**2. Extraction of phytochemicals**

#### **2.1 Solvent extraction methods**

#### *2.1.1 Principles*

Solvent extraction is the commonest method to extract plant material. The main purpose is to select the suitable solvent to extract target plant materials efficiently. During the extraction, the solvent has to diffuse into the cell membrane in the first instance, in the following step it has to dissolve the solutes, then intracellular and extracellular concentration difference is formed, and finally it has to diffuse out of the cells enriched in the extracted solutes [5].

Selecting suitable solvents is the key of the solvent extraction method. Using a solvent of an appropriate polarity according to the principle of "like dissolves like" is the main point to select solvent. Thus, hydrophilic solvents are used to solubilize hydrophilic chemical constituents and vice versa. The hydrophilicity or lipophilicity of solvents and chemical constituents could be predicted by polarity. The plant compounds, such as terpenoids and steroids, possess low polarity, and could be dissolved into lipophilic solvents such as chloroform and ether, while chemical constituents, such as carbohydrates and amino acids, possess rather high polarity and could be dissolved into water and aqueous ethanol.

Solvents commonly used for extracting chemical constituents of plants are in the order of weak to strong polarity as follows: petroleum ether < carbon tetrachloride < benzene < dichloromethane < chloroform < ether < ethyl acetate < n-butanol < acetone < ethanol < methanol < water.

Water is a cheap, easy to get, and nontoxic solvent with strong polarity. It could be used to extract phytochemicals with strong polarity, such as inorganic salts, saccharides, amino acids, tannins, proteins, organic acid salts, alkaloid salts, and glycosides. Acid or alkaline water is applied sometimes to increase the solubility of certain specific components. Acid water could extract alkaline materials, such as alkaloids, via the formation of salts. Similarly, organic acids, anthraquinoids, flavonoids, coumarinoids, phenols, and other acidic materials could be extracted via the formation of salts. The disadvantage to extract chemical constituents with water is that the aqueous extract is easy to go moldy, so difficult to preserve. Additionally, water possesses high boiling point, and the water extract needs to be concentrated for a rather long time. Furthermore, the water extract contains many impurities such as proteins, pectins, tannins, mucilages, and inorganic salts, which make the extraction of target components difficult.

Hydrophilic organic solvents are strong-polarity and water miscible, such as methanol, ethanol, and acetone. Ethanol is the most commonly used hydrophilic organic solvent. Chemical constituents could be extracted by ethanol of different concentrations according to their properties. Furthermore, ethanol is inexpensive,

**47**

*Analytical Methods of Isolation and Identification DOI: http://dx.doi.org/10.5772/intechopen.88122*

into plant cell tissues.

*2.1.2 Immersion method*

*2.1.3 Percolation method*

*2.1.4 Decoction method*

*2.1.5 Refluxing method*

tion roughly by whether heating or not.

safe, and concentrated easily. Additionally, ethanol extract is not readily moldy and glycosides are hard to be hydrolyzed in ethanol extract. Thus, ethanol is one of the most commonly used solvents in laboratories and industrial production. Methanol possesses similar property to ethanol and lower boiling point. However, methanol has rather strong toxicity, so we have to pay attention to safety when it has to be used. Acetone is a good solvent to extract lipid-soluble chemical constituents. However, acetone is easy to volatilize and flame, and it possesses certain toxicity. Petroleum ether, benzene, chloroform, ether, ethyl acetate, dichloroform, and so on are lipophilic organic solvents and are not miscible with water. They could be applied to extract lipophilic components, such as volatile oils, fats, chlorophyll, lactones, phytosterols, some alkaloids and some aglycones (aglycones of flavonoids, anthraquinoids, steroids, and so on). These solvents possess low boiling points and are easy to concentrate. However, strong-volatility, large loss, flammability, toxicity, and high price are their disadvantages. Additionally, they are difficult to permeate

Solvent extraction methods could be classified as cold extraction and hot extrac-

