A comparative study of various oil extraction

Rev Chem Eng 2014; 30(6): 605–626

Jibrin Mohammed Danlami, Agus Arsad*, Muhammad Abbas Ahmad Zaini and Hanizam Sulaiman

A comparative study of various oil extraction techniques from plants Abstract: Researchers have shown that techniques such as microwave-assisted extraction, ultrasound-assisted extraction, pressurized liquid extraction, and supercritical fluid extraction developed for extraction of valuable components from plants and seed materials have been successfully used to effectively reduce the major shortcomings of the traditional method such as Soxhlet extraction. These include shorter extraction time, increase in yield of extracted components, decrease in solvent consumption, and improvement of the quality of extracts. This review presents a detailed description of the principles and mechanisms of the various extraction techniques for better understanding and summarizes the potential of these techniques in the extraction of oil from plants and seed materials. Discussions on some of the parameters affecting the extraction efficiency are also highlighted, with special emphasis on supercritical fluid extraction. A comparison of the performance of traditional Soxhlet extraction with that of other extraction techniques is also presented. Keywords: microwave assisted extraction; pressurized liquid extraction techniques; Soxhlet extraction; supercritical fluid extraction; ultrasonication assisted extraction. DOI 10.1515/revce-2013-0038 Received November 18, 2013; accepted July 30, 2014; previously published online September 4, 2014

*Corresponding author: Agus Arsad, Enhanced Polymer Research Group (ENPRO), Faculty of Chemical Engineering, Department of Polymer Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Malaysia, e-mail: [email protected] Jibrin Mohammed Danlami and Muhammad Abbas Ahmad Zaini: Centre of Lipid Engineering and Applied Research (CLEAR), Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Malaysia Hanizam Sulaiman: Centre for Information and Communication Technology (CICT), Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Malaysia

1 Introduction Extraction is an important step for the separation, identification, and use of valuable compounds from different plants (Stevigny et al. 2007). The choice of an acceptable technique to obtain maximum yield and highest purity varies according to the nature of the target compound. Numerous chemical and mechanical processes like solvent extraction and steam distillation are used for the extraction of compounds from plants (Shirsath et al. 2012). The existing techniques used for the extraction of essential oils, fat, and oils include Soxhlet, hydrodistillation, and maceration with alcohol (Wang and Weller 2006). The mass transfer resistances due to the involvement of more than one phase within the system repeatedly limit the use of traditional Soxhlet extraction techniques (Jadhav et al. 2009). This separation method requires a very long time depending on the diffusion rates of solvents. Furthermore, standard extraction techniques are energy intensive (Puri et al. 2012). These techniques are manual processes, and reproducibility is a major challenge (Shen and Shao 2005). Thermally sensitive components are deteriorated by the heating process, resulting in low extraction yields. These active molecules might be altered by the pH, temperature, and pressure conditions used. The limitations mentioned above, combined with the significant increase in the demand for bioactive components, essential oils, fat, and oils, have prompted the need for appropriate, selective, cost-saving, and eco-friendly extraction technologies that are rapid, produce higher yields, and comply with relevant legislation (Ibáñez et al. 2012). This has led to the development of novel extraction processes, such as supercritical fluid extraction (SFE), to enhance the product quality and the quantity of the active natural products (Sajfrtová et al. 2010, Bimakr et al. 2012). In the last few years, SFE has received significant attention as a promising alternative to conventional technology for separation of various valuable compounds from natural sources (Gomes et al. 2007, Liu et al. 2010). This is because the technique is generally performed at low temperatures and short extraction times and a little amount of solvent is used as compared with traditional extraction

