PHYTOCHEMISTRY Notes / “Phytochemistry Explained: How Plant Chemicals Power Medicine and Health” / “What Is Phytochemistry? A Complete Guide to Plant Chemical Science”

PHYTOCHEMPHYTOCHEMISTRYISTRY

9.1.1. Introduction

Phytochemistry involves the study of chemicals (mainly the secondary
metabolites) the plants produce as a measure to protect themselves from insects,
pests, pathogens, herbivores, UV exposure, and
environmental hazards.
Phytochemistry includes the structural compositions, the biosynthetic pathways,
functions, mechanism of actions in the living systems, and the medicinal,
industrial, and commercial applications of secondary metabolites.
Phytochemicals (derived from the Greek word
phyto meaning plant) are
naturally occurring and biologically active chemical compounds present in
plants. They provide health benefits for humans in addition to those provided by

the macronutrients and micronutrients.

9.1.2. Extraction

The process of separating medicinally active constituents of plant and animal
tissues with the help of selective solvents and standard procedures is termed
extraction. The extracted products of plant tissues obtained in liquid or semisolid
state (after removing the solvent) or in dry powdered form are complex mixtures
of metabolites; these products are meant for oral or external use. The extracted
preparations include decoctions, infusions, fluid extracts, tinctures, pilular
(semisolid) extracts, or powdered
extracts; these preparations are named as
galenicals after Galen (a Greek physician of 2nd century).
The standardised extraction procedures involve treatment with a selective solvent
(menstruum) to yield the therapeutically
active constituents of crude drugs,
removing the inactive ones. The undissolved residue left behind is termed marc.
The extract obtained by this process, after standardisation is either used as a
medicinal agent in the form of tinctures or fluid extracts, or is further processed
for incorporation in tablet and capsule forms.
The drug extraction process is divided into the following four steps:
1) The solvent penetrates the drug,
2) The drug constituents dissolve in the solvent,
3) The solution within the cells diffuses out, and

4) The dissolved portion separates from the exhausted drug. 

The efficiency of extraction process depends on the following factors:

1) Nature and properties of drug and its extractable constituents,
2) Particle size of the powdered drug,
3) Solvent nature, and
4) The state of contact between the solvent and drug particles.
Drug extraction process is also influenced by various
physical factors , like
gravitation, diffusion, osmosis, adhesion, capillarity, convection, solubility, and
surface tension. A solvent’s wetting property can be improved by displacing the
air entrapped within the capillaries and by using a surfactant, which also
increases the solubility of cellular contents by solubilisation in the menstruum.
With the help of agitation, equilibrium bet
ween the outer and inner cellular
contents can be attained fast; this in turn facilitates dissolution of the desired
constituents in the menstruum.
Extraction at elevated temperatures has the following advantages:
1) Decreased viscosity reduces the boundary layers.
2) Convection currents act similar to agitation during the extraction process.
3) Diffusion coefficients are directly proportional to absolute temperature and
inversely proportional to viscosity, thus diffusion rate is influ
enced by
increased temperature.
4) Solubility of the constituents in the menstruum increases with increased
temperature.
The menstruum used for extraction should have the following properties:
1) Chemically and physically inert,
2) Non-toxic,
3) Inexpensive, and
4) Selective, i.e., it should disso
lve the desired active constituents with a
minimum of the inert material.
Water, ethanol, and their mixtures are most commonly used as they fulfil the

above mentioned considerations.

 

 

9.1.2.1. Principles of Extraction

The principles and mechanisms of extracting cr

ude drugs by maceration,

percolation, and infusion are similar to those of extracting soluble constituents

from solid materials using solvent (referred to as leaching which involves simple

physical solution or dissolution).

Extraction procedures get affe cted by the transport rate of solvent into the mass,

the solubilisation rate of the soluble constituents by the solvent, and the transport

rate of solution out of the insoluble material. Extraction of crude drugs gets

facilitated by increase in the surface area of the material to be extracted and

decrease in the radial distances travelled between the crude drug particles.

The theory of mass transfer states that maximum surface area is obtained by

size reductions that involve reduction of materials into in dividual cells; however,

this theory is not acceptable in many cases of vegetable material. It has been

proved that hundreds of unbroken cells having intact cell wall are present in 200

mesh particles. Therefore, these unbroken cells are used to carry out extraction so

that an extract with a high degree of purity can be obtained. This also provides

sufficient time to the solvent to diffuse through the cell wall for dissolution of the

desired solute (or groups of constituents) and to the solution (or the ext

ract) to

diffuse through the cell wall surface.

