Solvents are used in large amounts in different industries such as food and pharmaceutical to separate different valuable bioactive compounds. Over the last decade, the design of green, efficient and sustainable extraction methods has received a great attention from researchers. Great improvements can be achieved with the use of non-conventional techniques such as supercritical carbon dioxide extraction(SC-CO₂), Ultrasound Assisted Extraction (UAE) and Microwave-Assisted Extraction (MAE). In past decades, environmental friendly techniques are being interested to develop the ‘‘Green Chemistry” concept [1]. Therefore, an improved or better extraction technique is necessary. In the last 17 years (2000-2017), the extracts of more than 300 plant species have been studied using Supercritical Fluid Extraction (SFE) technology. The major share of SFE research covers plant material [2,3]. Many valuable pure components obtained from these plants are already in use for human nutrition and health purposes [4]. The most used solvent in supercritical state is carbon dioxide (CO₂) due to its great versatility, non-explosive, non-flammable, non-toxic and cost-efficient properties and easy to remove from the solutes [5]. CO₂ is classified as a low-critical temperature solvent. A good feature of low-critical temperature solvents, as compared with conventional liquid solvents, is that they operate at moderate temperature and provide thermo degradation of thermally labile compounds. These solvents are highly preferred in pharmaceutical and natural-product industries. A major advantage of low-critical temperature solvents is their easy separation from the extract [6]. Generally, supercritical carbon dioxide (SC-CO₂) which possesses properties of both liquids and gases has several major advantages compared with liquid solvents. Figure 1 shows the pressure-temperature phase diagram of CO₂. A pressure-temperature phase diagram of CO₂ indicates the temperature and pressure conditions necessary for the various states of substance to exist. The critical temperature for carbon dioxide is 31.1°C, and the critical pressure is 73 atm. above the critical temperature and pressure, the fluid is called supercritical fluid. The isothermal compressibility of a fluid near its critical point is infinity, which translates into rapid change in its density as a function of temperature and pressure. The density of a supercritical fluid which affected the dissolving power is strongly tunable by varying the temperature or/and pressure. Therefore, selective fractions can be extracted from natural sources by carefully choosing the temperature and pressure operation [7,8].

list of fluids which have been proposed as supercritical fluid solvent was shown in Table 1. It has been shown that N2O cause violent explosions when used for samples having high organic content and should, therefore, be used only when absolutely necessary.The possible practical applications of super critical H2O has been limited due to its high critical pressure and temperature (T >374 ºC and P > 221 bar) together with the corrosive nature of H2O at these conditions [2].Other solvents (e.g. ethane, propane) have also been used in studies but their use is limited compared with CO₂ and for this reason the emphasis of this review is on SC-CO₂.

deMelo et al. (2014) presented a review regarding the plant species that have been studied under the scope of SFE. It has been stated that supercritical fluids have been mainly applied to the extraction valuable compounds from seeds and leaves which are followed by fruits, roots, flowers, rhizomes and bark [4]. Different valuable bioactive compounds including triglycerides, fatty acids, fatty alcohols, terpenoids, phytosterols, tocopherols, tocotrienols, and phenolics can be obtained applying SC-CO₂ from plant matrices.

A triglyceride is an ester derived from glycerol and three fatty acids. Triglycerides are the main constituents of body fat in humans and animals, as well as vegetable fat. Furthermore it is known that their abundance in extracts lead to high quality biodiesel. Based on our knowledge there is lack of information about SFE application covering this purpose. Free fatty acids are also a specific family of compounds that occur independently in SFE extracts. In some cases the control of free fatty acid concentration may be helpful to quality control of the oils. Moreover, these compounds are key stones for soaps, oleochemical esters, oils and lubricants [9]. These compounds have been studied widely as target compounds for extraction, such as in SFE of borage (Borage officinalis L.) [10], primrose (Oenotherabiennis L.) [10], chinese star anise (Illiciumverum) [11], palm (Elaesguineensis)[12], pine (Pinussylvestris L.) [13], pupunha (Guilielmaspeciosa) [14], winter melon (Benincasahispida) seed [15, 16], spearmint (Menthaspicata L.) [17-19] and Strobilanthescrispus (pecahkaca) [20].

