The word percolation refers to the “First pass through”. Percolation can be defined as ‘Short successive maceration or extraction by the method of displacement’. Percolation can be performed into the conical percolator, cylindrical percolator, or steam jacketed percolator.
The various stages involved in the percolation are following
To assure the complete exhaustion of the crude drug, the drug should be suitable size reduced. Size reduction also increases the surface area of the crude drug and more surface area of the crude drug will be available to react with the menstruum. Size reduction is also helpful for uniform packing of the crude drug in the percolator and reducing the moment of menstrual in the percolator.
Imbibition is a process in which the powdered drug is kept along with menstruum for 4 hours in a well-stoppered container. During this duration, the menstruum penetrates the cell wall. This initial moistening of the crude drug powder is very important because it reduces the chances of choking of percolator though the dried drug swells when comes into contact with menstruum. The swelling of the drug reduces the porosity of the powder and chokes the percolator. Imbibitions replace the interstices air which can otherwise affect the flow of menstruum. Imbibitions also prevent the washing of fine particles of the crude drug during percolation.
After imbibition, the lump of the crude drug must be broken. The lower end of the percolator should be plucked with cotton and then place the drug powder layer by layer. The packing should be perfect, neither too tight nor too loose which may affect the flow of menstruum. Two third of the percolator should be covered with the drug on which place the piece of filter paper and wash sand should be placed on the top of the filter paper. This prevents any type of disturbance of the crude drug by the flow of menstruum.
A sufficient amount of menstruum should be added after packing the percolator. During the addition of menstruum, the lower tap of the percolator should be open so that air present in the percolator is displaced by the menstruum. When menstrual starts to come from a lower tap close the tap and allow to stand, the percolator for 24 hours as the primary maceration take place. The menstrual level should be above the drug bed.
Open the lower tap after 24 hours of maceration and collect the menstruum from the lower end until the three fourth portion of the final product is obtained. Meanwhile, sufficient menstruum is added over the powdered drug so that the packed drug does not become dry. Completion of the percolation process should be checked by various methods like:
- By checking the specific gravity of the end few ml of percolate and compare with the specific gravity of fresh menstruum.
- Evaporate the last few ml of percolate to dryness and see if any residue comes after evaporation or not.
- Perform the specific chemical test for the last few ml of percolate like tannins, glycoside, alkaloids, etc whether it is positive or negative.
Pressing the marc:
In the last, the marc should be pressed, and obtain liquid is added into the collected menstruum. More menstruum should be added to obtain the desired volume. Allow the liquid to stand and separate the suspended particles by filtration or decantation. Example- Strong tincture of ginger, tincture of belladonna, etc.
In this type of percolation, the initial portion (near about three fourth of the percentage of menstruum) which has the major quantity of active ingredient is kept separate just like the simple percolation process and the last one-fourth amount of menstruum is collected in another vessel and concentrated by the method of evaporation. This concentrated syrupy fluid should be added into the previous three fourth portion of menstruum. Adjust the
final volume. The last one-fourth portions contain very less amount active ingredients so, with the help of evaporation generally, we concentrate the menstruum. Example- Liquid extract of licorice.
The process has the advantage that we obtain the concentrated menstruum without heating the major portion (three fourth portions) which have more active ingredient and just heat the last one-fourth of portions which also save energy as well as handles less amount fluid for the process.
In the modified percolation process we reduce the drug/percolate ratio to 1:3 which is generally 1:4 in the common percolation process. By this, we can save menstruum, time, and heat. Percolation is a type of displacement method. It has been observed that a stationary menstruum dissolves the more active constituents because stationary menstruum may remain in contact with more time to drug compare to the moving menstruum. Hence more quantity of menstruum is required to exhaust the drug in the case of a simple percolation process. But if a percolation process is broken into the various step of maceration the drug/ product ratio will be reduced to 1:3.
Soxhlet Extraction/Continuous Hot Percolation Process
This extraction process is more suitable where menstruum has less penetration into the cellular tissue. Active constituents are not readily extracted with a solvent. The quantity of solvent for extraction of the active constituents is very less.
Soxhlet extraction is preferable in the above-mentioned conditions. Here the hot menstrual cycle is passed again and again over the crude drug and solubilizes its active ingredient until the drug becomes exhausted. The apparatus consists of three important parts:
- A round bottom flask (RBF) contains menstruum.
- Extraction chamber in which drug is kept in a thimble.
The size reduction of the crude drug was performed and packed the size reduced drug was into the thimble (made of filter paper or cellulose) which is helpful to prevent choking of the extractor.
Place the solvent in the round bottom flask (RBF) and boil it by the heating mantle. The vapor of the solvent goes to the condenser via a side tube. The condenser condenses the vapor of solvent and this solvent penetrates the packed drug. It percolates the crude drug and extracted the active ingredient. The level of menstruum is increased in the extractor until it reaches to maximum point from where it is siphoned into the round bottom flask.
This menstruum is again heated in RBF where active constituent remains in RBF and vaporized solvent again passes through the side tube and condenser which penetrates the drug present in the extractor. The process will continue till the drug become exhausted. Some drugs like resin, opium, gum block the Soxhlet apparatus so this is not a suitable method for these drugs.
Thermolabile drugs and constituents are not suitably extracted by this method because the active ingredient can be destroyed. Pure solvent or only those solvent mixtures which have constant boiling temperature can be used for extraction purposes.
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