INTRODUCTION:

The world, as we know it, is faced with the grave problem of depleting levels of non-renewable energy resources. At the same time, the demand and consumption of energy is on the rise. According to the IEA (International Energy Agency), the transportation sector dominates the world oil consumption. The transportation sector of today is mostly dependent on petroleum. Intensive utilization of fossil fuels such as petroleum and gasoline, has led to the generation and release of greenhouse gases (GHG) into the atmosphere, which is the primary cause of global climate change and global warming. More than 70% of all GHG emissions come from transportation sector, especially road transport [1].

With the world’s oil reserves running low, there has been a dramatic increase in the price of petroleum worldwide. Due to concerns about global warming, low reserves of fossil fuels and health issues, the search for renewable sources of energy, which would help reduce GHG emissions, has become a matter of utmost importance.

Compared to petroleum, biofuels or biomass-based fuels offer many advantages [2]. Raw materials for biofuel production are easily available. Biofuels are considerably environment friendly, they are biodegradable, nontoxic, and has low emission profiles, and so it is environmentally beneficial and contribute to sustainability[3]. Bioethanol, which is a biofuel, is a potential alternative to petroleum [4]. It was first used by Nikolas Otto in the internal combustion engine (ICE) in 1897 [5]. Bioethanol has also been used as fuel in Brazil since 1925. Gasoline-ethanol blends can be used in all types of gasoline run engines [6]. Ethanol is a harmless substitute to methyl tertiary butyl ether (MTBE) that is used as an additive to gasoline for obtaining a cleaner combustion [7]. MTBE is a lethal compound that contaminates ground water. Bioethanol is a renewable source of energy that can be derived from a range of feedstock like wheat, straw, sugar beet, rice, corn and wood. Lignocellulosic biomass is a potential starting material for ethanol. It includes agricultural wastes, herbaceous crops, forestry wastes, waste paper and other cellulosic wastes [8]. However, the present bioethanol production process has some significant barriers that need to be addressed. For instance, there is a deficit of efficient enzymes that can function well in varying temperatures and pH, yet are cheap and economical. However, there are a few major bottlenecks in the current production of biofuels. Also, inexpensive fermentation processes are required for efficient break-down of derived sugars obtained from cellulose [9]. The process requires enzymes to be stable and active at low or high pH.

Other names of bioethanol are ethyl alcohol, grain alcohol and ETOH. It is chemically represented as CH₃-CH₂-OH. Bioethanol is an oxygenated fuel that contains 35% oxygen. This boosts the combustion process and cuts down the production of GHG, hydrocarbons and particulate matter. Octane number of bioethanol is 108. When compared to gasoline, it has a broad flammable limit, high flame speed, and high heat of vaporization, which allows for a greater compression ratio, lesser burn time and thinner burn engine that give efficiency advantages over gasoline in an ICE [10]. Energy contained in ethanol is 66% of that in gasoline. Bioethanol is erosive, has little flame brightness, and has low vapour pressure that makes cold starts troublesome. It mixes quite well with water but is hazardous to ecosystems [11]. The most favoured fuel mixture for light-duty vehicles is E85 containing 85% bioethanol and 15% gasoline [12].

This review aims at describing the various raw materials, particularly the lignocellulosic materials used in the large scale production of bioethanol commercially.

Feedstocks for bioethanol production:

Sources of raw materials for producing bioethanol are many, of which three major sources are: sucrose containing feedstocks (eg. sugarcane, fruits, sugar beets, etc.), starchy materials (wheat, rice, barley, corn, etc.) and lignocellulosic materials (wood, straw, grasses). Bioethanol from rice, wheat, corn and sugarcane straws is now gaining popularity worldwide [13]. Currently, the major emphasis is on generating bioethanol from agricultural wastes. Though there are a few concerns related to the raw materials to be used for bioethanol production like chemical make-up of the biomass, farming methods, vacancy of fields for farming, use of pesticides, soil erosion, formation and upkeeping of employment, resource consumption, issuance of GHGs, acidic and ozone degrading gases, loss of biodiversity and aesthetic values, biomass costs, transportation and storage costs, water requirements and availability, location, and seasonal availability of feedstocks [3]. Globally, Brazil utilizes sugarcane, while the United States and Europe utilizes starch from wheat, barley and corn for the production of bioethanol.

