Microwave-Assisted Extraction of Bioactive Compounds (Review)

In recent times, bioactive compounds from plant samples are extracted using a microwave extractor. This is because traditional methods of extraction are need of higher volume of solvents, degrade thermal-sensitive bioactive compounds, and consume much time of extraction. Hence, this chapter unveils the importance of the microwave-assisted extraction (MAE) technique in the recovery of bioactive compounds from plants. The involving extraction steps need to recover higher yields, faster, consumption of lesser extracting solvents, and ensure stable heat-sensitive bioactive compounds. The factors affecting MAE in the recovery of bioactive compounds from plant materials are as well discussed. Additionally, some of the previously reported bioactive compounds from plant samples using MAE are highlighted.


Introduction
Extraction involves separating dissolvable substances from non-dissolvable residues using solvent(s); it can be in form of liquid or solid [1]. There are two categories of extraction which are traditional and modern; the former includes Soxhlet, soaking, maceration, ultra-sonication, turbo-fast blending, and solvent permeation; the latter includes ultrasonic-assisted, subcritical, supercritical CO 2 , enzyme-assisted, pressure-assisted, and microwave-assisted methods [2][3][4][5][6]. The traditional methods are mainly associated with an extended time of extraction, destruction of heat-sensitive bioactive compounds, and enormous consumption of solvents [3,7]. It is then important to explore modern methods of extraction to overcome the setbacks associated with the traditional methods. Out of all the modern methods of extraction, microwave-assisted extraction (MAE) has received the greatest attention due to its reduced consumption of solvent, shorter operation time, reproducibility, improved recovery yield, good selectivity, and reduced sample manipulation [8,9]. Gedye et al. and Giguere et al. were groups that first described the usage of microwave energy in 1986, it was employed in organic synthesis; microwave energy was also employed in the extraction of biological samples for analyzing organic compounds [10][11][12].
MAE method is being used in different kinds of samples which include geological, environmental, and biological matrices. In recent times, MAE is generally used in obtaining bioactive compounds from plant samples, this has greatly improved the total interest in development and research areas. This method allows for faster recovery of solutes from plant samples with appreciable extraction efficiency as compared to traditional techniques. MAE is one of the modern methods, and employed shortened time of extraction, minimal solvent consumptions, and secure thermolabile compounds. It is a green technology that is effective for extracting bioactive compounds from plant samples [13]. Based on the importance of MAE, this method has provided two sub-classes which are microwave solvent-free extraction (MSFE) and microwave-assisted solvent extraction (MASE).
Microwave irradiation employs a specific frequency of electromagnetic field in a way closely to photochemical-activated reaction; the frequency falls between 300 MHz and 300 GHz [14]. Nevertheless, few frequencies are allowed for medical, scientific and industrial usages; this falls within 0.915 and 2.45 GHz worldwide. Dielectric heating from MAE is appropriate for heat-sensitive bioactive compounds [15]. It had been provided that the used water for extracting phenolic compounds is not effective compared to traditional techniques due to reduced dissipation factor and higher dielectric constant associated with water relative to other solvents; hence, using solvents that possess higher dissipation and dielectric factors is advisable in MAE. Furthermore, extractability is proportional to the solvent used in extracting bioactive compounds from plants and kind of plant sample [16]. Table 1 presents the dielectric losses, dielectric constants, and loss tangents for different solvents used in MAE. Rapid heating is generated in MAE when ionic species or polar molecules are used, this heating generates collisions with molecules from surrounding which do not require higher pressure. In most cases, the extraction time and microwave power fall within 30 s to 10 min and 25 to 750 W, respectively [17]. Several studies had reported the use of MAE for recovering phenolics from plant samples including bitter leaf, purple fleabane, roselle, tea leaf, vanilla, radix, flax seeds, scent leaf, siam weed, and among others [6,8,9,[18][19][20][21][22].
Thus, the chapter presents the working principle, factors influencing this method, and previously reported bioactive compounds extracted through MAE. the cell structure to change. Microwave-assisted extraction works with a principle by which polarizable materials and dipoles of polar solvent interact with microwave radiation whereby the forces between magnetic and electric components change direction rapidly. The molecules of polar solvent get heated when they orient in the changing field direction. In the case of non-polar solvents that do not have polarizable groups, the heating is poor. This thermal effect at the molecular level is rapid but limited to the depth near the surface and a small portion of the samples. The remaining part of the samples is heated up by conduction. Therefore, this is the major drawback of the MAE because large samples or agglomerates of small samples cannot be heated uniformly. There is a possibility of using high power sources in order to enhance the depth of penetration but microwave radiation involves an exponential decay once inside a microwave-absorbing solid [23].

