Comparison of Microwave-Assisted Extraction with Other Solid–Liquid Extraction Techniques - So sánh chiết vi sóng với một số kỹ thuật tách chiết Rắn - Lỏng khác

Comparison of Microwave-Assisted Extraction with Other Solid–Liquid Extraction Techniques

To introduce bioactive plant extracts in pharmaceutical and cosmetic formulations,

industries are looking for green and ef fi cient extraction processes free of toxic solvents.

Methodologies using biodegradable and nontoxic solvents such as water and

ethanol are being developed [ 52 ] .

The traditional techniques of solvent extraction of plant materials are based on

the correct choice of solvents and the use of heat or/and agitation to increase the

solubility of the desired compounds and improve the mass transfer. Soxhlet extraction

is the most common and is still used as a standard in all cases [ 53 ] . As a result

of several secondary metabolites, the development of high performance and rapid

extraction methods is an absolute necessity [ 54 ] . The new extraction techniques

with shortened extraction time, reduced solvent consumption, increased pollution

prevention, and with special care for thermolabile constituents have gained attention.

In the many published papers comparing MAE with other advanced and conventional

extraction methods, MAE has been accepted as a potential and powerful

alternative for the extraction of organic compounds from plant materials [ 55 ] .

The ideal extraction technology depends on the type of compound to be extracted,

whereas the extraction method ef fi ciency is based on the highest recovery, especially

of the effective constituents, the shortest processing time, the lowest production

cost, and use of minimum organic solvent [ 56 ] . There have been numerous

reviews and research on the advances of different extraction techniques, comparing

their results. In the extraction of bioactive compounds from plants, MAE was

reported to be more ef fi cient compared to conventional techniques such as Soxhlet

and advanced methods of extraction including ultrasound-assisted extraction (UAE),

pressurized liquid extractions (PLE), and supercritical fl uid extraction (SFE), which

have emerged as energy-saving technologies. Over the years the procedures based

on MAE have replaced some conventional extraction methods and have been

adopted over decades in laboratories and industry.

In addition, the progress in microwave extraction gave rise to other categories of

techniques to improve its performance: (1) microwave-assisted distillation (MAD) for

the isolation of essential oils from herbs and spices [ 57 ]; (2) microwave hydrodiffusion

and gravity (MHG), a combination of microwave heating and distillation at atmospheric

pressure that requires less energy and no solvent and simply combines microwaves and

earth gravity at atmospheric pressure [ 58 ] ; (3) vacuum microwave hydrodistillation

(VMHD), which uses pressures between 100 and 200 mbar to evaporate the azeotropic

mixture of water–oil from the biological matrix [ 59 ]; (4) microwave-integrated Soxhlet

extraction (MIS), a combination of microwave heating and Soxhlet [ 60 ]; and (5) solvent-

free microwave extraction (SFME), based on the combination of microwave heating

and distillation, which is performed at atmospheric pressure [ 61 ]. If these techniques

are explored scienti fi cally, they can be proven to be ef fi cient extraction technologies for

ensuring the quality of herbal medicines worldwide [ 13 ] .

As already mentioned, MAE is increasingly employed in the extraction of natural

products as an alternative to traditional techniques of extraction for several reasons:

reduced extraction time, reduced solvent consumption, and less environmental

pollution as a result of increased ef fi ciency and clean transfer of energy to the

matrix; improved extraction yield and product quality, because materials can be

rapidly heated, and often processed at lower temperatures; up to 70% energy saving

compared to conventional energy forms from the high energy densities and the

direct absorption of energy by the materials; compact systems, as small as 20% of

the size of conventional systems; and selective energy absorption resulting from the

dielectric properties of the material and applicator design [ 52, 55, 62 ] .

On the other hand, some disadvantages can also be mentioned: additional

fi ltration or centrifugation is necessary to remove the solid residue after the process;

the ef fi ciency of microwaves can be poor when the target compounds or solvents are

nonpolar, or when they are volatile; and the use of high temperatures that can lead

to degradation of heat-sensitive bioactive compounds [ 63 ] .

Considering these advantages and drawbacks of MAE compared to other techniques,

a discussion on MAE performance compared to conventional and advanced

techniques as Soxhlet, SFE, UAE, and PLE is appropriate. Table 2.2 presents their

advantages and drawbacks;