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                          TECHNICAL PAPER #40
                        UNDERSTANDING PRESSURE
                             James Casten
                          Dr Harry E. Snyder
                          Technical Reviewers
                           Dr. Earl Hammond
                             Jon E. McNeal
                             Robert Ridoutt
                   1600 Wilson Boulevard, Suite 500
                     Arlington, Virginia 22209 USA
                Tel:  703/276-1800 . Fax: 703/243-1865
               Understanding Pressure Extraction of
                          Vegetable Oils
                        ISBN: 0-86619-252-2
            [C]1985, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in Technical
Assistance to provide an introduction to specific state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their situations.
They are not intended to provide construction or implementation
details.  People are urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis.  Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.  VITA staff included Betsey Eisendrath
as editor, Suzanne Brooks handling typesetting and layout,
and Margaret Crouch as project manager.
The authors of this paper are VITA volunteers.   VITA Volunteer
James Casten is a chemical engineer with experience in oil extraction,
and has worked in Africa, South America, and Europe.
The co-author; VITA Volunteer Dr. Harry E. Snyder is a Professor
of Food Science at the University of Arkansas in Fayetteville,
Arkansas.  The reviewers are also VITA volunteers.  Dr. Earl
Hammond is a Professor of Food Technology at the University of
Iowa in Ames, Iowa. Jon E. McNeal is an analytical chemist with
the United States Department of Agriculture in Washington, D.C.
Robert Ridoutt is employed with Heinz, USA in Pittsburgh, Pennsylvania,
and has had several years' experience in extraction
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries.   VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations.  VITA maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
              by VITA Volunteers James William Casten
                        and Harry E. Snyder
Since the beginning of history, people have made use of the oils
obtained from seeds and nuts.
The principal use of these oils is as food.   They are eaten raw
and cooked, are a useful ingredient in baking, and serve as a
means of transfer of heat in frying.   Oils are a source of
calories and of fat soluble vitamins.
Oils also have a number of nonfood uses.   They serve  as
lubricants, and as a drying base for paints.   They are boiled
with alkali to make soaps, and are an ingredient in many
Commercial Nuts and Seeds Used for Oil
The table below lists the seeds most commonly used commercially
to obtain oil by means of mechanical pressing.(*)
                           Oil Content
Seed                   (%)         Use
Almond                         50            Food, salad oil, soap
Castor                         50            Medicine, lubricant
Cotton seed                    30           Food, paint, resin
Hemp seed                     35            Paint, varnish, soap
Linseed                        40           Paint, soap, varnish, linoleum
Olive                          40            Salad oil, cooking oil
Peanuts (groundnuts)           50           Salad oil, cooking oil
Perilla seed                   50           Drying oil for paint, resin
Poppy seed                     50           Salad oil, cooking oil
Rape seed (colza)              40           Salad oil, cooking oil
Sesame seed                    50           Salad oil, cooking oil
Sunflower seed                 35           Salad oil, cooking oil, soap
Tung nuts                      20           Paint
(*) Soybeans are  not included in the list because their oil content
of only 20 percent makes it impractical to extract oil from them
by mechanical pressing.  Soybean oil is recovered by solvent
Commercial Nuts and Seeds Used for Fat
Vegetable fats are semisolid at room temperature, whereas oils
are liquid.  Fats have a higher melting point than oils, and thus
are heated before pressing.  The table below lists common sources
of vegetable fats.
                            Fat Content
Sees                    (%)           Uses
Cocoa (cocoa) butter             40           Chocolate, food
Coconut oil from copra           50           Food, chemicals, soap
Hahua (illipe) butter            60           Food, candles, soap
Japan wax                        30           Lubricant, leather dressing
Palm nut oil                     50           Food, chemicals, soap
Shea butter                      55           Food, candles, soap
Oil can be extracted from nuts and seeds by heat, solvents, or
pressure.  Extraction by heat is not used commercially for
vegetable oils.  Extraction by solvents is dealt with in
Understanding Solvent Extraction of Vegetable Oils by Nathan
Kessler, in this same series.   This paper deals with extraction
by pressure.