Sucrose-containing feedstock:

This group mostly contains biomass from sugarcane and sugar beet [14]. While sugarcane is found in equatorial counties, sugar beet is found in temperate counties only. Brazil is the single biggest manufacturer of sugarcane in the world. In Asian countries sugarcane is grown on small fields owned by small-scale cultivators [15]. European countries mostly use beet molasses as the primary sucrose containing raw material [16]. The advantages of sugarcane and sugar beet include a smaller crop cycle, bigger harvest, greater resilience towards climatic variations, less water and fertilizer requirements. Sugar beets need 35-40% less water and fertilizers than sugarcanes [17].

Starchy feedstock:

Starch is a homopolymer, made up of D-glucose monomers [18]. For production of bioethanol using starch sources, it is necessary to split the chains of this carbohydrate into glucose units that is fermented by yeasts to yield ethanol. Hydrolysis converts starch into fermentable sugar using water. [19]. It could be either enzymatic or acid hydrolysis. When starch is hydrolysed by the enzyme amylase, the process is industrially known as liquefaction. Factors like choice of substrate, enzyme activity and reaction conditions like temperature and pH affect the enzymatic hydrolysis of starch [20].

Corn and wheat fall under the starchy feedstock category and are majorly used for bioethanol production in North America and Europe [16]. About 5% of corn is wasted worldwide. This wasted corn, if utilized properly, can produce 9.3GL of bioethanol that can replace 6.7GL of gasoline [21]. However, since corn prices vary annually from place to place, it poses a small disadvantage for the industry [22]. Like corn, barley can also be used as feedstock for bioethanol production. Wasted barley, amounting to 3.4% worldwide, has the potential of producing 1.5GL of bioethanol and replace 1.1GL of gasoline [21].

China is the largest producer of wheat, closely followed by India. Since wheat straw is available in plenty as agriculture waste, it can be used as an attractive raw material for producing bioethanol [23].

Lignocellulosic feedstock:

Lignocellulosic biomass includes agricultural wastes such as corn stover, wood, rice and wheat straws. Lignocellulosic materials are the most appealing raw materials for manufacturing bioethanol due to its abundance and reproduciblility [24]. Advantages of lignocellulosic feedstocks are good produce, less expenditure, suitability to less fertile soil and low environmental impacts. Amount of lignin in most feedstocks is nearly 27%, although bit grasses contain substantially less [25]. One of the main challenges of producing bioethanol from lignocellulosic biomass is the high cost of hydrolysis process [26].

Around 731 million tons of rice straw is produced annually worldwide which has the potential to produce around 205 billion litres of bioethanol annually, making it a major lignocellulosic raw material [27, 28].

Sources of Lignocellulosic feedstock:

Forest woody feedstocks:

Woody feedstocks are of two types: hardwoods and softwoods. Softwoods are derived from coniferous trees and gymnosperms. Unlike hardwoods, softwoods have less solidity and develop quickly[29]. Gymnosperms mostly consist of evergreen plants like cedar, pine, spruce, cypress, redwood, fir and hemlock[30]. Hardwoods on the other hand are angiosperms, mostly deciduous trees like oak, aspen, willow, maple and elm[31]. Woody raw materials have flexible reaping times and avoid long periods of dormancy in storage [32]. Woody feedstocks have more lignin and less ash content than agricultural residues. Forestry wastes like sawdust from saw mills, wood chips and dry branches can also be used as raw material [33].

Agricultural residues and municipal solid wastes (MSW):

Rice and wheat straws and corn stalks have the maximum potential to be used as feed for bioethanol production. Crop residues have more hemicellulose content than woody biomass (25-35% approx.) [34]. Agricultural residues are environmentally friendly and prevents dependency on woody biomass and hence decrease deforestation. Switch grass is a kind of herbaceous prairie grass grown as an energy crop in the northern parts of North America and Canada. Due to low costs of financing, large quantities, disease resistance, low maintenance and high sugar yields per acre, these perennial grasses are potential candidates for the production of bioethanol. Miscanthus giganteus is yet another fast growing grass that is a potential candidate for bioethanol production [35].

Apart from cellulosic feedstock, solid wastes from municipality and industries can be used as raw materials for bioethanol production. The use of such wastes restricts environmental problems like disposal of household garbage, processing of industrial wastes like papers, food, liquors and pulps, etc. [36]. Even though enough biomass can potentially be available to replace 30% of the petroleum-derived gasoline by the year 2030, potential lands and feedstocks if not overseen and developed properly will result in the price of biomass being considerably high, which will be a serious drawback [33, 36].