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606 J.M. Danlami et al.: A comparative study of extraction methods (Liza et al. 2010). Supercritical carbon dioxide (SC-CO2) extraction has attracted a lot of interest because carbon dioxide (CO2) is an inert, low-cost, nontoxic, and environmentally-friendly solvent that permits extraction at low temperatures and comparatively low pressures. In addition, CO2 can evaporate instantly when exposed to atmospheric conditions (Herrero et al. 2010). As CO2 is a nonpolar solvent, adding a little quantity of polar solvents as cosolvent can greatly enhance the extraction efficiency of polar compounds. Among the prominently used solvents, ethanol (EtOH) is the most commonly used because of its high miscibility with CO2, nontoxicity, and allowed use in the food and pharmaceutical industries (Herrero et al. 2010). SC-CO2 has been found to be selective in the isolation of desired compounds without leaving any toxic residues in the extracts and with no risk of thermal degradation of the processed product. In reality, SC-CO2 extracts are most often recognized as safe to use in food products (Gerard and May 2002). Extraction with SC-CO2 has become comparatively mature with potential applications for the extraction of valuable compounds from solid plant matrices and seed oil. Numerous review articles have been published on Soxhlet extraction (Luque de Castro and Priego 2010), microwave-assisted extraction (MAE) (Kaufmann and Christen 2002, Tripti et al. 2009), ultrasound-assisted extraction (UAE) (Vinatoru 2001, Patist and Bates 2008, Vilkhu et al. 2008), pressurized liquid extraction (PLE) (Kaufmann and Christen 2002, Smith 2002), and SFE of vegetable materials (Sovová and Stateva 2011). The present review focuses on issues arising from the applications of several extraction techniques such as Soxhlet extraction, MAE, UAE, PLE, and SFE. The principle and mechanisms of each extraction technique and factors affecting SFE, such as temperature and pressure, effects of modifier, extraction mode, and extraction time, are discussed. Finally, potential applications and demerits of these extraction methods are also reviewed.

extraction, is the most referenced technique for evaluating the performance of other solid-liquid extraction methods except in restricted fields of applications, such as the extraction of thermolabile compounds (Luque de Castro and Priego 2010). An overview of Soxhlet extraction of solid materials has been reported by Luque de Castro and Priego (2010). Figure 1 shows the standard Soxhlet system. The seed materials (solids) are placed in the thimble-holder and filled with condensed fresh solvent from a distillation flask. As the liquid reaches an overflow level, a siphon aspirates the solution of the thimble-holder and unloads it into the distillation flask, carrying extracted solutes into bulk of the liquid. In the solvent flask, solutes are separated from the solvent using distillation. Solutes are left in the flask and fresh solvent passes into the solid bed. The operation is repeated until complete extraction is achieved. Soxhlet extraction and heat reflux extraction are not the same process. Heat reflux extraction can be performed simply by boiling the material in the solvent, where a chilled surface is used to condense the rising solvent vapors as they boil off and return them to a liquid state in the container, without boiling away. The extract continues to concentrate in the solvent and is reduced to essence later. Soxhlet extraction, on the other hand, is for separating parts that are soluble in a solvent.

Condenser

Extractor

2 Extraction techniques

Siphon Sample

2.1 Traditional Soxhlet extraction 2.1.1 Principles of operation

Distillation flask

Basic techniques for the extraction of fat and oils from seed matrices are based primarily on the selection of solvent, including the use of heat and agitation. Soxhlet extraction, which has been the oldest method of

Heat source

Figure 1 Soxhlet extractor.


J.M. Danlami et al.: A comparative study of extraction 607

2.1.2 Solvent selection An appropriate extracting solvent ought to be selected for the extraction of targeted component using the Soxhlet extraction technique. Different solvents will yield entirely different extracts and extract compositions (Zarnowski and Suzuki 2004). The most generally used solvent for extraction of edible oils from plant sources is hexane. This is due to its low boiling temperature and easy recovery and since most oils are soluble in hexane. The disadvantage of n-hexane is its hazardous air pollution level (Mamidipally and Liu 2004). The use of alternative solvents such as isopropanol, ethanol, hydrocarbons, and water has increased because of environmental, health, and safety considerations. d-Limonene and hexane have been employed in the extraction of oil from rice bran (Mamidipally and Liu 2004). It was observed that d-limonene extracted a considerably higher quantity of oil than hexane did under any given set of conditions. At a pH of 12, water (H2O) was used to extract rice bran oil (Hanmoungjai et al. 2000). The oil extracted using the aqueous medium had a lower content of free fatty acid (FFA) and color imparting than did oil extracted using hexane. Low FFA offers low initiation of products’ oxidation and coloring of materials. However, using different solvents often results in less recovery because of a decrease in molecular affinity between solvent and solute. The costs of alternative solvents such as acetone and ethanol (hexane is the standard) may be higher. A cosolvent is commonly added in order to increase the polarity of the liquid phase. A mixture of solvents such as isopropanol and hexane has been reported to increase the yield and kinetics of the extraction (Li et al. 2004).