Pressure extraction separates the oil from the solid particles by
simply squeezing the oil out of the crushed mass of seeds.  The
simplest method is to fill a cloth bag with ground seed pulp and
hang the bag so that it can drain.   Some of the oil, called free
run oil(*), flows out; the rest must be pressed out mechanically.
The simplest way is by placing heavy rocks on the materials.  Or
bags of oil seed pulp can be placed one above another in a box or
cylinder, and great pressure can be slowly brought to bear on the
whole mass.  A long lever such as the one shown in Figure 1 can

upe1x3.gif (600x600)

exert up to 100 pounds per square inch.
Since greater pressure provides greater oil recovery, the lever
has often been replaced by heavy and strong mechanical jacks of
several designs (screw jacks, ratchet jacks, and hydraulic
jacks).  A 20-ton jack can exert 1,000 pounds per square inch on
a small cylinder of seeds.
(*)  Terms in boldface are defined in the glossary at the end of
this paper.
A batch press is a press that processes one batch of seeds at a
time.  Batch presses range from small, hand-driven presses that
an individual can build to power-driven commercial   press
capable of processing many tons of seeds a day.
Small Batch Presses
Small batch presses are simple, but inefficient.   However, they
do work.  They can be used in remote areas and can help determine
whether there is a market for oil produced locally.   Few
resources are needed for an operation on this scale:   wood fires
for heating, and hand labor for pressing.   Much hand labor is
required to produce a small amount of oil this way.
If you plan to build a press in a remote area using only wood or
locally available materials, VITA can send you some papers about
processing oil seeds.  Most companies listed in the Appendix sell
batch presses, especially the Anderson and French firms in the
United States.
Advantages of small batch presses:
    o   They can be made of locally available materials.
    o   They can produce a good quality product.
    o   They are easy to repair.
    o   Their cost is low.
     o  They do not require trained operators.
Disadvantages of small batch presses:
    o   They are labor intensive.
    o   Complete recovery of the oil from the seeds is difficult.
       If seeds are plentiful, this is not a serious problem.
Commercial Batch  Presses
Once a business is started, the market and cash flow may grow
quickly.  If that happens, the simple equipment just described
may be outgrown.  You must then get information on larger
equipment from commercial sources.   Larger, commercial batch
presses are available from most of the companies listed in the
The smallest commercial presses cost several hundred U.S. dollars
and are hand-operated.  Write to manufacturers for price and
If electric power is available, hydraulic presses are available
for any capacity required.  When writing to a manufacturer,
describe the kind of seeds or nuts that are available and the
amount you plan to process.  Also give the type of electricity
available, 50 cycle 220 volts for example.
At this scale of operation, seed storage and disposal of oil cake
need to be considered carefully.
Hydraulic presses, which are suitable only for batch processing,
may be powered either by hand or by electricity.   In many parts
of the world, they are the most practical and economical way to
extract oil from seeds.
A hydraulic press <see figure 2> is simple in operation.  The ground seed

upe2x4.gif (600x600)

material or wet plant tissue is placed in the press in layers,
with each layer separated from the next by a press cloth.
Pressure is applied, slowly at first, and then increased as the
oil content in the tissue decreases.   Maximum total pressure is
2,000 pounds per square inch for one inch layers.   Total time to
load the press, apply the pressure, and remove the cake, is
approximately one hour.  Drainage of the oil while under pressure
may  require 30 to 45 minutes.  The amount of raw material that
can be handled depends on the size of the press, which in turn
depends on whether it is a hand press or is operated by
electrical power.
Advantages of commercial-size batch presses:
    o   They can be driven by hand or by electricity.
    o   They are economical to operate.
    o   They are simple to operate and maintain.
    o   Operators require only minimum training.
    o   Recovery of oil from seeds is excellent.
Disadvantages of commercial-size batch presses:
    o   The cost of the machinery is substantial, and delivery
       time may be long.
    o   Spare parts are difficult to obtain in remote areas
       (though they can be airmailed almost anywhere).
    o   Electric power, or generators to produce it, must be
       available to operate the larger models.
