Collection of medicinal plants
• Drugs may be collected from wild or
cultivated plants.
• It is known that the active constituents of
medicinal plants are affected by many
factors and may vary during the course of
plant growth.
• Proper time of collection is very important to
obtain a drug of a good quality.
Factors affecting collection
1. Time of the year:
The plant may contain a substance in winter
that is not present in summer, or its amount
varies markedly e.g. Rhubarb contains no
anthraquinone in winter, instead it contains
anthranols, which in summer, are oxidized to
anthraquinones.
Colchicum corm is free from bitterness and is
devoid of the alkaloid colchicine in autumn,
hence is used in Austria as a food, instead
of potatoes. Bitterness starts to appear in
spring and early summer when it is used as
a drug.
2- Time of the day:
Some drugs, like Digitalis, contain different
amounts of active constituents in different
times of the day. Being highest in the
afternoon.
3- Stage of maturity and age:
• The value and content of active
constituents of many drugs depends on
the stage of maturity and age.
• Conium fruits contain coniin when fruits
are mature and unripe.
• Santonica flowers are rich in santonin,
when unexpanded, when it starts to open,
the santonin content decreases.
Drying of crude drugs
Reasons for drying:
1. To help in their preservation.
2. To fix their constituents, by preventing reactions
that may occur in presence of water.
3. To prevent the growth of micro-organisms such as
bacteria and fungi.
4. To facilitate their grinding.
5. To reduce their size and weight.
Insufficient drying favors spoilage by micro-organisms
and makes it possible for enzymatic destruction.
Methods of drying
Drying is carried out either by natural or artificial
methods.
1- Natural drying: this is accomplished by natural
air in sun or shade.
2- Artificial drying: this is a rapid method done at
well-controlled temperature and is accomplished
by:
• direct fire.
• Use of heated stones.
• Use of stoves.
• Drying chamber
• Drying ovens
• Lyophilization (Freeze drying):
Frozen material is placed in an evacuated apparatus
which has a cold surface maintained at -60 to -80 °C.
Water vapour from the frozen material passes
rapidly to the cold surface.
It is used for drying heat-sensitive substances e.g.
antibiotics and proteins.
• Chemical drying using desiccators
• An absolutely dried drug is that completely freed
from water, when exposed to air it absorbs 8-10% of
moisture and is called air-dry drug.
Stabilization
• On long storage, enzymatic reactions will
slowly destroy the constituents, because the
last traces of water can never be removed.
• In order to avoid this degradation, the
enzymes should be destroyed before drying,
a process usually called stabilization.
• The most common method being brief
exposure (a few minutes only) of the plant
material to ethanol vapor under pressure (0.5
atm).
Fermentation
• Enzymatic transformation of the original plant
constituents is sometimes desirable. e.g. Vanilla pods
• The fresh material is placed in thick layers,
sometimes covered and often exposed to raised
temperatures (30-40 °C) and humidity, so as to
accelerate the enzymatic processes.
• The fermented product must be dried afterwards to
prevent attack by microorganisms, e.g. moulds.
Preservation and protection of crude drugs
Storage represents the last stage of preparing crude drugs. drugs
usually deteriorate along the time of storage, except in few cases
e.g. Cascara and Frangula should not be used except after certain
period of storage.
Certain drugs as Nux vomica are hardly affected by storage.
Generally, changes that take place during storage of crude drugs
are objectionable, e.g. drugs containing volatile oils gradually lose
their aroma.
Improper methods of storing and inadequate protection during storage
can cause a pronounced deterioration.
There are two principal reasons for deterioration:
Physiochemical: moisture, heat, air and light.
Biological: fungi, bacteria, insects and rodents.
Physicochemical factors
1. Moisture: moisture sometimes affects drugs adversely
through activating the enzymes (as in cardiac
glycosides).
2. Heat: rise of temperature up to 45 activates the
enzymes causing decomposition of active constituents.
Volatile oil containing drugs are also affected by higher
temperatures, their content decreases.
3. Air: oxygen of air oxidizes certain constituents of crude
drugs, e.g. linseed and lemon oil, it causes rancidity of
fixed oils and resinification of volatile oils.
4. Light: it affects drugs, especially those having marked
colours.e.g. yellow colour of Rhubarb changes to
reddish tint, white coloured corollas turn brown.
Biological factors
1. Bacteria: cotton fibres are rendered brittle by bacterial
attack which makes the cotton wool objectionable and
dusty.
2. Moulds: the mycelium of delicate hyphae produces an
unpleasant mass of clinging particles in powdered
drugs.
3. Insects: they seem to attack all drugs but have
preferences to certain drugs as ginger, belladonna, kola,
liquorice,.....
