**2.2 Working mechanism of MAE**

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).

*Solvents with their corresponding dielectric losses, dielectric constants, and loss tangents.*

**Solvent Dielectric loss Dielectric constant Loss tangent** Chloroform 0.437 4.8 0.091 Dimethyl sulfoxide 37.125 45.0 0.825 Dimethylformamide 6.079 37.7 0.161 Ethanol 22.866 24.3 0.941 Ethylene glycol 49.950 37.0 1.350 Hexane 0.038 1.9 0.020 Toluene 0.096 2.4 0.040 Water 12.3 80.4 9.889

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

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, 18–22].

**2. Operating principle and working mechanism of MAE**

**2.1 Operating principle of MAE**

**4**

**Table 1.**

Thus, the chapter presents the working principle, factors influencing this method, and previously reported bioactive compounds extracted through MAE.

The fundamental of MAE technique is different compared to traditional techniques, this is because MAE happens based on electromagnetic waves that causes

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:


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

*Microwave-Assisted Extraction of Bioactive Compounds (Review)*

*DOI: http://dx.doi.org/10.5772/intechopen.96092*

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

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*

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-to-

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

to microwave [29].

**3.2 Irradiation time**

time [21].

*atrum* yielded 5 min for each cycle [33].

**3.4 Microwave power and temperature**

solid ratio and minimized the mass transfer barrier [13].

**3.3 Effect of stirring**

**7**

**Figure 2.** *Pictorial diagram of yield against time in the extraction [14].*

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].