2.1.3 B enefits and drawbacks of Soxhlet extraction techniques The benefits of conventional Soxhlet extraction method include (a) keeping the system far from equilibrium by constantly exposing the solid matrix to fresh solvent, (b) maintaining high extraction temperature to enable recovery of the compounds of interest, and (c) not requiring filtration after leaching. Additionally, the Soxhlet extraction is a very simple and a low-cost technique (Luque de Castro and Priego 2010). The major disadvantages of conventional Soxhlet extraction method include the following: (a) the extraction time is lengthy and the process is labor intensive; (b) a considerable amount of solvent is consumed; (c) agitation cannot be provided in the extraction device to speed up

the process; (d) the large solvent used needs an evaporation/concentration procedure; (e) there is risk of thermal decomposition of the target compounds; (f) there is no selective extraction; and (g) the process allows manipulations of limited variables. The time and the requirement of a large amount of solvent result in wide criticism of Soxhlet extraction technique (Luque de Castro and Priego 2010).

2.2 MAE A microwave is referred to as a nonionizing electromagnetic radiation that has a frequency of 300 MHz–300 GHz. MAE transfers energy to the heated solvent such as methanol or methanol/water mixture for polar compounds and hexane for nonpolar compounds by twin mechanisms of dipole rotation and ionic conduction. The target for microwave heating, in most cases, is dried plant materials; however, the plant cells still contain traces of moisture. The moisture, when heated up within the plant cell due to microwave effect, evaporates and generates high pressure on the cell wall and results in swelling of the plant cell (Vivekananda et al. 2007). This pushes, stretches, and consequently ruptures the cell wall. These facilitate leaching of active constituents from the cells to the solvent, thereby enhancing the yield of oil. This phenomenon could be intensified as the solvent is impregnated into the plant matrix at a high heating efficiency of the microwave. The bonds of cellulose (the main constituent of plant cell wall) break down (hydrolyzed) at high temperature attained by microwave radiation and are converted into soluble fractions in 1–2 min. The attainment of higher temperature by the cell wall during MAE leads to the dehydration of cellulose and thereby reduces its mechanical strength and also enhances solvent movement to the compounds inside the cell (Latha 2000). A study of cell damage in MAE experiments on tobacco leaf was conducted by scanning electron microscopy (Zhou and Liu 2006). Scanning of the untreated sample, MAE sample, and a sample from heat-reflux extraction revealed no structural distinction between heat-reflux extraction and the untreated samples, except slight ruptures of the sample surface. However, the sample surface was greatly destroyed after MAE. This suggests that microwave treatment affected the structure of the cell as a result of the sudden rise in temperature and internal pressure increase. During the process, exudation of the chemical substance occurring in the cell to the surrounding solvents takes place. This mechanism of exposing the sample to the solvent is not different from that of heat-reflux extraction


