Note: Descriptions are shown in the official language in which they were submitted.
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SOYBEAN OIL PROCESS
This application is a continuation-in-part of U.S. Patent Application Serial
No.
09/775,105, filed February 1, 2001.
Background of the Invention
( 11 Field of the Invention
The present invention relates generally to a process for extracting and
refining
soybean oil and to the resultant product, and in particular to the production
of non-
hydrogenated soybean oil having an acceptable frylife similar to that of
partially
hydrogenated soybean oil.
(2) Description of the Prior Art
Soybean oil production involves several steps that are necessary to render the
1 S soybean oil suitable for human consumption. These production steps may be
broadly
characterized as 1) soybean preparation, 2) oil extraction, and 3) oil
refining. Soybean
preparation generally includes the steps of cleaning, drying, cracking, and
dehulling.
The great majority of commercial soybean oil production processes extract or
separate the oil from the soybean meal by a process known as solvent
extraction. In
the solvent extraction process, the prepared beans are first flaked to provide
a large
surface area. A solvent, commonly hexane, is then pumped through the soybean
flakes to dissolve the oil in the hexane, separating approximately 99.5% of
the oil
from the meal. The hexane is then separated from the oil and recycled.
The crude oil resulting from the solvent extraction must then be subjected to
additional treatments, collectively called "refining", to remove various
materials in
CA 02383009 2002-04-23
order for the oil to be suitable for consumption. These materials include
hydratable
and non-hydratable phospholipids, free fatty acids, and various color and
flavor
components. Crude soybean oil contains phosphorous compounds called hydratable
phospholipids, and small amounts of calcium and magnesium that complex with a
portion of the phospholipids to form non-hydratable phospholipids. Hydratable
phospholipids are normally removed by a process known as "degumming", in which
the oil is agitated or otherwise intimately combined with water to precipitate
gums
from the oil. The gums are then removed by centrifugation.
These precipitated gums can be used as a feed additive, or evaporated to
remove moisture. The end product, lecithin, has various end uses such as food
emulsifier. The degummed oil is dried under vacuum to remove any water.
Removal
of non-hydratable phospholipids is considerably more difficult and expensive,
requiring further chemical treatment, typically chemical refining, to break
the
chemical bonds between the calcium or magnesium ions and t:he phospholipids,
followed with extensive bleaching of the oil.
In most processes, free fatty acids are removed from the oil by a process
known
as caustic refining, also called chemical or alkali refining, in which the oil
is mixed
with a caustic material, such as sodium or potassium hydroxide, which
undergoes a
saponification reaction with the acids, forming soaps that are then removed by
centrifugation. Non-hydratable phospholipids are removed along with the free
fatty
acids. Chemical refining soybean oil is an expensive process, requiring a
large
investment in capital equipment. In addition, a significant quantity of the
oil is
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captured by the soaps, adversely affecting oil yield. Also, the caustic
refining process
produces soapstock, which has little commercial value, and it is difficult to
dispose of
without environmental problems.
Conventional refining processes also involve some bleaching of the soybean oil
to remove color pigments that adversely affect the color of the oil. Finally,
chemicals
that add flavors to the oil are removed by a process known as "deodorization",
which
is essentially a form of physical distilling, in which the oil is subjected to
high
temperatures under a vacuum for a short period of time, which is sufficient to
remove
the flavor-causing components, but insufficient to break down non-hydratable
phospholipids.
A major use of soybean oil is in deep frying of foods, such as chicken, fish,
french fries, etc., either in the production of pre-cooked packaged foods, or
in the
preparation of foods for on-premise or carry-out consumption in restaurants
and other
commercial establishments. In deep frying, a container or vat is filled with
cooking
oil that is heated to a flying temperature, normally around 35(I° to
375° F. The
uncooked food is then immersed in the hot oil for a sufficient time to effect
the desired
cooking, and then removed for serving or packaging.
Some of the oil in the vat is lost during cooking due to absorption and
evaporation. The oil is replenished by adding fresh oil to the oil remaining
in the vat,
and the oil is reused. This procedure is repeated until the oil becomes
unusable, as
indicated by darkening of the oil and the food cooked in the oil, and/or by
the
observance of an undesirable taste or appearance in the food being cooked.
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Non-hydrogenated soybean oil produced by solvent extraction and caustic
refining is unsuitable for use in commercial frying operations due to its
limited frylife.