Terpenoids are a large and diverse class of naturally occurring chemicals found in plant sources. These compounds are secondary metabolites whose role in plants is related to protection, pollination and growth mechanisms [21]. Plant terpenoids are used extensively for their aromatic qualities and play a role in traditional herbal remedies. The extraction of these compounds has been one of the major objectives driving SFE research. These compounds are classified to five subgroups according to the number of isoprene units. Monoterpenoids, with two isoprene units, such as geraniol [22,23]; sesquiterpenoids, three isoprene units, such as parthenolide [24]; diterpenoids, with four isoprene units, such as cafestol [25]; triterpernoids, with six isoprene units, such as ursolic acid [26-28]; tetraterpenoids, with eight isoprene units, such as lycopene [29,30].

Phenolic compounds could act as hydrogen donators, singlet oxygen quenchers and reducing agents because of their redox properties [20]. Literatures pointed out phenolic compounds are responsible to reduce the risk of coronary heart diseases, stroke, cancer, degenerative and atherosclerosis diseases attributed to oxidative stress [18]. Phenolic compounds which present in edible and non-edible plants show various biological effects such as antioxidant activity. Plant materials crude extracts which contain high amount of phenolic compounds lead to reduce the rate of lipid oxidation and improve the quality of food products are strongly favoured in food industry. In addition, it was revealed that phenolic compounds like flavonoids can act as scavengers of Reactive Oxygen Species (ROS) through oxido-reductases inhibition [20]. Depending on the vegetable species, extracts obtained by SC-CO₂ can comprise substances from several phenolic groups like coumarins [31], cinnamic acids [32], quinones [33], flavonoids [17-20], lignans [32].

Selection of Operating Conditions

There are several parameters such as plant material preparation, selection of supercritical fluids, modifiers and extraction conditions which affected strongly the efficiency of SFE process. Hence, they should be considered carefully to develop a successful process.

Sample preparation

Preparation of plant materials is a critical step for SFE process. Fresh plant matrices contain high moisture content which cause mechanical problems due to ice formation. Furthermore, the efficiency of SFE process will reduce due to the high watersolubility of compounds dissolved in aqueous phase while the water solubility (0.3%) in SC-CO2 is very low [34]. Therefore, it is necessary to control the moisture content of plant matrices by drying or mixing them with chemicals like sodium sulphate and silica gel. It has been stated that when the moisture content of Benincasahispida seeds was increased from 5 to 20% the crude extraction yield was decreased up to 38% [35]. Saldana et al., (2004) found that 10% moisture content was adequate to reach high recovery of β-carotene from apricot pomace by SCCO2 extraction. The plant matrices particle size could be considered as another critical parameter for SC-CO₂ extraction of nutraceuticals as the extraction process controlled by internal diffusion feed. The extraction time is extended by using larger particle size of samples. In contrast, fine powder can increase the rate of extraction; however, it is difficult to keep flow rate properly. By referring to different published literatures, it was believed that a smaller sample particle size results in greater extraction of lycopene by SC-CO₂ [36-39].

Pressure and Temperature

The solvent properties of supercritical fluids may be tuned by changing pressure and temperature values, directly influencing density. The solubility of targeted compounds in SC-CO₂ is mainly determined by the SC-CO₂ density. In general, SCCO2 density increases with pressure at constant temperature and decreases with temperature at constant pressure, where the density decrease becomes smaller at higher pressures. The majority of the studies indicated that an increase in the extraction pressure of the supercritical carbon dioxide leads to an increase in the amount of valuable bioactive compounds extracted [6]. Bimakr et al. (2011) utilized pressure of 100 to 300 bar in the extraction of bioactive compounds spearmint (Menthaspicata L.) leaves. They found that the extraction yield increased with pressure from 100 to 200 bar, which was due to increase of SC-CO₂ density at higher pressures. However, an increase in the pressure level above 200 bar led to an unexpected reduction in the extraction yield. Liza et al. (2010) also found that at pressure above 200 bar there was non-existent of any flavonoid compounds. Moreover, same behavior was reported by Liu et al. (2009) in SCE of pomegranate (Punicagranatum L.) seeds. They found that an elevation of pressure (up to around 320 bar) caused significant increase of crude yield extract. They mentioned that this result most likely was due to the improvement of solute solubility resulted from the increased solvent density [39].