Expellers, or continous screw presses, are used throughout the
world for the expression of oil from copra, palm kernals,
peanuts, cottonsees, flaxseed, and almost every other variety of
seed, wherever there is a large enough seed supply to justify a
continuous operation.
Expellers achieve the pressure needed to express the oil by means
of an auger that turns inside a barrel.   The barrel is closed,
except for an opening through which the oil drains.
An expeller can exert much greater pressure on the seed cake than
a hydraulic batch press can.   This increased pressure permits the
recovery of a larger proportion of the oil:   generally, about 3 to
4 percent of the oil is left in the cake with an expeller,
compared to 6 to 4 percent with a hydraulic press.   The expeller
is an essential part of almost all modern oil seed extraction
plants.  It is used both by itself and as a pre-press before
solvent extraction.  Expellers vary in size from machines that
process 100 pounds of seed per hour, to machines that process 10
or more tons of seed per hour.   A three-horsepower machine for 40
kilograms per hour is shown in the Appendix.
Advantages of expellers:
    o   They are the most common type of mechanical extraction
       equipment in use commercially today.
    o   They require less labor than any other method.
    o   Where power is reasonable in cost, and labor is
       expensive, continuous expellers are economical.
    o   Plant capacity is higher than with batch equipment.
    o   Expellers extract a greater proportion of the oil than do
       hydraulic batch presses.
Disadvantages of expellers:
    o   Equipment must be purchased.
    o   Maintenance costs are high, and maintenance requires
       skilled mechanics.  It is always best that the chief
       mechanic be sent to the machinery supply factory for
    o   More energy is required than for batch processing.
    o   Electric Power is required for the press and for
       auxiliary equipment.
    o   The press must operate continuously for at least eight
       hours; intermittent operation is unsatisfactory.
    o   Oil from an expeller has more impurities than oil from
       a batch press, and must be heated and filtered to
       obtain a clean oil.
The type of press that is appropriate depends largely on the size
of the operation.  Oil processing operations range in size from
cottage industries processing only a few pounds of seed per day,
to factories processing as much as 3 or 4 thousand tons of seed
per day.
For small operations (processing less than 1 ton of seed per
day) , the right equipment is almost always a form of batch press.
If 1 or more tons per day are to be pressed, the right equipment
is most often an expeller.
The sequence of operations in processing oil seed for pressing is
as follows:
The seeds, nuts, or plant tissue containing the oil must be
properly stored and prepared for extraction, to maintain high
quality in the final product.
If the oil-bearing material is dry, it must be stored so that it
remains dry, for optimum extraction and quality of the oil.  If
the oil-bearing material is wet plant tissue, it should be
processed for oil-extraction as soon as possible after harvest so
that storage time is kept to a minimum.   Oils in the presence of
water deteriorate rapidly, forming free fatty acids and rancid
After the oil-bearing materials have been removed from storage,
the first step in preparing them for oil extraction is to clean
them.  The cleaning is done so that the oil is not contaminated
with foreign materials, and so that the extraction process can
proceed as efficiently as possible.
Inspect the seeds carefully and remove stones, sand, dirt, and
spoiled seeds.  Dry screening is often used to remove all
material that is over or under size.   Washing is possible, but it
is important to try to avoid wetting tissue that would have to be
dried later.
After raw material has been cleaned, it may be necessary to
remove its outer seed coat.  There are a couple of reasons for
doing this.  The seed coat does not contain oil, so including it
makes extraction less efficient.   Also, the next step will be
grinding to reduce particle size, and any tough seed coats
interfere with this process.
Some seeds, such as peanuts, can be shelled by hand.   Some
others, such as sunflower seeds, are usually hulled in machines.
Still others, like safflower and colza, cannot be shelled.  If
the seed coat is a small part of the whole seed and presents no
problem in grinding the seed, it may be left on.
