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May 2003



Herbs in Africa - Part 7
 Extraction Apparatus

By Ivor Hughes.

Part seven in a series of articles which outline the basic requirements, for small scale, sustainable cultivation, and processing techniques, for rural communities continues. See links below for previous articles in this series.

The procedures and apparatus required for extraction purposes are simple. Skills required would be carpentry, and light engineering skills, such as metal cutting, drilling, and welding.

When considering the economics of small scale processing plant, there are four factors that need to be balanced.

1. Construction Cost ~ The costs may be reduced by utilizing used dairy or food processing equipment. The main cost is for welding services.

2. Operational Cost ~ Energy use is a prime cost. Unwanted heat loss will add considerably to energy use, as will bad design. Faulty design produces increased labour costs, e.g., difficult load/unload procedures.

3. Maintenance ~ Parts and fittings that are awkward and difficult to clean will add considerably to labour costs.

4. Durability ~ Fragile parts such as sight glasses should have adequate protection. Seals and breakable joints that have to be dismantled should be of good quality. Stopcocks, taps and valves should be corrosion proof.

Contamination problems may easily arise because of the nature of the substances involved in the processing, from chemical action of one substance on another, e.g., heavy metals leached from the equipment are in themselves toxic contaminants, which may then trigger a further reaction in the substance being operated on.

Great care should be taken in the selection of materials that will be in contact with solvents or herb extracts. If using plastics or rubbers, then ask the supplier for the specifications of use. Do not use glaze ware unless you know what type of glaze it is. There are also several physical factors that need to be considered, for example:

A. Strength and Weight ~ Will the equipment be fixed or portable? Will the equipment be able to withstand any stresses placed upon it?

B. It's Durability ~ Parts that are in contact with liquids and vapors must be resistant to corrosion. Metals that are prone to rust should as far as possible be avoided.

C. Thermal Expansion and Conductivity ~ When mating materials, which are different, remember that they will have differing thermal expansion rates. That will produce stress or fatigue with an increased risk of fracture. Distillation equipment and condensers should possess good thermal conductivity.

D. Cleansing and Sterilizing ~ Smooth polished surfaces will simplify cleaning and sterilizing and help in the prevention of the formation of heat resistant films.

Two of the most commonly used materials for plant construction are copper and stainless steel. If considering copper, then it is most important that all linings in contact with the herbal materials, liquids or vapors, must be tin plated. Copper is a heavy metal that can cause liver damage. (hepatoxic).

Stainless steel will meet all criteria. Costs may be kept to a minimum by purchasing and modifying used vats, fittings and tubing.

Austenitic Stainless Steels

When nickel is added to stainless steel in sufficient amounts the crystal structure changes to "austenite". The basic composition of austenitic stainless steels is 18% chromium and 8% nickel. Austenitic grades are the most commonly used stainless steels accounting for more than 70% of production, type 304 is the most commonly specified grade. A higher grade is type 316

Basic properties:

excellent corrosion resistance; excellent weldability (all processes)
excellent formability, fabricability and ductility
excellent cleanability, and hygiene characteristics
good high and excellent low temperature properties
non magnetic (if annealed)
hardenable by cold work only

Percolation is a method of extraction achieved by the downward displacement of soluble extractive by a suitable solvent through a suitably comminuted drug plant. The process is a combination of maceration and percolation and is sometimes referred to as a process of "Macero-Percolation'. Not all plant drugs are suitable for the process. There are 7 distinct operations involved, they are in order of operation;

1. Comminution. The principles of size reduction are covered in section 11-16. Remember if the particles are too fine a solid cake may occur, this will effect the downward flow of menstruum and will most certainly lead to the formation of 'dry pockets' within the body of the material which will escape extraction. It the material is too coarse then interstices are formed through which there is a speedy percolation of menstruum which produces an incomplete extraction and will require excessive volumes of menstruum to exhaust the marc. 

The Principles of Size Reduction
Diagram of cube
The methods of size reduction (comminution) and the classification of particles obtained are covered later in the article. A major factor in the solubility of a substance is the amount of surface area exposed to a solvent. The greater the exposed surface, the faster and more complete is its entry into solution, e.g.

The cube has 6 sides each side is 100 cm2

-: area exposed is 6 x 100 cm2 = 600 cm2.

We slice the cube into 8 equal portions.
The surface area for each cube is

6x 25 cm2 = 150 cm2 -:- 8x 150 cm2 = 1200 cm2

Therefore we have doubled the surface area.

The degree of comminution required will also depend on the composition of the crude drug e.g. is it hard or soft ?, is it thick or thin ? If the drug is leaves or petals then it will be easily penetrated by the solvent, therefore the degree of comminution need not be great, whereas hard and woody substances will require a greater reduction in size. Some substances such as aloes or gum resins need only be crushed, therefore it is a matter of becoming familiar with the material being operated on. The following categories will serve as a general guide.

(A) Broken or Crushed. Gums, resins and inspissated juices. Most seeds and fruits.

(B) Sliced or Small Cut. Rinds, skins, pith, stalks.

(C) Rasped. This type of size reduction is of dubious value and is only officially used for quassia which is a hard wood. From personal experience the tedium may be avoided by pulverizing such substances.