Working mechanism of MAE
The mechanism at which microwave-assisted extraction works is different from other types of extraction methods because the extraction occurs as a result of changes in the cell structure caused by electromagnetic waves [3]. As provided in Figure 1, this process of extraction involves a synergistic combination of mass and heat transfers working in the same direction whereas the mass transfer in conventional methods occurs from inside to outside of the substrates and heat transfer occurs from the outside to inside of the substrate [13]. The series of phenomenological steps that occur during the microwave-assisted extraction (MAE) are as follows: a. The irradiation heat from a microwave is transferred to the solid through the microwave-transparent solvent without absorption; b. The intense heating of the (a) above results in residual microwave-absorbing in the solid being heated up; c. The heated moisture evaporates and creates a high vapor pressure; d. The high vapor pressure breaks the cell of the substrate; and e. Cell wall breakage enhances the releases of the extract from the samples [13].

Figure 1.
Heat and mass transfer mechanisms in conventional and microwave extraction [13].
Additionally, the extracting solvent is absorbed into the plant sample through diffusion, causing the dissolution of solutes into the solvent until saturation. This solution diffuses to the plant surface through effective diffusion and then transfer to the bulk solution (Figure 2). Several forces that include physicochemical relations and interactions can be seen during the process (chemical interactions, driving forces, interstitial diffusion, and dispersion forces), and the strength and persistence of properties can be related to the characteristics of the extraction solvent (polarity, solubility in water, purity, solubilization, and among others) [4].

Essential factors influencing MAE and mechanism of action
Several studies had been done on optimizing MAE factors to achieve optimal yields from the considered plant samples. The operative parameters influencing MAE include solvent-to-feed ratio, solvent composition, characteristic of the plant sample and its water content, microwave power, irradiation time, stirring effect, microwave energy density, and extraction temperature. These operative parameters determine the efficiency of MAE. Hence, understanding the influences and interactions of these parameters on the extraction process is paramount.

Solvent-to-feed ratio
The selection of solvent is the most significant factor that affects microwaveassisted extraction. Adequate solvent selection will produce an efficient extraction process. The solubility of the compound of interest, mass transfer kinetics of the process, and solvent penetration that occurs from the interaction between the dielectric effect and sample matrix are inevitable parameters [24,25]. Chan et al. reported that the selection of extraction solvent depends on the capacity of that solvent to absorb microwave energy [26]. If the solvent has a high dielectric constant and dielectric loss, the solvent capacity to absorb microwave energy will be high [25]. Tatke and Jaiswal reported that solvents such as methanol, ethanol, and water are excellent microwave-absorbing solvents which possess sufficient polarity to be heated up through microwave power [27]. Studies had shown that the addition of a small quantity of water to polar solvent resulted in higher diffusion of water into the cells of the matrix, leading to effective heating and thus facilitating the transport of compounds into the solvent at higher mass transfer rates [24,26,28]. Veggi et al. had reported that the extraction solution must not exceed 30-34% (w/v) [29]. In the past studies, the solvent-to-feed ratio between 10:1 (mL/g) and 20:1 (mL/g) had been reported to give optimal yields [29,30]. The volume of extracting solvent is another important factor, a large volume of solvent requires more energy and time to condense extraction solution in the purification process. MAE may give lower recoveries because of non-uniform distribution and exposure to microwave [29].

Irradiation time
The irradiation time is another important factor that affects microwave-assisted extraction. One of the importance of MAE over conventional methods is that the extraction time is very short. The usual time ranges from a few minutes to half an hour depending on the plant matrix so as to avoid possible oxidation and thermal degradation [13,25,27]. The irradiation time is affected by the dielectric property of solvent used. Solvents such as ethanol, water, and methanol may heat up rapidly on longer exposure which can result in degradation of thermolabile compounds in the extracts [4,26]. Increased time of irradiation can improve the recovery yield; nevertheless, the increased yield can decline at prolonged irradiation time [21].
Sometimes, if the extraction will take a longer time, the plant materials are extracted through multiple stages by utilizing consecutive extraction cycle. Here, a new solvent is introduced to the residues, the procedure is then repeated to ensure exhaustion of the plant sample. The use of this process helps higher recovery yield with no excessive heating [26,31]. The nature of plant sample and solute determines the number of extraction cycles. A study presented that 3 cycles of 7 min were adequate in extracting triterpene saponins from yellow horn through MAE [32]. The optimization MAE to obtain triterpenoids saponins from Ganoderma atrum yielded 5 min for each cycle [33].

Effect of stirring
Mass transfer processes in the solvent phase are usually enhanced by stirring. The equilibrium between the vapor and aqueous phases is achieved more rapidly. The use of a stirrer in MAE accelerates the extraction process by increasing the dissolution and desorption of bioactive compounds in the sample matrix [13,27]. Thorough stirring can reduce the drawbacks possess when using a low solvent-tosolid ratio and minimized the mass transfer barrier [13].