Marine algae:

Algae has been seen as a potential feedstock for biofuel production in the U.S. since 1978. Assessment of several aspects of algal biomass like water uptake, productivity per acre, by-product and co-products recovered during bioethanol production were carried out, but further improvements to its efficiency and competence were limited in the 20^th century thus hindering its development as a feasible raw material. But at present, algal biomass is gaining importance as third generation feedstock for biofuel production [37]. Besides being a prospective raw material for bioethanol production, algae can also be used as feedstock for biodiesel and aviation fuel production [38]. Algae are capable of generating 10x the volume of ethanol produced per growing area by corn [39]. Algal biomass can be grown in unclean water like sewage or polluted water systems. Due to its capability of absorbing large volumes of CO₂while growing, it acts as a very efficient carbon sink [40].

Composition of Lignocellulosic material:

Lignocellulosic materials have three main components: cellulose (~40%), hemicellulose (~25%) and lignin (~15%) [41]. The hemicellulose and cellulose are firmly bound to the lignin component by covalent and hydrogen bonding, making the structure tough, inflexible and resistant to treatment.

Hemicellulose:

Hemicellulose, also known as polyose, is made up of heteropolymers of hexoses and pentoses. It sometimes also contain sugar acids like D-glucuronic, D-galacturonic and methylgalacturonic acids [42], and has a backbone of xylan β(1à4) linkages [41]. Xylan composition is never the same in two different feedstocks [43]. Due to the presence of different sugars, hemicellulose requires many enzymes to be completely hydrolysed to free monomers. However, hemicellulose is easier to breakdown than cellulose due to its branched structure and amorphous nature [44].

Cellulose:

Cellulose fibres provide strength to wood and comprise 40-50% weight of dry wood [45]. Cellulose from plant cell walls contains long chains of glucose units linked by β(1à4) glycosidic bonds, with extensive hydrogen bondings in between molecules, giving a crystalline and sturdy matrix structure [46]. Each cellulose molecule can have up to several thousand glucose units. The molecules are strong and compact due to the cross linkages of numerous hydroxyl groups. Cellulose requires a temperature of 608°F and a pressure of ~247 atm to shift from its crystalline structure to an amorphous structure in water [47].

Lignin:

Lignin is a complex biopolymer that is aromatic and rigid in nature. It contains covalently bonded xylan that provides a good amount of solidity to plant cell walls [48]. Among woody biomass, softwood barks contain the most lignin (~45%) followed by hardwood barks (~42%) while agricultural wastes contain the least (~9%) [49, 50]. Lignin components have been received with little interest previously, but are now becoming popular since it can dilute fermentation in bioreactors [51]. Degradation of lignin results in phenolic groups that significantly deactivates cellulolytic enzymes. This negative effect of lignin has led to interests in lowering the lignin negative effect [52]. Although lignin modification via genetic engineering practises that target biosynthetic pathways could be performed to decrease lignin generation and enhance ethanol production, this process may be problematic as lignin acts as the major plant defence system against pathogens and insects. Thus modifications can lead to disruption of the plants’ defence mechanisms [53, 54].

Lignocellulosic feedstock processing to bioethanol:

Although processing of lignocellulosic materials involve quite a number of steps, industries carry of different combinations of these steps to achieve the desired percentage of ethanol. Some of the major methods of processing have been described in this article.

Pre-treatment:

The primary step in conversion of lignocellulosics is size reduction and pre-treatment [55]. Pre-treatment is carried out to remove structural or compositional defects so as to enhance the action of enzymes during the fermentation stage [56]. Pre-treatment can be quite expensive but extremely necessary. During pre-treatment, long chains of cellulose and hemicellulose need to be fragmented to decrease crystallinity and increase amorphousness of cellulose, so as to enhance catalysis by enzymes [57]. Pre-treatment is done to advance sugar formation using enzymes, prevent deterioration of carbohydrates, prevent the production of inhibitory by-products, to decrease energy consumption and also to bring down the production costs [13, 26]. Pre-treatment of lignocelluloses requires several physical, physico-chemical, chemical and biological processes.