608 J.M. Danlami et al.: A comparative study of extraction that solidly depends on permeation and solubilization processes of bringing the analytes out of the matrix. A destructive change in orange peel as a result of microwave treatment was also observed using scanning electron micrographs (Kratchanova et al. 2004). The changes in plant tissue because of microwave heating gave an increased yield of extractable pectin. Furthermore, the movement of dissolved ions increased solvent penetration into the matrix, thereby facilitating the release of chemicals. It was also observed during the extraction of essential oils from plant sources that MAE permits the desorption of compounds of interest out of the plant. This is a result of the targeted heating of the free water molecules in the gland and vascular systems, which leads to localized heating leading to enlargement and subsequent rupture of their walls, thereby allowing oil to flow toward the organic solvent (Garcia-Ayusa et al. 2000, Kubrakova and Toropchenova 2008). In fact, microwave energy is dependent on the dielectric susceptibility of the solvent and plant matrix. Most of the time, the sample is immersed in a solvent that absorbs microwave energy strongly. An increase in the solvent temperature allows its penetration into the sample and the constituents are released into the hot solvent (Routray and Orsat 2012). However, in some cases, only selective heating of the sample matrix is brought about by immersing the sample in a microwave transparent solvent (hexane and chloroform). This approach is useful for thermolabile compounds to prevent their degradation (Vivekananda et al. 2007). There are two types of MAE systems, the closed vessel and the open vessel. Closed vessels are for the extraction of target compounds at high temperature and pressure conditions, while open-vessel systems are for extractions carried out at atmospheric pressure conditions (Kaufmann and Christen 2002). MAE extraction technology has been applied for both laboratory-scale level and large-scale industrial operations, that is, full-scale commercialized extraction applications (Ying et al. 2013). During the last few years, the MAE technology has been used for isolating essential oils, fats, and oils (Deng et al. 2006, Bayramoglu et al. 2008). Microwave technology has been found to be a rapid, safe, and low-cost technique for the extraction of essential oils, fats, and oils and does not need samples devoid of water (Chemat et al. 2006, Bousbia et al. 2009). In an attempt to improve Soxhlet performance, the most successful has been the use of microwaves, which has provided a wider variety of approaches. In fact, microwaveassisted Soxhlet extraction remains the most interesting improvement of conventional Soxhlet extraction (Li et al. 2006). Microwave-assisted Soxhlet extraction of solids of low-polar and nonpolar extractants comprises heating

these up to their boiling point by using microwaves while stirring with a magnetic stirrer to absorb microwave radiation. Therefore, unlike classical conductive heating methods, microwave heats the whole sample simultaneously. In this way, solvent vapors penetrate through the sample and are condensed on arrival at the condenser, as shown in Figure 2. With water running through the condensation pipe of the MAE system, the sample is mixed with the extraction solvent, and the suspension is irradiated with microwaves in a presetting procedure to reach the desired temperature. The microwave irradiation power is set as required until a complete extraction is achieved. Results from numerous biological samples obtained by this technique are qualitatively and quantitatively comparable with the steam distillation (Ferhat et al. 2006, Bendahou et al. 2008, Sahraoui et al. 2008, Farhat et al. 2009, 2011). However, it still uses organic solvents, such as hexane, and therefore cannot be considered as a green technology.

2.2.1 Solvent choice and volume A major factor that affects extraction is the selection of the appropriate solvent. The choice of solvent is based mostly upon the solubility of the required analyte, the ability of the solvent to interact with the matrix, and its absorbance of microwaves (Chen et al. 2008). The solvent selected should have a high selectivity of the analyte of interest over the matrix components and should be compatible with further chromatographic analytical steps. Transparent solvents are not heated under the microwave, and those with high absorbing capability get heated faster to enhance the extraction. Hexane is referred to be

To drain Energy attenuator

Condensation pipe

Connecting tube

Cooling water Shield thermistor

250 ml boiling flask

Temperature recorder

Time presetting Microwave cavity

Time controller Magnetic stirrer

Figure 2 Schematic diagram of microwave-assisted Soxhlet extractor.



a transparent solvent of the microwave, while ethanol is regarded as an excellent microwave absorbing solvent. Researchers have used solvent mixtures of high and low microwave absorbability to get the optimum extraction yields. There should be sufficient solvent to immerse the plant matrix throughout the irradiation process. In conventional extraction, higher solvent volume to solid matrix gives higher extraction yields, whereas in MAE, higher solvent might not give higher yield because of nonuniformity and exposure to microwaves.