9.1.2.2. Choice of Solvents

The solvent used for extraction process is significantly important for determining

the biologically active compounds from plant materials. The choice of solvent is

affected by the quantity of phytochemicals to be extracted, the extraction rate,

diversity of inhibitory compounds extracted, ease of handling of the extracts,

toxicity of the solvent in the bioassay, and the potential health hazards of the

extractants. Since traces of residual solvent are always present in the end product,

the solvent used should be non-toxic and should not interfere with the bioassay.

An ideal solvent used for extraction should have the following properties:

1) It should be economical.

2) It should be non-toxic.

3) It should be stable, i.e., physically and chemically inert.

4) It should not be too volatile or inflammable.

5) It should be selective in nature, i.e., the desired amount of active ingredient

can be extracted using minimum amount of inert material.

The different solvents used for the extraction process are discussed below:

1) Water: It is a universal solvent that is used for extracting plant products

having antimicrobial properties. The traditional healers use water extract, but

the plant extracts from organic solvent s provide more reliable antimicrobial

action. On the other hand, the water

-soluble flavonoids (mostly

anthocyanins) have no antimicrobial activity and water -soluble phenolics are

only important as antioxidant.

2) Acetone: It is miscible with water, is volatil e, has a low toxicity, and is a

very useful extractant for antimicrobial studies in which more phenolic

compounds are to be extracted. Acetone dissolves many hydrophilic and

lipophilic components from plants. In a study it was revealed that tannins and

other phenolics can be better extracted using aqueous acetone than the

aqueous methanol. However, acetone and methanol are both useful in

extracting saponins having antimicrobial activity.

3) Alcohol: The activity of ethanolic extracts is much higher than that o

f the

aqueous extracts due to the presence of higher amounts of polyphenols. It

means that the ethanolic extracts more efficiently degrade the non -polar cell

walls and seeds to release polyphenols from the cells. The decreased activity

of aqueous extract can also be due to the polyphenol oxidase enzyme, which

degrades polyphenols only in water extracts, while it becomes inactive in

methanolic and ethanolic extracts. Besides, water also provides a better

medium for microbial growth in comparison to ethanol. The concentration of

bioactive flavonoids was found to be more with 70% ethanol due to its higher

polarity than the pure ethanol. However, the polarity of pure ethanol can be

increased by adding water to it up to 30% for preparing 70% ethanol.

Moreover, e thanol can also easily penetrate the plant cellular membrane to

extract the intracellular ingredients. Almost all the known plant components

that are active against microorganisms are aromatic or saturated organic

compounds, thus they are often obtained th

rough primary ethanol or

methanol extraction. Methanol is although polar than ethanol, but its

cytotoxic nature makes it inappropriate to be used for extraction a

s it may

give improper results.

4) Chloroform: Terpenoid lactones can be obtained by successive e xtractions

of dried barks using hexane, chloroform, and methanol with activity

concentrating in chloroform fraction. Occasionally tannins and terpenoids are

found in the aqueous phase, but they are more often obtained by extraction

using less polar solvents.

5) Ether: It is commonly used for extracting coumarins and fatty acids.

6) Dichloromethanol: It is mainly used for the selective extraction of

terpenoids.

9.1.2.3. Modern Methods of Extraction

Extraction of crude drugs can be carried out by various processes, and th

e

selection of process depends on the chemical properties of the drug’s active

constituents.

Factors Affecting Choice of an Extraction Process

1) Nature of the Drug: The detailed pharmacognosy of the drug to be extracted

helps in selecting the extraction process with optimum results. For extracting

hard and tough drugs ( e.g., nux vomica), percolation method is used; while

for soft and parenchymatous drugs ( e.g., gelatin), drugs which cannot be

easily powdered ( e.g., squill), and for unorganised drugs ( e.g., benzoin),

maceration method is preferred.

2) Therapeutic Value of the Drug:

For extracting less therapeutically

important drugs [e.g., flavours (lemon), bitters (gentian)], maceration method

is used; while for drugs with significant therapeutic value ( e.g., belladonna),

percolation method is preferred.

3) Stability of the Drug:

For drugs whose constituents are thermolabile,

continuous extraction method should be avoided.

4) Cost of Drug: For costly drugs (e.g., ginger), percolation method is used (as

it is economical); while for cheaper drugs, maceration method is preferred.

5) Solvent: For drugs whose desired constituents require a solvent other than a

simple boiling solvent or an azeotrope, reserved percolation method (and not

continuous extraction) is used.

