**1. 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 CO2, enzyme-assisted, pressure-assisted, and microwave-assisted methods [2–6]. 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 [10–12].

MAE method is being used in different kinds of samples which include geological, environmental, and biological matrices. In recent times, MAE is generally used

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


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

The mechanism at which microwave-assisted extraction works is different from

a. The irradiation heat from a microwave is transferred to the solid through the

b. The intense heating of the (a) above results in residual microwave-absorbing

e. Cell wall breakage enhances the releases of the extract from the samples [13].

c. The heated moisture evaporates and creates a high vapor pressure;

d. The high vapor pressure breaks the cell of the substrate; and

*Heat and mass transfer mechanisms in conventional and microwave extraction [13].*

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

exponential decay once inside a microwave-absorbing solid [23].

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

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

microwave-transparent solvent without absorption;

**2.2 Working mechanism of MAE**

in the solid being heated up;

follows:

**Figure 1.**

**5**

**Table 1.**

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

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, 18–22].

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

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

#### **2.1 Operating principle of MAE**

The fundamental of MAE technique is different compared to traditional techniques, this is because MAE happens based on electromagnetic waves that causes *Microwave-Assisted Extraction of Bioactive Compounds (Review) DOI: http://dx.doi.org/10.5772/intechopen.96092*

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].
