TECHNICAL PAPER #40
UNDERSTANDING PRESSURE
EXTRACTION OF VEGETABLE OILS
By
James Casten
Dr Harry E. Snyder
Technical Reviewers
Dr. Earl Hammond
Jon E. McNeal
Robert Ridoutt
VITA
1600
Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 . Fax:
703/243-1865
Internet: pr-info@vita.org
Understanding Pressure Extraction of
Vegetable Oils
ISBN: 0-86619-252-2
[C]1985,
Volunteers in Technical Assistance
PREFACE
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
technology.
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.
UNDERSTANDING
PRESSURE EXTRACTION OF VEGETABLE OILS
by VITA
Volunteers James William Casten
and Harry E. Snyder
I. INTRODUCTION
USES OF VEGETABLE OILS
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
cosmetics.
VEGETABLE SOURCES OF OIL AND FAT
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
extraction.
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
II. METHODS OF
EXTRACTING OIL FROM NUTS AND SEEDS
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.
BATCH PRESSES
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
Appendix.
The smallest commercial presses cost several hundred U.S.
dollars
and are hand-operated.
Write to manufacturers for price and
size.
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 CONTINUOUS SCREW PRESSES
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
training.
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.
CHOOSING YOUR METHOD
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.
III. SEQUENCE OF
OPERATIONS
The sequence of operations in processing oil seed for
pressing is
as follows:
STORAGE
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
off-flavors.
CLEANING
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.
DEHULLING
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.
GRINDING OR ROLLING
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.
HEATING
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.
PRESSING
The materials prepared in these ways are pressed, usually in
a
lever press, hydraulic press, or expeller, to remove the
oil.
REFINING
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
deodorization.
IV. FINAL
CONSIDERATIONS
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
oils.
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.
EQUIPMENT MANUFACTURERS: OIL
PROCESSING
Anderson International Corporation
19699 Progress Drive
Strongsville, Ohio 44136, USA
Crown Iron Works
P.O. Box 1364
Minneapolis, Minnesotta 55440, USA
CeCoCo
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.
Roorstraat
Post-Bon 3007
Amsterdam, HOLLAND
Rose, Downs and Thompson, Ltd.
Old Foundry
HU11, ENGLAND
Officine Meccaniche Angelo e Tullio Bosello
VIllatera de Saonara
Padova, ITALY
Mathias Reinartz Maschinewfabril
P.O. Box 137, Industriestrasse 14
404 Neuss, WEST GERMANY
IBG Monforts and Reiners, P.O. Box 516
4050 Monchengladbach 2, WEST GERMANY
ORGANIZATIONS INVOLVED WITH OILSEED PROCESSING
CANOLA
301433 Main Street
Winnipeg, Manitoba
CANADA R3B 1B3
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
Lahore, PAKISTAN
GLOSSARY
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
liquid.
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