It is a method to dissolve out phytochemicals with appropriate solvents at room or low temperatures (<80°C). It is suitable to extract phytochemicals easily to be destroyed at high temperature. The plants with abundant starches, pectins, gums, or mucilages could also be extracted with this method. Firstly, plant powder or pieces should be loaded in the adequate container, and then the suitable solvents (water, ethanol, aqueous ethanol, and so on) are added into it to immerse the material for the given length of time. Discontinuous stirring or shaking during the process could accelerate dissolution rate. The immersion method is simple but inefficient, and the extraction ratio is also low. Furthermore, aqueous extract is easy

to go moldy, so addition of appropriate preservatives is necessary.

The coarse particles of plants should be loaded in percolation apparatus and immersed with suitable solvent for 24–48 h, then collect the percolates at the bottom of percolation apparatus. New solvent should be added at the top of percolation apparatus constantly during the percolation process. It possesses higher efficiency than the immersion method because of the sustained concentration difference during the process. However, this procedure is complex and consumes rather much solvent and long time.

Load short segments, thin pieces, or coarse powder into an appropriate container, add water, and heat it to boiling; the components are then extracted. It is easy to operate; most of the constituents could be extracted in various degrees. Nevertheless, rather much nontargeted components could also be extracted, and it is not suitable to the extraction of volatile compounds and thermal unstable compounds. Furthermore, it is not suitable to extract plants with lots of starches [6].

It is a method to extract plant chemical constituents by organic solvent using heating and refluxing. Refluxing apparatus is necessary so as not to waste solvents,

#### *Analytical Methods of Isolation and Identification DOI: http://dx.doi.org/10.5772/intechopen.88122*

*Phytochemicals in Human Health*

active phytochemicals.

**2. Extraction of phytochemicals**

the cells enriched in the extracted solutes [5].

and could be dissolved into water and aqueous ethanol.

acetone < ethanol < methanol < water.

extraction of target components difficult.

**2.1 Solvent extraction methods**

*2.1.1 Principles*

The chemical structures of plant compounds must be identified, which may provide the necessary basis for further study on the bioactivities, structure-activity relationships, metabolisms *in vivo*, structural modification, and synthesis of the

The structural studies are often difficult to carry out with classical chemical methods, such as chemical degradation and derivative synthesis, because of the minute amount of compound isolated from plants. Therefore, spectral analysis is mainly used. That is, consuming sample as little as possible to obtained structural information as much as possible by measuring and analyzing various spectra [4].

Solvent extraction is the commonest method to extract plant material. The main purpose is to select the suitable solvent to extract target plant materials efficiently. During the extraction, the solvent has to diffuse into the cell membrane in the first instance, in the following step it has to dissolve the solutes, then intracellular and extracellular concentration difference is formed, and finally it has to diffuse out of

Selecting suitable solvents is the key of the solvent extraction method. Using a solvent of an appropriate polarity according to the principle of "like dissolves like" is the main point to select solvent. Thus, hydrophilic solvents are used to solubilize hydrophilic chemical constituents and vice versa. The hydrophilicity or lipophilicity of solvents and chemical constituents could be predicted by polarity. The plant compounds, such as terpenoids and steroids, possess low polarity, and could be dissolved into lipophilic solvents such as chloroform and ether, while chemical constituents, such as carbohydrates and amino acids, possess rather high polarity

Solvents commonly used for extracting chemical constituents of plants are in the order of weak to strong polarity as follows: petroleum ether < carbon tetrachloride < benzene < dichloromethane < chloroform < ether < ethyl acetate < n-butanol <