Physical pre-treatment:

Mechanical comminution:

Comminution is a technique for reducing a material to smaller fragments or particles. Wastes can be comminuted by chipping, grinding, and milling to decrease cellulose crystallinity [26] and improve the efficiency of downstream processing. The size of the materials after chipping is ~20mm, and ~1.1mm after grinding or milling [58]. There are quite a few milling techniques used industrially, like vibratory ball mills, wet mills, dry mills and compression mills [13]. Vibratory mills are more effective than ordinary ball mills for decreasing cellulose crystallinity and enhancing digestibility [59]. Power requirements depend on the initial and final particle size, moisture content, and nature of the waste [60]. Size reduction may be problematic for subsequent steps by generating clumps in steps involving liquid, and may thereafter lead to channelling [13]. Mechanical pre-treatment techniques can take long time to finish, are energy demanding and expensive. [8].

Pyrolysis:

Pyrolysis the process of thermo-chemical degradation of a compound at high temperature, usually in the absence of oxygen. Here, biomass is heated to temperatures beyond 300°C, which rapidly disintegrates cellulose to produce H₂, CO, and residual char [61]. This process is slow and often give rise to less volatile products at low temperatures [57, 62]. Residual char can be leached with water or mild acid [13]. Studies show a high conversion rate of 80-85% of cellulose to reducing sugars with glucose content of around 50% using pyrolysis [63].

Physico-chemical pre-treatment:

Steam explosion:

Steam explosion, also known as autohydrolysis, is one of the biomass fractionation processes. Here, small pieces of biomass are treated to steam at high pressure. The pressure is then rapidly decreased that leads to an explosive decompression of the materials [26]. The process is initiated at 320-500°F along with a pressure of 0.7-4.8MPa for a few minutes after which the material is exposed to atmospheric pressure. Effects of steam explosion on lignocellulosics are [65]: (1) increase in crystallinity, (2) hydrolysis of hemicelluloses, (3) promotes delignification, and (4) increases cellulose hydrolysis. Factors affecting steam explosion are temperature, particle size, residence time and moisture content [66]. Steam explosion require low energy and has no environmental costs. 70% more energy is required by mechanical methods to obtain the same size reduction as steam explosion [67]. Steam explosion may cause degradation of portions of xylan, generation of inhibitory compounds and partial splitting of lignin-carbohydrate matrix [68].

Ammonia fibre explosion (AFEX):

AFEX is an alkaline physico-chemical pre-treatment process. Here, the material is treated with liquid ammonia at high temperature and pressure, followed by rapid decompression. This results in quick saccharification of lignocellulosic material [69]. Normally, 1-2 kg of liquid ammonia is used per kg of dry biomass at 363K and incubated for 30 minutes [26]. This process allows enzymes to act on polymers and reduce them to sugars [70]. AFEX works moderately and is not advisable for biomass that contains larger amounts of lignin. Since grasses contain relatively lower amounts of lignin (15-20%) than hardwood and softwood (20-35%), AFEX can be used for grasses [71]. Ammonia should be recycled to reduce cost and prevent environmental degradation.

Liquid hot-water pre-treatment (LHW):

This method has been used for several decades in pulp industries. Here, biomass is subjected to hot water at high pressure and around 473-503K for approximately 15 minutes. This process produces less quantities of inhibitors and has high recovery rates for pentoses [72]. Around 40-60% of the total mass is dissolved in this process [73].

Chemical pre-treatment:

Ozonolysis:

Ozonolysis uses ozone gas to split up lignin and hemicellulose chains and enhance cellulose digestion. It is carried out at room temperature and removes lignin without formation of toxic compounds [74]. Ozonolysis is used worldwide to reduce lignin content of agricultural and forestry wastes [75]. However, the process is not very economical as large quantities of ozone is required.

Alkaline hydrolysis:

Alkaline solutions are used in this process to expel lignin and various uronic acid substitutions from hemicelluloses. This improves enzyme accessibility and enhances biocatalysis [76]. Saponification of inter-molecular ester bonds cross-linking xylan hemicelluloses and other components like lignin is the underlying mechanism of alkaline hydrolysis. When cross-links are removed, the porosity of the biomass also increases [77]. This process requires lower temperatures and pressures when compared to other pre-treatment technologies.