Seed is not usally pressed whole, since oil extraction is more
efficient if the seed is in smaller particles.   Grinding the oil
seed is one effective way to reduce particle size.   A hand-operated
mortar, millstone grinder, or even a kitchen meat
grinder can be used to convert the seeds to a coarse meal.  Small
hammer mills, motor or hand-powered, are also good.
Another way to reduce particle size is to roll the oil seeds to
produce flakes for extraction.   Many commercial extraction plants
find this the most effective approach.   With large oil seeds it
may be necessary to grind the seed first, and then put the pieces
through flaking rollers.
Either process makes the actual pressing more efficient.  The
final piece size that leads to most efficient extraction can best
be determined by experiment, as the size will vary depending on
the kind of seed and the kind of pressing operation.   Generally,
smaller-size pieces are better for oil removal.   But if the
pieces are too small, they may contaminate the oil and be
difficult to remove from the final product.
A final step in raw material preparation is heating the ground or
flaked oil seed.  The exact reason that heating improves oil
extraction is unknown, but it does increase yields.   Also,
heating is useful if there are enzymes in the plant tissue that
have a deteriorating effect on the oil quality.   If the oil seed
cake (that is, the residue remaining after oil removal by
pressing) is to be used for feed or food, heating may be useful
in increasing protein availability.
Sometimes oil-bearing material is pressed without being heated.
Oil extracted in this way is called cold press oil.
The materials prepared in these ways are pressed, usually in a
lever press, hydraulic press, or expeller, to remove the oil.
Cold press oil can be of such high quality as to need no refining
if it comes from seeds that are fresh and of good quality.
All other oil, especially that which has been pressed from lower
quality feedstock, is likely to have some undesirable cloudiness,
color, or flavor that needs to be removed.
Removal of Cloudiness
Pressed oils need to be filtered to remove particles from the
pressing operation, if the oil is to be clear and clean.
If the cloudiness is caused by gums precipitating, the gums can
be removed by washing the oil with about 2 percent water.  For
this process to be effective, the oil should be heated, and the
hot oil mixed with water, with active stirring.   Next the water
and oil must be separated.  For this, a centrifuge is most
effective.  The degummed oil should be dried by heating to drive
off all moisture, for the reasons cited previously.
Removal of Excess Color
For the removal of excess color, bleaching earths are effective.
The oil is heated and mixed with 1-2 percent of its weight of an
effective bleaching earth purchased from a reliable supplier for
this purpose.  After a contact time of approximately one hour,
the bleaching earth is separated by filtration.   Activated carbon
can also be used.
Removal of Unwanted Flavors
Unwanted flavors are more difficult to remove.   They may be due
to excessive free fatty acids.   If the oil-bearing material is
stored at a high moisture level, or if they material is bruised
or broken or moldy, it becomes more difficult to press, and the
free fatty acid content of the oil extracted from it usually
increases.  Free fatty acids in fresh olive oil vary from 1/2 of
1 percent to 3 percent.  Acidity of over 10 percent is common; if
over 20 percent acid, the oil is good only for making soap.
Free fatty acids can be removed from the oil by washing the oil
with alkali:  put 25 to 30 gallons of the rancid oil in a 55-gallon
steel drum with an open top.   Add 15 gallons of water with
2 1/2 pounds of soda ash dissolved in it.   Mix well with a big
wire whip or paddle to mix the oil and water solution.   The fatty
acid will react with the soda to form soap, which stays in the
water phase.
Let the layers separate for several hours and siphon off the oil
layer.  If the oil still contains fatty acids, you should repeat
the operation.  There will be a loss in volume because the free
fatty acids may account for 10 to 20 percent of the original
volume.  If emulsions form, you can heat the mixture, which will
usually cause a separation.  It is a good idea to heat the
refined oil to drive off any remaining water.   This method works
well without your needing to send the oil to a laboratory for
analysis to determine how much soda ash to add.   An experienced
operator is the best substitute for a chemical laboratory.
It is important to use clean equipment, so wash all the utensils
well at the end of the day.  Also, allow no copper in the plant.