6) Concentration of the Product:

Dilute products (like tinctures) can be

prepared by maceration or percolation method. Percolation is used for

preparing semi -concentrated products ( e.g., concentrated infusions) and

concentrated products ( e.g., liquid extracts or dry

extracts). In case the

solvent is suitable and the desired constituents are heat

-stable, continuous

extraction method is used.

Various extraction methods employed are:

1) Maceration,

2) Digestion,

3) Percolation,

4) Continuous hot extraction (Soxhlet extraction),

5) Supercritical fluid extraction,

6) Counter current extraction,

7) Microwave assisted extraction,

8) Ultrasonic assisted extraction,

9) Infusion and decoction,

10) Pressure cooker extraction,

11) Extraction by passage through a colloid mill,

12) Use of surface active agents in drug extraction, and

13) Expression and diacolation.

9.1.2.4. Maceration

The word maceration denotes softening. The maceration process (or Process

M) is used for producing tinctures, extracts, and concentrated infusions. It is the

simplest method of crude drug extraction, which was official in I.P., 1966.

Classification

1) Simple Maceration: It is a method for preparing tinctures from organised

drugs, e.g., roots, stems, leaves, etc.

2) Modified Maceration: It is a method for

preparing tinctures from

unorganised drugs, e.g., oleo-resins and gum resins.

3) Multiple Maceration: It is a method for preparing concentrated extracts.

This method includes:

i) Double maceration, and

ii) Triple maceration.

Simple Maceration

Simple macerati on involves extraction of organised drugs having specific cell

structures, e.g., roots, stems, leaves, flowers, etc. It is a very simple method,

which does not require trained operators. Tincture of myrrh and compound

tincture of benzoin are examples of products prepared by simple maceration.

Principle

In simple maceration, solid ingredients and the solvent are taken in a stoppered

container, and left undisturbed for at least 3-7 days with frequent agitation. When

the soluble matter dissolves in the solvent, the resultant mixture is passed through

sieves or nets. The marc retained in the sieves is pressed, the liquids are

combined, and filtered or decanted after standing.

Apparatus

Simple maceration is performed using a wide mouthed bottle or any other

container which can be closed tightly to prevent evaporation of the menstruum

(figure 9.1).

Procedure

In simple maceration, the crude drug to be

extracted and the menstruum are placed in close

contact in a closed vessel for 7 days with

frequent agitation. After 7 days, the resultant

mixture is strained through sieves, the marc

retained is pressed, and the liquids are combined

and filtered. The drug

should be properly

comminuted.

The menstruum penetrates the cellular structure

of drug, and softens and ultim ately dissolves the

soluble portions. A closed vessel is used for

preventing menstruum loss by evaporation.

Occasional shaking is recommended to maintain a rapid equilibrium between the

intra- and extra -cellular fluids. The degree of pressing the marc may vary, thus

the final product is not adjusted to a fixed volume. Simple maceration process

may take around 14 days to complete. The ratio of drug to menstruum should be

1:10. When the drug and menstruum are left undisturbed, sediment may form,

which can be avoided by standing the fluid product for a few days prior to use.

Simple maceration method cannot be used for extracting all the drugs, thus other

maceration processes have to be employed.

Modified Maceration

Modified maceration is used for extraction of drugs having no cellular or tissue

structure, i.e., unorganised drugs ( e.g., gums, resins, gum-resins, and oleo gum -

resins). During the extraction of unorganised drugs, the marc forms a compact

mass and retains no macerate; thus pressing the marc is not required. In modified

maceration, the final product is adjusted to a definite volume for extracting

unorganised drugs, because complete extraction of constituents occurs within a

short time period. Since the unorganised drugs lack cellular structure, the soluble

components are in direct contact with the menstruum, thus quickening the

extraction process.Modified maceration occurs in the following steps:

1) The unorganised drug to be extracted is reduced to minute particl

es and

placed in a closed vessel with the menstruum (equal to 80% of the bulk of the

finished product) for 2-7 days.

2) This mixture is frequently agitated.

3) After a week, the mixture is strained and the strained liquid is filtered.

4) The marc left behind is was hed with fresh menstruum (less than 20% of the

finished bulk) and the washings obtained are passed through the strainer and

filter used previously (in step 3).

5) Menstruum is passed through the filter in amount sufficient to make up the

desired volume.

6) A dry receiver or a receiver washed with the menstruum

is used for

collecting the filtrate; this is because unorganised drugs contain water

-

insoluble resinous matter which gets precipitated, thus affecting the clarity of

the finished product