Acid hydrolysis:

Acid hydrolysis is a technique used to obtain high sugar yields from biomass, using concentrated or dilute acids like H₂SO₄, HNO₃ or HCl at temperatures between 130-210°C. This process removes hemicelluloses and uncovers cellulose for better action of enzymes [78]. The concentrated acids used should be recycled to make the process affordable [79]. There are two categories of dilute acid pre-treatment processes: (1) temperature higher than 160°C, continuous flow process for low solids loading (5-10% w/w) [80], and (2) temperature lower than 160°C, batch process for high solids loading (10-40% w/w) [81].

Wet oxidation:

In wet oxidation, biomass is treated with water in presence of oxygen at temperatures above 120°C [82]. Water is added at a ratio of 1 litre per 6 grams of biomass. Hemicelluloses are converted from solid to liquid phase by this technique. Sugar oligomers are the end products of hemicellulose hydrolysis by wet oxidation.

Oxidative delignification:

Peroxidase enzyme is used to degrade lignin in presence of H₂O₂ [83]. Studies have shown that this process enhances the susceptibility of lignocellulosic materials to enzymatic hydrolysis. Peroxidases have the ability to solubilize about 50% lignin and most hemicelluloses in presence of 2% H₂O₂ at 30°C within 8 hours. Efficiencies can be as high as 95% glucose production from cellulose when treated with cellulase at 45°C for 24 hours [84].

Biological pre-treatment:

In biological pre-treatment processes, microorganisms especially different types of fungi are used to degrade lignin and hemicelluloses in waste materials [85]. Brown rots attack cellulose while white and soft rots attack both cellulose and lignin. Although in most cases the rate of hydrolysis is very low, this method is safer, saves more energy and requires mild conditions due to less mechanical support [23]. It does not need any chemicals but low hydrolysis and less yields hinders its implementation. Bio-delignification generally requires longer time.

There are several factors that are responsible for the low cost and an advanced pre-treatment [86], like high yield for multiple crops depending on site, ages and harvesting times; highly digestible pre-treated solids; negligible sugar degradation; minimum quantities of toxic compounds; size reduction not being actually necessary; process should be carried out in reasonably sized and moderately priced reactors; non-production of solid waste residues that may create disposal issues; effectiveness of the process at low moisture content; fermentation compatibility; lignin recovery; and, minimum mechanical support like heat and power requirements.

Enzymatic hydrolysis:

Cellulase is the primary enzyme used to carry out enzymatic hydrolysis of cellulose. Reducing sugars like glucose are the final products of hydrolysis. When compared to acid or alkaline hydrolysis, the cost of enzymatic hydrolysis is quite less. It is carried out usually at mild conditions with pH of around 4.8 and temperature ranging between 45-50°C. This technique does not have a corrosive problem [66]. Since microorganisms like bacteria and fungi are capable of producing cellulases for their own metabolic activities, these organisms can be used to produce cellulose for enzymatic hydrolysis for cellulose on an industrial scale. These microbes can be aerobic or anaerobic, mesophilic or thermophilic. Bacteria belonging to species like Bacillus, Erwinia, Clostridium, Cellulomonas, Ruminococcus, Dacteriodes, Acetovibrio, and Streptomyces can produce cellulases [86]. Anaerobes such as Clostridium produce cellulases with high specific activity, but often it is found that these enzymes have low titre values [66]. Since anaerobes have a low growth rate and need to grow in the absence of oxygen, most research for commercial cellulase has been focussed on fungi [66].

Fungal species like Trichoderma, Aspergillus, Schizophyllum, and Penicillium [87] have been reported to produce cellulases. The most extensively studied organism for cellulose production is Trichoderma. This species has been widely used for enzymatic hydrolysis because of the production of a substantial amount of cellulase. However, it is unable to produce sufficient amounts of β-glucosidase, and hence needs to be used in a concoction along with other fungal species that are able to produce β-glucosidase.

There are three major groups of cellulases required in enzymatic hydrolysis: (1) endoglucanase, which attacks in the middle of the cellulose polymer in regions of low crystallinity, creating free chain-ends; (2) exoglucanase or cellobiohydrolase, which acts on the free chain ends and degrades the molecule further by removing cellobiose units; and (3) β-glucosidase, which hydrolyses cellobiose to glucose [88]. During enzymatic hydrolysis, cellulose is broken down by cellulases to form reducing sugars that can be fermented by yeast or bacteria to give ethanol. Factors affecting enzymatic hydrolysis are substrate concentration, cellulase activity, and reaction conditions. Low substrate concentrations can lead to more yields, while high substrate concentrations may lead to substrate inhibition resulting in decreased rates of hydrolysis [89, 90]. Lignin hinders hydrolysis by obstructing access of cellulases to cellulose and by forming irreversibe bonds with hydrolytic enzymes. Thus, removal of lignin for efficient hydrolysis is essential. Cellulase activity is inhibited by cellobiose and glucose.