Water is a cheap, easy to get, and nontoxic solvent with strong polarity. It could be used to extract phytochemicals with strong polarity, such as inorganic salts, saccharides, amino acids, tannins, proteins, organic acid salts, alkaloid salts, and glycosides. Acid or alkaline water is applied sometimes to increase the solubility of certain specific components. Acid water could extract alkaline materials, such as alkaloids, via the formation of salts. Similarly, organic acids, anthraquinoids, flavonoids, coumarinoids, phenols, and other acidic materials could be extracted via the formation of salts. The disadvantage to extract chemical constituents with water is that the aqueous extract is easy to go moldy, so difficult to preserve. Additionally, water possesses high boiling point, and the water extract needs to be concentrated for a rather long time. Furthermore, the water extract contains many impurities such as proteins, pectins, tannins, mucilages, and inorganic salts, which make the

Hydrophilic organic solvents are strong-polarity and water miscible, such as methanol, ethanol, and acetone. Ethanol is the most commonly used hydrophilic organic solvent. Chemical constituents could be extracted by ethanol of different concentrations according to their properties. Furthermore, ethanol is inexpensive,

**46**

safe, and concentrated easily. Additionally, ethanol extract is not readily moldy and glycosides are hard to be hydrolyzed in ethanol extract. Thus, ethanol is one of the most commonly used solvents in laboratories and industrial production. Methanol possesses similar property to ethanol and lower boiling point. However, methanol has rather strong toxicity, so we have to pay attention to safety when it has to be used. Acetone is a good solvent to extract lipid-soluble chemical constituents. However, acetone is easy to volatilize and flame, and it possesses certain toxicity.

Petroleum ether, benzene, chloroform, ether, ethyl acetate, dichloroform, and so on are lipophilic organic solvents and are not miscible with water. They could be applied to extract lipophilic components, such as volatile oils, fats, chlorophyll, lactones, phytosterols, some alkaloids and some aglycones (aglycones of flavonoids, anthraquinoids, steroids, and so on). These solvents possess low boiling points and are easy to concentrate. However, strong-volatility, large loss, flammability, toxicity, and high price are their disadvantages. Additionally, they are difficult to permeate into plant cell tissues.

Solvent extraction methods could be classified as cold extraction and hot extraction roughly by whether heating or not.

## *2.1.2 Immersion method*

It is a method to dissolve out phytochemicals with appropriate solvents at room or low temperatures (<80°C). It is suitable to extract phytochemicals easily to be destroyed at high temperature. The plants with abundant starches, pectins, gums, or mucilages could also be extracted with this method. Firstly, plant powder or pieces should be loaded in the adequate container, and then the suitable solvents (water, ethanol, aqueous ethanol, and so on) are added into it to immerse the material for the given length of time. Discontinuous stirring or shaking during the process could accelerate dissolution rate. The immersion method is simple but inefficient, and the extraction ratio is also low. Furthermore, aqueous extract is easy to go moldy, so addition of appropriate preservatives is necessary.

#### *2.1.3 Percolation method*

The coarse particles of plants should be loaded in percolation apparatus and immersed with suitable solvent for 24–48 h, then collect the percolates at the bottom of percolation apparatus. New solvent should be added at the top of percolation apparatus constantly during the percolation process. It possesses higher efficiency than the immersion method because of the sustained concentration difference during the process. However, this procedure is complex and consumes rather much solvent and long time.

#### *2.1.4 Decoction method*

Load short segments, thin pieces, or coarse powder into an appropriate container, add water, and heat it to boiling; the components are then extracted. It is easy to operate; most of the constituents could be extracted in various degrees. Nevertheless, rather much nontargeted components could also be extracted, and it is not suitable to the extraction of volatile compounds and thermal unstable compounds. Furthermore, it is not suitable to extract plants with lots of starches [6].

#### *2.1.5 Refluxing method*

It is a method to extract plant chemical constituents by organic solvent using heating and refluxing. Refluxing apparatus is necessary so as not to waste solvents,

#### *Phytochemicals in Human Health*

and the toxicity to operators or ruin the environment is deduced. It is applicable to extraction of lipophilic phytochemicals, such as steroids, anthraquinoids, and terpenoids. It is an extraction method of high efficiency but complex, and consumes much more solvent. This method is not applicable to extract thermal unstable chemical constituents because of long time heating.