2.2.2 Benefits and drawbacks of MAE MAE is a viable possibility for the extraction of essential oils, fats, and oils from seeds due to its distinct benefits over solvent extraction techniques (Cintas et al. 2013). MAE is comparable with other alternative modern extraction techniques due to its simplicity and low cost of equipment. Other benefits of this extraction technique include the following: (a) reduced extraction time; (b) allowing of simple, rapid, and low solvent usage; (c) ability to add reagents during treatment; (d) extraction reaction controlled by temperature and pressure control; (e) in processing applications, ability to instantaneously shut the heat source, which makes enormous difference to the product quality and, hence, production economics; (f) higher volume of raw material is processed over a given period; (g) safe, with no potential hazard; (h) selective heating of the sample solvent mixture; and (i) use of the technology for chemical reaction such as hydrolysis. However, compared with other modern extraction techniques, an additional stage (filtration or centrifugation) is required for the removal of solid residues (Shah and Rohit 2013). Furthermore, the efficiency of microwaves can be very poor when either the target compounds are nonpolar or they are volatile. In addition, MAE is not suitable for use when considering heat-sensitive compounds because they will be denatured. The analyte recovery is low, high pressure usage may lead to explosion, and large cooling or venting times are needed after the extraction process.

2.3 UAE Ultrasound waves are regarded as high-frequency sound waves above human hearing, that is, above 20 kHz. These waves are propagated by rarefactions and compression. This expansion causes negative pressure in the liquid. If the pressure exceeds the tensile strength of the liquid, formation of vapor bubbles occurs. These vapor bubbles

go through implosive collapse in ultrasound fields, which is called cavitation (Luque-García and Luque de Castro 2003). The implosion of cavitation bubbles generates macroturbulence, at higher-velocity interparticle collisions, and perturbation in microporous particles of the biomass (Shirsath et al. 2012). Cavitation close to the liquid-solid interfaces directs a fast-moving stream of the liquid through the cavity at the surface. Impingement by these microjets leads to surface peeling, erosion, and particle breakdown, facilitating the release of bioactives or the targeted compound from the biological matrix. This results in an increase in the efficiency of extraction by increasing mass transfer by eddy and internal diffusion mechanisms (Vilkhu et al. 2011). The mechanical effects of ultrasound enable a larger penetration into cellular materials and thereby improve mass transfer. Ultrasound disrupts biological cell walls to facilitate the release of contents. Therefore, cell disruption and effective mass transfer are cited as major factors enhancing extraction with ultrasonic power. Ultrasound permits changes in processing conditions like a decrease in pressure and temperature from those utilized in other extraction techniques, allowing the extraction of thermolabile compounds (Shah and Rohit 2013). For solid-hexane extraction of pyrethrines from pyrethrum flowers without ultrasound, extraction yield increases with extraction temperature, and maximum yield is achieved at 339K. With ultrasound, the effect of temperature on the yield is negligible in the range of 313–333K, so that optimal extraction occurs at the temperature range of 313–333K. Therefore, the use of UAE is advisable for two compounds, which may be altered under Soxhlet and heat reflux extraction operating conditions due to high extraction temperature (Romdhane and Gourdon 2002). There are two types of ultrasound equipment that may be used for extraction purposes, an ultrasonic water bath and an ultrasonic probe system fitted to horn transducers (Ibáñez et al. 2012). Figure 3 shows a UAE system using an ultrasonic probe. UAE extraction technology has been used from laboratory-scale level to larger-scale industrial operations, for commercialized extraction applications (Vilkhu et al. 2008, Chemat et al. 2011). UAE has been widely used for the extraction of nutritional material, like lipids (Metherel et al. 2009), proteins (Zhu et al. 2009), flavoring (Chen et al. 2007, Da Porto et al. 2009), essential oils, fats, and oils (Kimbaris et al. 2006), and bioactive compounds (e.g., flavonoids) (Ma et al. 2008), carotenoids (Sun et al. 2006, Yue et al. 2006), and polysaccharides (Iida et al. 2008, Chen et al. 2010, Wei et al. 2010, Yan et al. 2011). UAE was also reported for the extraction of oil from rapeseed (Ibiari et al. 2010) and Monopterus


