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Aqueous Solvent (Water) Extraction:
Infusion
This method is used for those herbs which are
soft in nature. The boiling water is commonly used as solvent, since
it has a greater solvent action than cold water. Water is pouring
over the herbs and then allowing it to keep in contact with water
for fifteen minutes with occasional stirring and finally filtering
of the liquid.
Apparatus: - Coffee pot or tea pot is
the simplest form of apparatus used for preparing infusions but
special pots are also available known as infusion pots. The herb is
placed at the bottom of the pot water is added and it is well
stirred three or four times during the period of infusion. Infusion
can also be prepared by enclosing the herb in the peace of muslin
bag and then suspending it just below the level of the water in a
beaker. Stirring is not required in this case because the water
slowly circulates due to increase in specific gravity of water near
the herb. When water comes in contact with the herb, it dissolves
the active constituents and becomes heavier. The heavier layer comes
down and their place is taken by fresh water as result, there is
slow circulation of water and hence stirring is not required.
Infusion pots consist of a loose perforated shelf resting
on a projection near the top of the pot. The herb is placed on the
perforated shelf, the pot is filled with water and the perforated
shelf is adjusted below the surface of the water.
Types of Infusion: - There are two types
of infusion –
-
Fresh Infusion:
- A fresh infusion is an aqueous solution of active constituents
of a vegetable herb prepared by the process of infusion. Coarse
powder of the herb is used in the preparation of the infusion.
Water is used as menstruum because it has more penetration power
and dissolves the active constituents of the herb.
-
Concentrated
Infusion: - Concentrated infusions are prepared by double or
triple maceration process. Concentrated infusions are eight times
stronger than the fresh infusions. Alcohol in concentration of
20-25% is used as menstruum. These preparations can be stored for
a longer period due to preservative action of alcohol.
Decoction
A process in which the water soluble and heat
stable constituents of hard and woody crude herbs are extracted out.
Water is used as menstruum and the herb cut in the small pieces is
actually boiled with the menstruum for ten to fifteen minuets. After
boiling, the liquid is strained and water is passed through the
content of the strainer to make the required volume.
The advantages of water extraction are that it is quick, cheap and easy.
It does not require special expertise or special equipments and
ingredients. Its disadvantages are that it does not extract all
therapeutically useful plant constituents and finished product does
not keep well infusion and decoction last only twenty four hours in
the refrigerator before bacterial contamination starts. Also hard to
standardized the end product.
The processes like infusion, decoction are now obsolete hence are rarely
used with few exceptions for extraction of herbs. Maceration and
percolation processes are of particular importance and most
pharmacopoeias refer these processes for the extraction of crude
herbs.
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