#### *2.1.6 Constant refluxing method*

It is a method developed based on the refluxing method. Soxhlet extractor is the most frequently used constant refluxing apparatus. This method avoids disadvantages of consuming too much solvent and complex operation. However, as a refluxing method, constant refluxing method is not applicable to extract thermally unstable compound either because of long time heating.

#### *2.1.7 Supercritical fluid extraction method*

In the supercritical state, the supercritical fluid is contacted with the plant tissues. By controlling different temperatures, pressures and different kinds and contents of entrainers, the supercritical fluid can selectively extract the components of different polarities, boiling points, and molecular weights successively. This method is called the supercritical fluid extraction (SFE) method [7].

The critical point of a pure substance is defined as the highest temperature and pressure at which the substance can exist in vapor-liquid equilibrium. At temperatures and pressures above this point, a single homogeneous fluid is formed, which is known as supercritical fluid (SF). SF is heavy like liquid and has low viscosity like gas meanwhile. SF possesses rather large diffusion coefficient and could dissolve many compounds well. A number of materials could be used as SFs, such as ammonia, ethane, difluoro-dichloromethane, heptane, and so on, while the most widely used SF is CO2. The critical temperature of CO2 (*T*c = 31.26°C) is close to room temperature, and the critical pressure (*P*c = 7.2 MPa) is not too high. CO2 also has a series of other advantages, such as nontoxicity, odorless, nonflammable, chemical stability, and low cost, which allowed it to be the most commonly used solvent in SFE. CO2 is a nonpolar substance and applicable to extract lipophilic compounds. However, its dissolvability is weak compared to strong polar substances. Hence, entrainers are always added to improve the solubility of SF CO2 during the extraction of polar compounds. Entrainers, which are added into SF little, could enhance solubility of SF significantly. The commonly used entrainers are methanol, ethanol, water, acetone, ethyl acetate, acetonitrile, and so on.

The extraction of nonpolar and medium-polar components by SFE can avoid the sample loss and environmental pollution caused by solvent recovery in traditional extraction methods, especially for the extraction of volatile compounds with thermal instability.

The biggest advantage of SFE is that it can be performed at near-room temperature, and almost all the active ingredients in the product can be retained. There is no residual organic solvent in the process. The product has high purity and high yield. Additionally, the operation is simple and energy saving.

Compared with other conventional separation methods, SFE possesses the following advantages: (1) No residual organic solvents, fast extraction speed, simple process, high yield, and easy operation; (2) no flammable and explosive dangers, no environmental pollution. Low extraction temperature, suitable for the extraction of thermal unstable components; (3) the dissolution properties

**49**

temperature [9].

*Analytical Methods of Isolation and Identification DOI: http://dx.doi.org/10.5772/intechopen.88122*

cleaning equipment is difficult.

*2.1.8 Ultrasonic extraction method*

acids, glycosides, terpenoids, and so on.

structures of chemical constituents meanwhile.

*2.1.9 Microwave-assisted extraction method*

of SF are easy to improve, only the pressure needs to be changed at a certain temperature; (4) entrainers can be added to change the polarity of the extraction medium to extract polar substances; extraction medium can be recycled with low cost; (5) it could be applied combined with other chromatographic techniques, such as GC, IR, GC–MS, and HPLC, to extract, separate, and determine phytochemicals efficiently and quickly, so as to achieve the integration of extraction and quality analysis. However, supercritical extraction has some limitations: strong solubility of fat-soluble components, weak solubility of water-soluble components, high cost of equipment, resulting in higher product costs, and

Supercritical fluid extraction (SFE) technology has achieved gratifying results in the fields of medicine, chemical, food, light industry, and environmental protection. Especially, it has been widely used in phytochemical extraction field, such as the extraction of alkaloids, volatile oils, phenylpropanoids, flavonoids, organic

