okra biodiesel properties The biodiesel produced from okra seed oil contained more than 90% methyl ester and passed the EN 14214 standard for biodiesel (Table 3). Furthermore, it processed well in the ultra-sonic reactor. Higher viscosity biodiesels tend to have weaker vaporization and larger fuel droplets, which can cause undesirable combustion in the engine [68–70]. Among okra ecotypes, the Isfahan biodiesel exhibited the highest viscosity (2.463 mm2 s−1). Previous studies revealed that
3.2. Seed oil profile Okra seed oil consists of both saturated and unsaturated fatty acids. The seed oil of our ecotypes mostly contains C18:2 (Linoleic acid) (38–40%), which is also found in olive and peanut oils [63,64]. Palmitic acid (C16:0) (29–30%) is okra seed oil's second major constituent 521
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Fig. 7. Illustrations of okra biodiesel production. 21
the kinematic viscosity of different biodiesel derived from vegetable oils would fall within the narrow range of 4–5 mm2 s−1. Biodiesel from some vegetable oils has a slightly lower viscosity—coconut derived biodiesel showed a kinematic viscosity value of 2.75 mm2 s−1 [70]. The ASTM D6751 viscosity specification of 1.9–6.0 mm2 s−1 was satisfied with okra derived biodiesel, which had a kinematic viscosity range of 2.3–2.4 mm2 s−1.
Oil content (%)
20.5 20 19.5 19 18.5 Ahwaz
Isfahan
Mashhad
Okra Ecotype
Fig. 8. Iranian okra ecotypes seed oil content.
4. Conclusion 330
Different climatic conditions and vast areas of the country are pushing Iran toward becoming a highly populated country in the Middle East. As a result, it is a rapidly developing country with increasing energy demands. The environmental pollution and greenhouse gas emission concerns about fossil fuels, in addition to the limited number of fossil fuel reservoirs, have brought renewable and clean energy resources to the center of attention in the energy sector. Vegetable oils with suitable oil quality and environmental adaptability not only can help to provide clean and renewable energy, but also can create new job opportunities in both the agriculture and energy sectors of the country. Okra is a crop that has adapted well to Iran's climatic
Oil yield (kg/ha)
310
290
270
250 Isfahan
Ahwaz
Mashhad
Okra Ecotype
Fig. 9. Oil yield of Iranian okra ecotypes.
Table 2 Fatty acid composition of okra seed oil. Fatty acid composition
Myristic acid (C14:0) Palmitic acid (C16:0) Palmitoleic acid (C16:1) Margaric acid (C17:0) Stearic acid (C18:0) Oleic acid (C18:1) Linoleic acid (C18:2) Linolenic acid (C18:3) Arachidic acid (C20:0) Behenic acid (C22:0) Heneicosylic acid (C21:0)
Iranian okra ecotype
USA okra germplasm
Malaysian okra
Ahwaz
Isfahan
Mashhad
[67]
[9]
0.82 30.52 1.03 0.94 3.89 19.04 40.13 1.97 1.03 — 0.85
0.554 29.944 0.674 0.744 3.694 21.564 39.784 1.764 0.744 — 0.524
0.652 29.412 0.752 0.802 3.792 22.612 38.272 2.002 0.862 — 0.642
0.31 30.42 0.39 – 3.93 21.085 37.78 0.17 0.494 0.25 —
0.32 29.58 0.45 0.13 3.80 19.72 44.21 0.28 0.52 0.22 —
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Table 3 Biodiesel properties produced from okra and other vegetable oils. Crop
Ecotype
Methyl ester content (%)
Viscosity (mm2 s−1)
Flash point (°C)
Okra
Ahwaz Isfahan Mashhad [71] [72]
93.48667224 90.16709033 93.1295075 96 96.9
2.304 2.463 2.379 5.241 4.2
155 158 156 164 171
Sunflower Soybean
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