Copper and certain other heavy metals cause undesirable changes
in oils.  For example, heating butter in a copper vessel will
quickly impart a fishy taste to the butter.   Even a copper bolt
in a press can damage the flavor of your product.   Use cast iron,
or stainless steel, but no copper or copper-bearing materials.
Other types of flavors than those of fatty acids can be removed
from oil, but an expensive and difficult process known as
deodorization is used.  It involves distilling off the unwanted
flavors under high heat and high vacuum.   Normally the oils being
processed by small-scale pressing would have the flavors of the
raw material from which they came, and there would be no need for
One of the best sources of information about oil pressing on a
small scale is the small oil processor in the region of interest.
Very seldom will an oil processor be the first in a region to
attempt oil extraction.  Locate those individuals already in the
business of extracting oil from vegetable materials and learn
what kinds of equipment and raw materials they have had success
with, and what kinds of problems they have run into.
In remote villages where oil seeds are plentiful, but mechanics
and machinery are not, a lever press or hydraulic press can be a
convenient means of supplementing expensive imported cooking
Commercial-scale edible oil production, however, is not a cottage
industry.  The extraction and refining of oils and fats suitable
even for local markets is a highly technical and capital intensive
process.  It is large, efficient plants that are the ones
likely to make a reasonable profit.   The vegetable oil extraction
industry is a highly competitive commodity industry in which the
price of oil is established and the price of oil seeds fixed by
the commodity market.  If domestically grown oil seeds are exported,
then a local oil press will have to pay the same price
for seeds as its foreign competitors do.   The small local firm's
costs of doing business are likely to be as high or higher than
those of its competitors abroad.   Tariffs or subsidies may be
required to protect the home industry.   An expeller plant can
sometimes allow a small country to become independent of imported
oils, but the cost of the oil extraction plant may be higher than
the cost of the imported oils.
Anderson International Corporation
19699 Progress Drive
Strongsville, Ohio 44136, USA
Crown Iron Works
P.O. Box 1364
Minneapolis, Minnesotta 55440, USA
P.O. Box 8, Ibaraki City
Osaka Pref. 567, JAPAN
French Oil Millers
P.O. Box 920
Piqua, Ohio 45356, USA
Hander Oil Machinery Corporation
Osaka, JAPAN
S.P. Engineering Corporation
P.O. Box 218, 79/7 Latouche Road
Kampur, INDIA
Stork Company Apparatenfabriek, N.V.
Post-Bon 3007
Amsterdam, HOLLAND
Rose, Downs and Thompson, Ltd.
Old Foundry
Officine Meccaniche Angelo e Tullio Bosello
VIllatera de Saonara
Padova, ITALY
Mathias Reinartz Maschinewfabril
P.O. Box 137, Industriestrasse 14
IBG Monforts and Reiners, P.O. Box 516
4050 Monchengladbach 2, WEST GERMANY
301433 Main Street
Winnipeg, Manitoba
Cotton Development Board
P.O. Box 371
Tamale, GHANA
International Centre for Agricultural Research
P.O. Box 5466
Alleppo, SYRIA
Khadi Village Industries Commission
Irla Road
Vileparle, Bombay 56, INDIA
Makeni Ecumenical Centre
Box RW 255
Lusaka, ZAMBIA
Malkerns Research Station
P.O. Box 4
Malkerns, SWAZ ILAND
National Cottonseed Products Association
P.O. Box 12023
Memphis, Tennessee 38112, USA
National Horticultural Research Station
P.O. Box 220
Thika, KENYA
Nigerian Institute for Oil Palm Research
Benin-Lagos Road Benin City
Bendel State, NIGERIA
Punjab Vegetable Ghee Board
5 Bank Square
Free run oil - The natural accumulation and drainage of oil from
               seed pulp, without the use of a press.
Oil cake - The residue left after pressing.
Dry screening - The manual removal of under- or over-sized seeds
                before pressing.
Cold press - The pressing of oil bearing seeds, pulp, or cake
             that have not been heated.
Emulsions - Any colloidal suspension of a liquid in another