Herbs in Africa - Part 7
Extraction Apparatus
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
 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.
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
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.
Small Scale Extraction Apparatus.
 The
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. www.herbdatanz.com
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