It is a method of solvent extraction assisted by ultrasound. Ultrasonic wave is a kind of elastic mechanical vibration wave. The vibration frequency is as high as 20 KHz in elastic medium. The ultrasonic wave could vibrate the liquid medium. When the vibration is sparse, many small holes are formed in the medium. The instantaneous closure of these small holes can cause a pressure of up to thousands of atmospheric pressures. At the same time, the local temperature can rise to 1000°C. It can cause instantaneous rupture of the cell wall of plants and the whole organism, and make the solvent permeate into the cells of plants. This accelerates the dissolution of active ingredients in plants into solvents. Ultrasonic wave extraction could shorten the extraction time and improve the extraction efficiency, but could not change the

Ultrasonic extraction technology has been widely used in the extraction of natural products in recent years, for example, extraction of soy isoflavones; see [8].

of 0.1–100 cm (the corresponding frequency is 300–300,000 MHz), which is between infrared and radio waves. Polar molecules can absorb microwave energy, then release energy in the form of thermal energy, which makes the temperature inside the medium rise rapidly, causes the rather high pressure inside, and then the components flow out and dissolve in the solvent. On the other hand, the electromagnetic field produced by microwave can make some components diffuse to the interface of the extraction solvent, accelerating their thermal movement, which not only improves the extraction efficiency but also reduces the extraction

Microwave-assisted extraction has the advantages of less decomposition of chemical constituents, shorter time, lower energy consumption and less environmental pollution. Microwave-assisted extraction has been widely used in a series of fields of perfume, condiments, natural pigments, herbal medicine, cosmetics, soil and environmental analysis, and so on. In China, microwave-assisted extraction technology has been used in hundreds of Chinese herbal medicine extraction, such as *Pueraria lobata*, Panax notoginseng, Ginkgo, and so on, for example, the extrac-

tion of tea polyphenols and tea caffeine from green tea leaves; see [10].

Microwave refers to the electromagnetic wave whose wavelength is in the range

*Analytical Methods of Isolation and Identification DOI: http://dx.doi.org/10.5772/intechopen.88122*

*Phytochemicals in Human Health*

*2.1.6 Constant refluxing method*

chemical constituents because of long time heating.

unstable compound either because of long time heating.

*2.1.7 Supercritical fluid extraction method*

ethyl acetate, acetonitrile, and so on.

Additionally, the operation is simple and energy saving.

thermal instability.

and the toxicity to operators or ruin the environment is deduced. It is applicable to extraction of lipophilic phytochemicals, such as steroids, anthraquinoids, and terpenoids. It is an extraction method of high efficiency but complex, and consumes much more solvent. This method is not applicable to extract thermal unstable

It is a method developed based on the refluxing method. Soxhlet extractor is the most frequently used constant refluxing apparatus. This method avoids disadvantages of consuming too much solvent and complex operation. However, as a refluxing method, constant refluxing method is not applicable to extract thermally

In the supercritical state, the supercritical fluid is contacted with the plant tissues. By controlling different temperatures, pressures and different kinds and contents of entrainers, the supercritical fluid can selectively extract the components of different polarities, boiling points, and molecular weights successively. This

The critical point of a pure substance is defined as the highest temperature and pressure at which the substance can exist in vapor-liquid equilibrium. At temperatures and pressures above this point, a single homogeneous fluid is formed, which is known as supercritical fluid (SF). SF is heavy like liquid and has low viscosity like gas meanwhile. SF possesses rather large diffusion coefficient and could dissolve many compounds well. A number of materials could be used as SFs, such as ammonia, ethane, difluoro-dichloromethane, heptane, and so on, while the most widely used SF is CO2. The critical temperature of CO2 (*T*c = 31.26°C) is close to room temperature, and the critical pressure (*P*c = 7.2 MPa) is not too high. CO2 also has a series of other advantages, such as nontoxicity, odorless, nonflammable, chemical stability, and low cost, which allowed it to be the most commonly used solvent in SFE. CO2 is a nonpolar substance and applicable to extract lipophilic compounds. However, its dissolvability is weak compared to strong polar substances. Hence, entrainers are always added to improve the solubility of SF CO2 during the extraction of polar compounds. Entrainers, which are added into SF little, could enhance solubility of SF significantly. The commonly used entrainers are methanol, ethanol, water, acetone,

The extraction of nonpolar and medium-polar components by SFE can avoid the sample loss and environmental pollution caused by solvent recovery in traditional extraction methods, especially for the extraction of volatile compounds with

The biggest advantage of SFE is that it can be performed at near-room temperature, and almost all the active ingredients in the product can be retained. There is no residual organic solvent in the process. The product has high purity and high yield.