Some substrate-related factors limiting enzymatic hydrolysis are: (1) Cellulose crystallinity, where degree of polymerization and cellulose crystallinity determine the hydrolysis rates of relatively refined cellulosic substrates [91]; (2) Number of glycosyl residues per cellulose molecule, where decreased chain length reduces the degree of polymerization thereby having an impact on cellulose hydrolysis; (3) Available surface area on substrates, where increase in substrate surface area increases the accessibility of the substrate to the cellulolytic enzymes; (4) Lignin barrier, makes accessibility of cellulase enzymes to cellulose difficult thereby limiting the rate enzymatic hydrolysis; (5) Removal of hemicellulose widens the pore size of the substrate and thus enhances the accessibility and probability of the cellulose to become hydrolysed [92]; (5) Feedstock particle size affects the accessibility of cellulose to enzymes [26]; (6) Porosity, that helps cellulases to get trapped in the pores if the porosity is high, which improves hydrolysis; (7) Waxy barriers present in plant cell walls impedes penetration of enzymes; (8) Change in accessibility of glucan with conversion [93].

Fermentation:

Saccharified biomass is used for fermentation using several microorganisms. For ethanol production on an industrial scale, an ideal microorganism should be selected which has wide substrate specificity, high ethanol production, resistant to high temperatures and pH, tolerant to inhibitors and have cellulolitic activity. Generally, the process employs fermentation of lignocellulosic hydrolysate in SSF which is quite cost effective.

Fermentation is a process by which microorganisms use fermentable sugars as food and produces ethyl alcohol as by-product. These microorganisms typically utilize 6-carbon sugars like glucose. Yeast or Saccharomyces cerevisiae is the most useful fungi used in fermentation processes. Yeast has high bioethanol production from hexoses and has high tolerance to bioethanol and other inhibitory compounds. Bacteria showing the most promise for industrial exploitation are E. coli, Klebsiella oxytoca and Zymomonas mobilis [94]. Productivity of microorganisms depend on the process parameters like temperature, pH, productivity, yield, alcohol tolerance, osmotic tolerance, substrate specificity, growth rate, genetic stability, and inhibitor tolerance [94]. The microorganisms function best between 303-311 K [95]. They generally prefer a narrow pH range of 6.5-7.5 [96]. The majority of microorganisms survive in bioethanol concentrations above 10-15% (w/v) [95]. Fermentation can be carried out in batch, fed-batch or continuous reactors, depending upon the kinetic properties of microorganisms and the type of biomass used [96]. Fermenters helps in monitoring parameters like temperature, pH, agitation speeds, foam formation, dissolved and critical oxygen levels, etc., that are important for microbial growth in the industry.

DISCUSSION:

The demand for biofuels is on the rise due to the depletion of non-renewable energy sources and the need for clean, green fuels. Bioethanol is currently used as a biofuel worldwide, and is being developed as a transport fuel to be used as an alternative to gasoline in the near future. It is currently used as a blended automobile fuel to significantly reduce petroleum consumptions and exhaust greenhouse gas emissions. Bioethanol is derived from lignocellulosic feedstock like rice, wheat, sugarcane, sugar beets, grass straw and wood. A lot of studies are being carried out to find out good microbial sources for the enzymatic hydrolysis and fermentation of cellulose to produce bioethanol. It has been seen that biomass containing high levels of glucose or precursors of glucose are easiest to convert to bioethanol. Genetically engineered microbes are created to produce adequate amounts of cellulases and also to enable them to remain active in environments with higher product concentrations.

ACKNOWLEDGEMENTS:

I would like to thank the management of VIT University for the facilities provided and the constant support.

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Received on 27.06.2017 Modified on 08.07.2017

Research J. Pharm. and Tech. 2017; 10(8): 2750-2758.

DOI: 10.5958/0974-360X.2017.00488.7