Such oil has a maximum frylife of only about 4-5 fry cycles, a cycle being the
flying
of one batch of food. Replacement of the oil at this frequency is
uneconomical. In
order to extend the frylife of soybean oil to a commercially acceptable number
of fry
cycles, preferably at least about 30 fly cycles, refined soybean oil is
normally at least
partially hydrogenated. Hydrogenation of solvent extracted, caustic refined
soybean
oil reduces the percentage of C 18:3 acids. It is commonly believed that the
presence
of C 18:3 acids contribute to the rapid deterioration, and thereby limited
frylife, of
solvent extracted, caustic refined soybean oil.
Saturation or hydrogenation of C 18:3 acids, however, reduces the healthful
properties of soybean oil. Therefore, attempts have also been made to reduce
the C
18:3 or linolenic acid content of soybean oil by genetically modifying the
soybean,
thereby enabling the production of non-hydrogenated soybean oil having a C
18:3
content of significantly less than the 6-8~% by weight C 18:3 acids, based on
the total
content of free fatty acids, found in conventional soybeans. This approach, to
date,
has met with at most limited success.
Thus, there remains a significant commercial need for a soybean oil that has
an
acceptable fiylife, preferably at least 30 fry cycles, without the loss in
healthful
benefits resulting from hydrogenation as heretofore required. Non-hydrogenated
soybean oil with a commercially acceptable frylife would enable commercially
economical frying of foods while retaining the healthful benefits of the
soybean oil.
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Summary of the Invention
While the reasons are not fully understood, it has been found that an non-
hydrogenated soybean oil having a commercially acceptable frylife can be
produced
by a combination of mechanical extraction of the oil from the soybean meal
combined
with physical refining of the crude oil. While the resultant non-hydrogenated
oil has a
C 18:3 content similar to non-hydrogenated oil produced by solvent extraction
and
caustic refining, the frylife of the oil produced by the present process is
considerably
greater than that of non-hydrogenated oil produced by solvent extraction and
caustic
refining.
Mechanical oil extraction and physical refining are known separately but have
not been heretofore used in combination, and these resultant properties of the
resultant
oil have not been appreciated. A form of mechanical oil separation has been
practiced
in the prior art. In the prior art process known as expelling, the dehulled
beans are
extruded through a screw press to frictionally heat the beans and rupture the
oil cells.
Within the screw press, the beans are subjected to high pressures and
frictionally-
generated high temperatures for a short period. The crushed, oil-containing
meal is
then pressed to separate most of the oil from the meal. This process has
rarely been
used to process soybeans due to the fact that about 25% of the soybean oil is
left in the
meal.
Physical refining has heretofore been used for oils that are naturally low in
non-hydratable phospholipids, such as lauric oils, particularly palm oil. In
physical
refining, the oil is vacuum distilled at high temperatures, e.g., from about
450°F to
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about 500°F, to separate more volatile components from the oil. This
process is used
to remove various flavor components, and will also remove fi-ee fatty acids.
However,
the process has not been viable for removing free fatty acids from oils such
as soybean
oil, which contains higher levels, i.e., more than 20 ppm based on elemental
phosphorous content, of non-hydratable phospholipids. The high temperatures
required for physical refining tend to break down the non-hydratable
phospholipids
that are present in the soybean oil, producing chemical compounds that cause
an
unacceptable flavor and color.
In the present process, the soybean oil is mechanically separated from
prepared
soybeans by first rapidly heating the beans to a temperature of from about
300°F to
about 370°F, preferably from about 315°F to about 335°F,
followed by mechanically
pressing the oil from the beans. Surprisingly, it has been found that the
percentage of
non-hydratable phospholipids in the crude oil is considerably lower, e.g.,
less than 20
ppm based on the weight of elemental phosphorous, if the beans are heated to
within
this temperature range. Care should be exercised in using temperatures above
about
350°F, since the oil will tend to scorch, causing an off taste in the
final product and a
darker color. Also, heating the oil to less than 300°F will fail to
destroy sufficient
trypsin inhibitors in the meal.
The time during which the soybeans are heated is also important to the
invention. Heating of the beans to the desired temperature in at least 10
seconds has
been found to achieve maximum rupture of the oil cells, and thus maximum
extraction
of oil from the soybeans. However, heating of the beans for longer than about
60
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seconds degrades the desirable characteristics of the oil. Therefore, the time
to heat
the beans to the desired temperature should be less than about 60 seconds, and
preferably from about 10 to about 30 seconds.
Desirably, the beans are crushed during or after heating to assist in freeing
the
oil from the remainder of the soybeans, i.e., the meal. One way to
simultaneously heat
and crush the beans is to extrude the beans through an extruder in which the
beans are
subjected to high pressures that crush the beans while creating frictionally
heating of
the beans.