Compared with other conventional separation methods, SFE possesses the following advantages: (1) No residual organic solvents, fast extraction speed, simple process, high yield, and easy operation; (2) no flammable and explosive dangers, no environmental pollution. Low extraction temperature, suitable for the extraction of thermal unstable components; (3) the dissolution properties

method is called the supercritical fluid extraction (SFE) method [7].

**48**

of SF are easy to improve, only the pressure needs to be changed at a certain temperature; (4) entrainers can be added to change the polarity of the extraction medium to extract polar substances; extraction medium can be recycled with low cost; (5) it could be applied combined with other chromatographic techniques, such as GC, IR, GC–MS, and HPLC, to extract, separate, and determine phytochemicals efficiently and quickly, so as to achieve the integration of extraction and quality analysis. However, supercritical extraction has some limitations: strong solubility of fat-soluble components, weak solubility of water-soluble components, high cost of equipment, resulting in higher product costs, and cleaning equipment is difficult.

Supercritical fluid extraction (SFE) technology has achieved gratifying results in the fields of medicine, chemical, food, light industry, and environmental protection. Especially, it has been widely used in phytochemical extraction field, such as the extraction of alkaloids, volatile oils, phenylpropanoids, flavonoids, organic acids, glycosides, terpenoids, and so on.

#### *2.1.8 Ultrasonic extraction method*

It is a method of solvent extraction assisted by ultrasound. Ultrasonic wave is a kind of elastic mechanical vibration wave. The vibration frequency is as high as 20 KHz in elastic medium. The ultrasonic wave could vibrate the liquid medium. When the vibration is sparse, many small holes are formed in the medium. The instantaneous closure of these small holes can cause a pressure of up to thousands of atmospheric pressures. At the same time, the local temperature can rise to 1000°C. It can cause instantaneous rupture of the cell wall of plants and the whole organism, and make the solvent permeate into the cells of plants. This accelerates the dissolution of active ingredients in plants into solvents. Ultrasonic wave extraction could shorten the extraction time and improve the extraction efficiency, but could not change the structures of chemical constituents meanwhile.

Ultrasonic extraction technology has been widely used in the extraction of natural products in recent years, for example, extraction of soy isoflavones; see [8].

#### *2.1.9 Microwave-assisted extraction method*

Microwave refers to the electromagnetic wave whose wavelength is in the range of 0.1–100 cm (the corresponding frequency is 300–300,000 MHz), which is between infrared and radio waves. Polar molecules can absorb microwave energy, then release energy in the form of thermal energy, which makes the temperature inside the medium rise rapidly, causes the rather high pressure inside, and then the components flow out and dissolve in the solvent. On the other hand, the electromagnetic field produced by microwave can make some components diffuse to the interface of the extraction solvent, accelerating their thermal movement, which not only improves the extraction efficiency but also reduces the extraction temperature [9].

Microwave-assisted extraction has the advantages of less decomposition of chemical constituents, shorter time, lower energy consumption and less environmental pollution. Microwave-assisted extraction has been widely used in a series of fields of perfume, condiments, natural pigments, herbal medicine, cosmetics, soil and environmental analysis, and so on. In China, microwave-assisted extraction technology has been used in hundreds of Chinese herbal medicine extraction, such as *Pueraria lobata*, Panax notoginseng, Ginkgo, and so on, for example, the extraction of tea polyphenols and tea caffeine from green tea leaves; see [10].