Microwave power and temperature
Microwave power and temperature are important factors that affect the extraction yield when using MAE. The higher microwave power can lead to an increase in the temperature of the system resulting in the increase of the extraction yield until it becomes insignificant or declines [13,25,34]. An increase in temperature can result in solvent power increase because of a drop in surface tension and viscosity, enhancing the solvent to solubilize solutes, improving matrix wetting and penetration [13]. However, Spigno and De Faveri reported that the efficiency of MAE increases with the increase in temperature until an optimum temperature is reached [25]. Microwave power is also related to the quantity of sample and the extraction time required. However, the power provides localized heating in the plant matrix acts as a driving force for MAE to destroy the plant matrix so that the solute can diffuse and dissolve in the solvent. Therefore, increasing the microwave power will generally improve the extraction yield and result in a shorter extraction time [13,29,35]. On the other hand, if microwave power is too high, it can result in poor extraction yield leading to the degradation of thermally sensitive compounds in the plant matrix [29]. It is then important to select the appropriate microwave power to reduce the extraction time required to reach the set temperature and avoid a "bumping" phenomenon [13].

Characteristic of plant sample and its water content
The characteristic of the plant sample and its water content can influence MAE. The extraction efficiency improves as the contact surface area of the plant sample increases. Moreover, finer samples give room for deeper penetration of microwave irradiation [36]. Nevertheless, too much finest of the plant sample may generate some technical difficulties; hence, filtration or centrifugation is employed in the preparation of the plant samples [27,37]. During the sample preparation, the grinded sample is homogenized to improve contact between the solvent and the plant matrix. The plant particle sizes mostly fall within 2 and 100 mm [31]. Sometimes, the plant matrix is soaked before extraction to improve the yield; this is known as pre-leaching [37].
Mostly, the recovery of bioactive compounds from the plant matrix tends to increase through its moisture that acts as a solvent. This moisture is heated up, evaporated, causes pressure within the cell, and dispenses the solutes through rupturing of the cell wall; thus, increase the yield of bioactive compounds [38]. An increase in the polarity of solvent causes the addition of water to have a positive influence on microwave-absorbing capability; thus, encourages the heating procedure [26]. Extra water generates hydrolyzation and reduces the oxidation of bioactive compounds.

Microwave energy density
There are three heating operational modes employed in the performance evaluation of microwave-assisted extraction [28]. These include the constant-power heating mode, intermittent heating mode, and the constant temperature heating mode. Terigar et al. reported that the constant power heating mode presents the standard practice in the extraction of thermally sensitive active constituents of the plant matrix [35]. It is worthy to note that the microwave power alone does not provide an adequate explanation as to how energy is being absorbed in the extraction of the biological medium. Li et al. therefore studied the interrelationship between the microwave energy density and the extraction yield, it was concluded that for a unit of extracting solvent, microwave energy density is the most important factor affecting the extraction efficiency in a microwave-assisted extraction [39]. Gao et al. reported an accelerated effect on the ionic conduction and dipole rotation which in turn leads to an increase in the extraction yield [40]. This is due to the release of more microwave energy to the biological medium as the microwave power increases. Polar solvents rates of absorption improve with increasing power and ultimately resulting in higher heating and extraction rate [41]. Li et al. in [39] described the energy density of microwave heating as the power per unit quantity of sample under extraction as shown in Eq. (1).  [20]

Energy density W=mL
30.  Recovery of higher yield in a shorter time.

Influence of stirring
The influence of stirring can be linked to the mass transfer procedure in a solvent that causes convention. Hence, stability between vapor and aqueous phases can be obtained quickly. The process tends to accelerate through agitation, this enhances the dissolution and desorption of bioactive components in the plant sample [42]. Using a low solvent-to-feed ratio can be reduced as well as a reduction in the mass transfer barrier from solutes in a localized area emanating from inadequate solvent [26].

Previously extracted bioactive compounds from plants using MAE technique
MAE has been employed in several ways to extract bioactive compounds from different plant samples; the isolates from these plant samples are being used in nutraceutical and pharmaceutical applications. Microwave irradiation is mostly used to resolve some of the drawbacks associated with traditional methods. Table 2 presents some of the previous studies that employed MAE to extractive bioactive compounds from plant samples. In the presented results obtained from previous studies as presented in Table 2, it can be seen the use of microwave-assisted extraction technique recover improved quantities of global yields, different phenolic compounds, and bioactive compounds. These indicated the efficacy of MAE over other methods of extractions.

Conclusions
This chapter outlines the studies and many advances in development in the MAE of a number of plant compounds. The factors that influence the performance of MAE technique have been extensively discussed as well as some of the bioactive compounds previously reported from plant samples using the MAE. The previously reported results showed that MAE can recover higher yields of bioactive compounds relative to other extraction methods. Thus, MAE is a promising method in achieving substantial bioactive compounds from plant materials due to its importance over other techniques.