As mentioned above the density of CO₂ affected by the temperature. The density of CO₂ at constant pressure is reduced with increasing temperature and leading to reduce the solvent power of supercritical CO₂. Temperature also affects the volatility of the solute. Hence, the effect of a temperature elevation is difficult to predict because of its dependence on the nature of the sample. For a non-volatile solute, a higher temperature would result in lower extraction recovery owing to a decrease in solubility. A temperature increase may also cause breakdown of cell structure and increase the diffusion rate of the targeted compounds in the particles, therefore accelerating the extraction process [6]. Bimakr et al. (2011) studied the effect of temperature on SC-CO₂ extraction of bioactive compounds from spearmint leaves. They obtained that the extraction yield increased with temperature and the highest extraction yield (60.57 mg/g) was obtained at 60°C. In this manner, the solute vapour pressure played a key role leading to increase in the extraction yield [18].

Extraction Time

Extraction time is another important parameter that needs to be optimized for maximizing bioactive valuable compounds recovery from plant matrices. The extraction time could be helpful parameter to obtain a complete SC-CO₂ extraction. To enhance the efficiency of SC-CO₂ process it is necessary to prolong the contact time of the SC-CO2with the sample material. Due to the physical structure of the seed, the penetration of the solvent and the diffusion of targeted compounds in the particles are very slow. Therefore, extraction time is usually limited to the fast extraction period since the amount of extraction yield recovered in the slow extraction period is negligible. Bimakr et al. (2013) examined the effect of extraction time at 60, 90 and 120 min on valuable compounds recovery from Benincasahispidaseeds. They found that the highest crude extraction yield (176.30 mgextract/gdried sample) was obtained at 97 min. According to them, it was possible that 60 min of extraction time was insufficient for a complete extraction, while thermal degradation occurring at 120 min of extraction led to lowered yields of valuable bioactive compounds [16]. Reducing the extraction time could also reduce costs as well as improve energy efficiency [37]. In a study conducted by Ozkal et al. (2005) fast extraction period decreased from 183 to 64 and 32 min with a pressure increase from 30 to 45 and 60 MPa at 40°C. On the other hand, it decreased from 64 to 33 min with a temperature increase from 40°C to 50°C at 45 MPa. Furthermore, it was reported that, a 10-20 min static extraction prior to dynamic extraction improved the extract recoveries in SFE extraction of aflatoxins [40].

Bimakr et al. (2011) investigated the effect of dynamic time on the SC-CO₂ extraction of valuable compounds from Menthaspicata L. leaves. They concluded that the solvent power of SC-CO₂ is reduced at 100 bar pressure due to the lower CO₂ density so maximum yield was obtained during 90 min dynamic extraction time. Applying higher pressures (200 and 300 bar) the extraction rate is higher and as a consequence the extraction yield kept increasing up to 60 min dynamic extraction time[18]. In another study, Bimakr et al. (2016) shortened the extraction time using supercritical carbon dioxide extraction combined with pressure swing technique (SCE-PST). They found that application of pressurization-depressurization before continuous extraction time had a significant effect on improvement of extraction efficiency [15].

Fluid Flow Rate

The speed of the supercritical fluid flowing through the cell has a strong influence on the extraction efficiencies. The slower the fluid velocity, the deeper it penetrates the matrix. Papamichail et al. (2000) studied the SFE of oil from milled celery seeds using CO₂ as a solvent. They investigated the effect of flow rate of CO₂ on the extraction rate of celery seeds. They showed that the increase of the solvent flow rate leads to the increase of the amount of oil extracted versus extraction time [41]. Topal et al. (2006) tested SC-CO2 flow rates ranging from 1.5 to 4.5 mL/min and found the highest lycopene yield at a flow rate of 2.5 mL/min. Using the flow rate between 2.5 to 4.5 mL/min a decrease in the amount lycopene extracted was observed [38].