Insects which infest vegetable drugs include beetles, mites
and moths. They render drugs porous and powdery.
Methods for controlling insects
1. Heat treatment: it is the simplest method and
is done by exposing the drug to a temperature
of 60-65. it is effective especially for insect
eggs which are not affected by insecticides.
2. Fumigation: this is done by volatile
insecticidal agents in closed areas e.g. CCL4,
CS2, CN. Most fumigants do not kill eggs of
insects. It is advisable to repeat fumigation at
intervals to obtain better results.
3. Liming: liming of certain drugs as ginger,
nutmeg to protect against insect attack provides
only partial protection.
4. Low temperature storage: this method is
preferred to fumigants and liming. Adult insects,
pupae, larvae and eggs are sometimes killed by
very low temperatures.
5. Exposure to alternate periods of low and
high temperatures: frequently is more effective
for killing insects than a prolonged period of low
temperature exposure.
Rodents: they cause much spoilage of
crude drugs during storage, especially if
wrapped in paper, cloth or put in
cardboard or wooden containers. The
presence of rodent's filth, excreta, hairs
causes rejection of the drug.
Grinding of crude drugs
• Regardless of whether the crude drug is to be used
for isolation of a pure compound or for manufacture
of a simple preparation, the first operation that must
be performed is grinding of the plant material to a
powder of suitable particle size.
• It is important that the particles are of as uniform size
as possible.
• Excessive dust can clog percolators and result in a
turbid extract which is hard to clarify.
•
•
Large particles take a longer time for complete
extraction than small ones.
Large differences in particle size thus slow down the
extraction process.
Types of machines used for grinding crude drugs:
1. Hammer mill; a common type for grinding crude
drugs.
2. Knife mill; is useful for production of low-dust
powders of leaves, barks and roots for subsequent
percolation or maceration.
3. Tooth mill; is used for production of very fine
powders.
Notes:
• Grinding produces a certain amount of heat which
must be observed when grinding crude drugs
containing heat-sensitive compounds.
• Mills cooled with liquid nitrogen are available for
such purposes.
• Cold grinding is also preferable for crude drugs
containing volatile oils.
• Following grinding, the material must be sifted to
ensure the proper particle size.
Extracts
• Extracts can be defined as preparations of crude
drugs which contain all the constituents which are
soluble in the solvent used in making the extract.
• In dry extracts all solvent has been removed.
• Soft extracts and fluid extracts are prepared with
mixtures of water and ethanol as solvent.
• Tinctures are prepared by extraction of the crude drug
with five to ten parts of ethanol of varying concentration,
without concentration of the final product.
• Plant constituents are usually contained inside
the cells. Therefore, The solvent used for
extraction must diffuse into the cell to dissolve
the desired compounds whereupon the solution
must pass the cell wall in the opposite direction
and mix with the surrounding liquid.
• An equilibrium is established between the solute
inside the cells and the solvent surrounding the
fragmented plant tissues.
The speed with which this equilibrium is
established depends on:
1.
Temperature
2.
pH
3.
Particle size
4.
The movement of the solvent
Choice of solvent
• The ideal solvent for a certain pharmacologically
active constituent should:
1. Be highly selective for the compound to be extracted.
2. Have a high capacity for extraction in terms of
coefficient of saturation of the compound in the
medium.
3. Not react with the extracted compound or with other
compounds in the plant material.
4. Have a low price.
5. Be harmless to man and to the environment.
6. Be completely volatile.
• Aliphatic alcohols with up to three carbon atoms,
or mixtures of the alcohols with water, are the
solvents with the greatest extractive power for
almost all natural substances of low molecular
weight like alkaloids, saponins and flavonoids.
• According to the pharmacopoeias, ethyl alcohol
is the solvent of choice for obtaining classic
extracts such as tinctures and fluid, soft and dry
extracts.
• The ethanol is usually mixed with water:
1- to induce swelling of the plant particles
2- to increase the porosity of the cell walls which
facilitates the diffusion of extracted substances from
inside the cells to the surrounding solvent.
• For extraction of barks, roots, woody parts and seeds
the ideal alcohol/water ratio is about 7:3 or 8:2.
•
For leaves or aerial green parts the ratio 1:1 is
usually preferred in order to avoid extraction of
chlorophyll.
Extraction Procedures
There are many procedures for obtaining
extracts
1- Infusion
3- Percolation
5- Decoction
2- Maceration
4- Digestion
6- Continuous hot extraction
7- Solvent-solvent precipitation
8- Liquid-liquid extraction
9- Distillation
10- Specific procedures