610 J.M. Danlami et al.: A comparative study of extraction

Transducer

Ultrasonic probe Ultrasound generator

h-depth of probe in liquid Jacketed glass beaker

Figure 3 Schematic diagram of UAE (adapted from Shekhar et al. 2013, reproduced with permission from the American Chemical Society).

albus (Abdullah et al. 2010). Studies regarding the impact of various solvents and their mixture, impact of solvent volume, sonication time, and power indicated that UAE has the potential to enhance extraction efficiency and to also reduce the processing time, and the oil composition was not denatured by the use of ultrasound.

2.3.1 Benefits and drawbacks of UAE technique UAE is a simple, cost-effective, and efficient alternative compared with traditional extraction techniques. The benefits of using ultrasound in solid-liquid extraction are to increase the extraction yield and fasten the kinetics. Ultrasound facilitates the extraction of thermally sensitive compounds. The costs of equipment are lower than those of other new alternative extraction techniques. It could be used with a wider variety of solvents, including the aqueous extraction, for water-soluble components and other solvents like ethanol and methanol (Chemat et al. 2011). Compared with Soxhlet extraction methods, ultrasound extraction can enhance extraction efficiency and extraction rate, reduce extraction temperature, and increase the choice ranges of solvents (Vilkhu et al. 2008). For example, the ultrasound was found to have no effect on the composition of almond oil; however, the ultrasonic cavitating energy can cause structural breakage of the almond powder and greatly reduce the extraction time (Zhang et al. 2009). In view of its growing use for isolating or separating organic compounds and its advantages, the future introduction and dissemination of ultrasound equipment appear to be assured for rapid essential oil extraction. The shortcomings include wave attenuation in the dispersed phase and a decrease in the sound wave amplitude with distance, which is a major challenge in UAE technologies. In fact, the activated ultrasound zone

is restricted to a limited zone in the vicinity of the ultrasound emitter. These factors must be carefully considered in the design of UAE.

2.4 PLE techniques PLE is known also as accelerated solvent extraction, pressure fluid extraction, high-pressure solvent extraction, and enhanced solvent extraction (Nieto et al. 2010). The temperature and pressure conditions utilized in PLE are within the ranges of 323–473K and 3.5–20 MPa, respectively. The elevated pressure causes the solvent temperature to rise above the normal boiling point temperature. The increase in temperature tends to accelerate the extraction rate by increasing solubility and mass transfer rate. Also, the increased temperature reduces the viscosity and surface tension of solvents, helping them to spread evenly over the biological matrix and improve the extraction rate. In some cases, pressurized hot water is employed as a solvent for extraction rather than an organic solvent. This method is called pressurized hot water extraction or subcritical water extraction (Eskilsson et al. 2004). A schematic diagram of a PLE system is shown in Figure 4. The PLE equipment involves an extraction cell where the sample is introduced. The cell is filled with a solvent that is heated. High temperature and pressure are then maintained to facilitate faster extraction. The equipment has a pressure relief valve that guards against over pressurization of the cell. Nitrogen is used to purge all the residual solvents at the end of the extraction (Kaufmann and Christen 2002). Extraction solvent, temperature, pressure, numbers of cycles, and time are reported to influence extraction yield and rate (Nieto et al. 2010).

Purge valve

Pressure relief valve Oven

Pump

A

B

Extraction cell

C

Solvents Nitrogen Waste vial

Collection vessel

Figure 4 Schematic diagram of PLE (adapted from Shekhar et al. 2013, reproduced with permission from the American Chemical Society).



Oilseeds like canola, corn, flax, cotton, soybeans, etc., have been reported to be extracted in no time, with higher yield and improved quality through PLE compared with the traditional methods, such as Soxhlet extraction. David and Bruce (2010) examined the influence of elevated temperatures on the oil during the extraction by PLE. It was found that peroxide values were