Co-solvent Flow Rate

An important shortcoming of the use of SC-CO₂ is its low polarity. Considering the chemical nature of most natural bioactive compounds, generally polar compounds, CO₂ alone may not be able to extract them. To cope with this issue, co-solvents (also called modifiers) are employed during extraction at small proportions (typically, 1-10%). These co-solvents are solvents with higher polarity than CO₂, expanding the range of compounds attainable by increasing the polarity of the supercritical mixture [18]. According to Shi et al. (2009), due to the non-toxic properties of water and ethanol, these two compounds may be used in place of other organic solvents for the extraction of bioactive compounds [42]. In different studies it has been concluded that adding ethanol (5-30%) is needed to enhance the phenolic compounds solubility in SC-CO₂ of grape seeds [43], olive leaves [44], grape skin [45], pistachio hulls [46], apple, peach [47] and sour cherry pomaces [48]. Bimakr et al. (2011) investigated the effect of co-solvent flow rate (3-9 g/min) on antioxidant activities of SC-CO₂ extracts of spearmint leaves. They found that the highest antioxidant activity of SC-CO₂ extracts was obtained by using 9 g/min (63.13%). The obtained SC-CO₂ extracts antioxidant activity using 9 comparing to 6 g/min was higher but the difference between them is too low (1.46%) and it is not significant. Therefore, 6 g/min co-solvent flow rate is preferable due to its better effect on the antioxidant activity of obtained SC-CO₂ extracts and economic cost. They conclude considering the polarity of bioactive compounds such as phenolic compounds which contributed greatly on the antioxidant activity of spearmint leaves extracts by increasing co-solvent flow rate (ethanol) the antioxidant activity was increased [18].

Instrumentation

Depending on the process developed the instrumentation needed can be more or less complex. Figure 2 showed a schematic diagram of the SFE system. It is composed of solvent and modifier pumps, extractor vessel with temperature control, pressure restrictor and collection vessel. Circulated deionized water at 5°C was used for cooling different zones in the SCCO2 extraction system. The instrument may be also made more complicated by adding fractionation vessels equipped with independent pressure and temperature controls. To collect volatile compounds the system could be equipped with CO2 recycling system or refrigerated trap. It is evident that the complexity of the SC-CO2 system will depend on the scale. Several recent reviews dealt with the recovery of bioactive compounds from plants using SFE from a more or less broad perspective [49,50]. Huang et al. (2017) presented a good review abouttheoretical models for supercritical fluid extraction which could be helpful for researchers [51].

It must be kept in mind that the industrialization of SCE system is very limited due to the fact that CO2 possesses non-polar properties. In order to extract polar compounds it is necessary to add some polar solvents as modifier since they are soluble in carbon dioxide. As this new emerging technology is very expensive due to its high investment costs so it should compete with the worldwide opinion regarding the cost issues. Different studies focused on development of a scaling-up strategies and they concluded that for a process to reach an adequate scale-up level, it is expected that it has been previously assessed upon optimization of operating conditions, selection of preferable extraction times, and modeling of the process [4,12,17,25,30].

Comparison of supercritical fluid extraction to conventional methods

There are different classical methods to extract valuable compounds from plant matrices such as hydrostillation, Soxhlet and maceration. The principle of these methods is solvent choice coupled with heat and/or agitation. One of the most common extraction procedures which have been used over 100 years is conventional Soxhlet extraction. This method has developed by von Soxhlet in 1879 and is considered as the main reference to evaluate the other solid-liquid extraction methods performance [34]. The time for an average Soxhlet extraction ranges from 1 to 72 h. The completed extraction produces a high volume, dilute solution which usually needs to be concentrated prior to analysis. The main limiting step of this method is the extraction of thermo liable compounds which are degraded at high temperature during long extraction time [18].

The advantages of SC-CO₂ process over other conventional processes such as extraction by solvents and separation by distillation are automation, the reduction in operational steps, safe operation due to the use of nonorganic solvents and the use of moderate temperature in the critical range favorable for heat labile foods. The main advantage of SC-CO₂ extraction techniqueis the excellent quality of the resulting product [52].

Several researchers have compared supercritical fluid extraction with conventional extraction methods. A extraction yield obtained with SC-CO₂ method was similar to those from a hot hexane extraction [53]. Myer et al. (1992) reported that SC-CO₂ method obtained recoveries from 97% to 100% of a Soxhlet extraction in potato chips and puff-dried products [54]. King et al. (1996) concluded that there were no significant differences between fatty acids extracted from beef samples with an SFE method and with the solvent extraction [55].

Seed and nut oils are traditionally separated mechanical pressing of organic solvents. Undesirable solvent residue in the final product cannot be avoided using organic solvent extraction. SC-CO2 extraction is also used to extract seed and nut oils. It has been demonstrated that walnut oil extracted by SCCO 2 had a higher amount of tocopherols (405.7 μg/g oil) compared with the oil obtained with hexane (303.2 μg/ g oil [56]. In different studies it is stated that the oil extracted by SC-CO2 was clearer than that obtained by organic solvent extraction, indicating the need for less refining process [6,16].