(D) Powders. Rhizomes, roots, barks, woods, corms. There are 5 official grades of powder as seen in the table below.


For extraction purposes we may ignore the 80 and 120 mesh with the 25 and 45 mesh being most often used.

The final consideration for the degree of comminution needed is the menstruum or solvent to be used for the extraction. Remember that our solvents are water, alcohol or a combination, i.e., dilute alcohol.

The tissue of crude drugs in the dried state will contain around 4 to 5% moisture if it has been properly conditioned, therefore if water or a dilute alcohol is used as the menstruum it will penetrate and spread rapidly through the plant tissue; whereas strong alcohol i.e., 50% by volume or over in the initial stages will cause a shrinkage or hardening of the tissue because the water is pulled to the surface thus shrinking the interior. This phenomenon may be explained by the fact that ethanol molecules have a hydrophilic tail.

Apparatus for Size Reduction and Particle Classification.

Classification by Sieve.

Classification by sieve, may be used directly for particle reduction if using whole dried herb. Sieves are further used for size classification of materials that have been subjected to other means of size reduction.

Size Reduction.

Size reduction diagram

Small scale size reduction may be achieved by the use of a mortar or pestle or on a larger scale, the same method that is used to produce mealie from dried maize in a village situation.

For larger scale work a suitable ball mill may be constructed from scrap parts. It may be powered by hand, pedal, or other motive force such as a suitably geared lawn mower engine or the electric motor taken from a discarded washing machine. The general principle is :

The Ball Mill- diagrammatic: 
Rotating metal drum with ball bearings used as the impact and crushing agents

Ball mill diagram

Alternatively an agricultural feed mill will also serve the purpose but it will require an engine or electric motor to operate the apparatus. Size reduction is achieved by fixed or swinging beater arms revolving at speed within a chamber.

The Hammer Mill.
The hammer mill diagram

Small Scale Extraction Apparatus.

Small scale extraction apparatusThe diagram represents a typical piece of laboratory extraction ware. It will be seen that the principle is simplicity itself. Larger scale apparatus may be constructed from discarded milking equipment. 50 or 100 litre containers would meet the needs of the larger cultivation. It is not necessary for the extractor body to be tapered. However a domed bottom as shown in the diagram would facilitate the efficient extraction of the herbal material.

Choosing an Appropriate Extraction Process.

When considering a crude drug for extraction purposes thought must be given to whether the drug is classified as 'organized' or 'unorganized' . If the drug has no clearly defined cellular construction it is unorganized. The maceration process is the only method suitable for unorganized drugs such as gums, resins, oleo-resins etc. Such materials are unsuitable for percolation because the residues would block the percolation process. The same restriction will also apply to crude organized drugs that produce large amounts of mucilage e.g. flax or psyllium seed. In other cases if a drug for whatever reason cannot be reduced to a powder then it is not suitable for a percolation process. Some drug materials e.g. Garlic and squill are extremely hygroscopic and in the presence of water tend to fuse into lumps which make them unsuitable for the percolation process.

Tinctures from Unorganized Drugs.

Maceration is the only feasible method of producing a tincture from an unorganized drug such as gum benzoin or propolis resin. Further information on the exudates will be found in Sections 9-16, and 9-51 et seq. 9-57.

The marc from the exudates is usually slimy or gummy and may also consist of various types of debris e.g. insect parts, fragments of soil or plant parts. The gums are insoluble in alcohol while the resins or oleo-resins will pass completely into solution. The gum and debris will sink to the bottom of the maceration vessel. The separation of the supernatant liquid is usually by simple decantation or if required by filtration. There is no advantage to be gained by attempting to press the marc because all of the soluble constituents have entered into solution.

1. Imbibition. The word is derived from the Latin meaning 'to drink in'. The comminuted drug thoroughly moistened with a portion of the menstruum. This is best done in a lidded container of a suitable size. The moistened drug is allowed to stand for a period of four hours to allow the drug to imbibe the menstruum and thereby swell to its maximum capacity. The container used should be large enough to accommodate the expansion of the drug.

2. Packing. On completion of imbibition the drug should be passed through a number 10 sieve (Table 6-45A) to break up any lumps that may have formed. The drug is then transferred to the percolation vessel in portions. Each portion should be firmly packed but not so firmly that liquid is forced from the drug but sufficient to exclude any air pockets.

4. Maceration. Open the stop cock on the bottom of the percolator and pour in the menstruum in portions and allow to percolate through the packed drug. If the menstruum drips through the stop cock in less than 10 minutes, the drug is too loosely packed. If the first drop takes 25 minutes or more then the drug is too tightly packed. If all is well, then close the stop cock and pour in sufficient menstruum to leave a layer 1 or 2 cm deep over the drug. Cover the percolator and leave to macerate for 48 hours in a warm dark place at a temperature not exceeding 25°C.


For earlier articles in this series:

Part 6 Skin Care lotions and Medicinal Balms

Part 5 The Home Based Herbal Business. 

Part 4 Dehydration apparatus

Part 3 Extraction of herbal material

Part 2 Dehydration

Part 1 Conservation and cooperation

More information:

ŠIvor Hughes.

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