Donelian et al. (2009) compared extraction of patchouli (Pogostemoncablin) essential oil with SC-CO₂ and by steam distillation. They found that the extraction of patchouli essential oil with SC-CO₂ at 14MPa and 40°C gave the best yield (5.07%), which was higher than that of steam distillation (of 1.50%) [57]. Application of supercritical carbon dioxide extraction was investigated by Bimakr et al. (2013) to separate valuable compounds from Benincasahispida seeds. Optimal process condition were identified by using response surface methodology and the results demonstrated that the combined treatment of 234.25 bar of pressure, 46°C of temperature and 95.05 min of dynamic extraction time was required for maximizing crude extraction yield (174.91 mg/g) and antioxidant activity (52.96% inhibition of DPPH free radicals), and total phenolic content (40.45 mg GAE/g extract) of extracts. They concluded that the quantity and quality of extracts obtained under optimum condition of SC-CO2extraction was better than those obtained at optimized ultrasound-assisted extraction [16].

The efficiency of different environment friendly techniques including ultrasound-assisted extraction [58], supercritical carbon dioxide extraction [16], and supercritical carbon dioxide extraction combined with pressure swing technique [15] were compared for separation of valuable compounds from winter melon (Benincasahispida) seeds. They reported that application of various extraction techniques resulted in extracts that differ in quality and quantity of crude extract. They concluded that remarkable variations in crude extraction yield, radical scavenging activity, and total phenolic content of Benincasahispida seeds extract were found depending on the extraction technique. Higher crude extraction yield could be obtained by Soxhlet extraction with lower separation of bioactive compounds as they are thermo sensitive compounds and may be degraded during Soxhlet extraction. In contrast, using ultrasound- assisted extraction, SC-CO₂ extraction, and SC-CO₂ extraction lead to lower quantity of crude extraction yield with better quality in terms of valuable bioactive compounds separation. Piggott et al. (1997) investigated different extraction methods including steam distillation, solvent extraction, supercritical fluid extraction and liquid CO₂ extraction to obtain the volatile oil from Western Australian sandalwood. They revealed that the highest yields of extractable material and total volatile compounds was obtained applying SC-CO₂ [59].

Extraction of essential oils from Grapefruit flavedo were studied applying hydrodistillation and solvent extraction using different solvents such as pentane, ethanol and supercritical carbon dioxide [60]. Monoterpene hydrocarbons decrease in supercritical carbon dioxide extracts at 87-90% with respect to their quantity in pentane extracts (95%) and in hydrodistillate (97%) these levels in monoterpene hydrocarbons were related to the limonene content, the most abundant compound in grapefruit essence. Sesquiterpenes, aldehydes, alcohols and esters increased in supercritical carbon dioxide extracts obtained at a high fluid density.

Silva et al. (2016) provided a good review paper regarding different studies which applied SCE technique for extraction of different valuable bioactive compounds from natural sources. It can be very helpful for researchers in this field of study [61]. Furthermore, supercritical fluid extraction of different valuable compounds from various sources in different studies is presented in Table 2.

Conclusion

The use of supercritical fluids, especially carbon dioxide, in the extraction of bioactive components from natural sources has increased in recent years due to the key advantages of the supercritical extraction process. Carbon dioxide is widely applied in its supercritical state for extraction of bioactive compounds which could be due to its “health and safety” and environmental nature and relate to increased unease about the presence of organic solvent residues in material for human consumption.The supercritical fluid extraction operation is based on the solvating properties of supercritical fluid, the characteristics of which are achieved by employing pressure and temperature that surpasses the critical point of the fluid. Many studies have shown that SC-CO₂ is effective in extraction of valuable compounds. It has been stated that the extraction efficiency in terms of yield and composition are affected by different factors such as sample preparation (particle size and moisture content) and extraction parameters (pressure, temperature, solvent flow rate, extraction time, and use of a cosolvent). Moreover, SC-CO₂ extraction system can be used with supercritical fluid fractionation and supercritical fluid chromatography to purify and identify the extracted compounds. SC-CO₂ extraction can be a true alternative for obtaining high quality extracts from several natural sources.

Figure 1: Pressure-Temperature phase diagram for CO₂.

Figure 2: A schematic design of the supercritical fluid extractor unit [20].

 Table 1: Critical properties of fluids of interest in supercritical process.

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