Mechanical pressing of the beans removes only about 74% to about 76% of the
oil from the beans, leaving a soybean meal that includes from about 6% to 7%
soybean oil. However, this meal has a substantially higher nutritional value
than
soybean meal from conventional solvent extraction, with the resultant higher
selling
prices at least partially offsetting the oil loss. While not being restricted
to any
particular theory, it is believed that the oil residue left in the meal may
include
components that contribute to the limited frylife of solvent extracted,
caustic refined
soybean oil.
The crude soybean oil is then degummed by intimately mixing the crude
soybean oil with water, which may contain citric acid or a similar organic
acid, to
form gums of the hydratable phospholipids, which are then removed from the
crude
oil, e.g., by centrifuging. The degummed oil is then bleached with bleaching
materials, such as clay, silica gel, and if needed for damaged beans, sodium
metasilicate. The oil is then vacuum dried and filtered.
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At this stage, the oil is a useful product known as refined and bleached oil.
However, for many applications it is desirable to further process the oil to
remove free
fatty acids and components that contribute to the color and flavor of the oil.
In the
present process, it is possible to remove the free fatty acids without the
caustic
refining required in prior art processes. Instead, the free fatty acids are
removed at the
same time as the flavor and color components are removed during physical
refining.
As noted earlier, removal of free fatty acids by physical refining has not
been
feasible due to the large amount of non-hydratable phospholipids in the oil,
which
degraded under the high temperatures required for physical distillation. In
the present
process, however, the amount of non-hydratable phospholipids is generally less
than
2.0 ppm based on the weight of elemental phosphorous in the compounds as a
result
of the mechanical extraction. After silica treatment and bleaching, the
phosphorous
content will be less than 1 ppm. This insignificant amount of phosphorous has
no
affect on oil flavor or stability.
During the physical refining stage of the invention, free fatty acids and
flavor
components are removed from the oil by heating the oil in a distillation
column to a
temperature of from about 450°F to about 500°F, and preferably
for from about 460°F
to about 480°F, to distill off the free fatty acids and flavor
materials. By industry
standards, the final oil should contain less than about 0.05% free fatty
acids.
Detailed Description of The Invention
Various methods may be employed to heat and crush the soybeans. By way of
example, cleaned and dehulled soybeans were extruded through an extruder
having a
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15 inch conduit, with the dwell time within the conduit being about 30
seconds.
During extrusion, the beans were mechanically crushed and frictionally heated
to a
temperature of 315°F to 335°F. Crushed soybeans exiting the
extruder were then
pressed to separate most of the oil from the soybean meal. The separated oil
was
screened to remove remaining solids prior to the refining process.
The crude soybean oil was then degummed with water containing citric acid
and centrifuged to remove gums of the hydratable phospholipids. The degummed
oil
was then bleached, vacuum dried and filtered. The oil was then physically
refined by
heating the oil in a distillation column to a temperature of from about
460°F to about
480°F, to distill off the free fatty acids and flavor materials. 'Che
frylife of the
mechanically extracted and physically refined oil was then compared to
partially
hydrogenated soybean oil.
Example
Separate flyers were filled with a leading commercial, partially hydrogenated
flying oil (FryMax Supreme), and with non-hydrogenated soybean oil prepared in
accordance with the present process. The weight of oil in each fryer was
7000g, with
fresh makeup oil being added each day during the test to repl<~ce oil lost
through
evaporation and absorption. The oils were maintained at a frying temperature
of
350°F for eight hours each day. Ten batches of breaded chicken patties
were cooked
each day, with each batch containing 800g. Frying time was 3.5 minutes with a
3
minutes drain time between batches. Consumer tests were conducted on batches
#1,
#15 and #30. The fried chicken patties were then ranked on a scale of 1-10 by
a taste
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panel (10=like very much, 5=like somewhat, 1=dislike very much). The following
results were found:
Batch # Present Oil Commercial Oil
1 9 9
15 8 7
30 7 5
The commercial patties in batch #30 were observed to be darker and show burnt
spots.
These deficiencies were not observed in the patties fried in the present oil.
As seen from the following comparison, the C 18:3 content of the non-
hydrogenated oil resulting from the present process is in the range of at
least 6% up to
8% of the total fatty acids, and the total trans fatty acid content is less
than about 1 %.
In comparison, the partially hydrogenated soybean oil is substantially lower
in C 18:3
acids, and much higher in trans fatty acid content:
FAC (%w/w) Partially Hydrogenated Present Process
Soybean Oil Soybean Oil
C14:0 0.1 0.1
C 16:0 11.2 10.9
C 16:1 0.1 0.1
C 18:0 7.2 3.7
C 18:1t 8.2 -
C 18:1c 39.9 24.7
C 18:2t 4.4 0.4
C 18:2c 27.1 53.5
C 18:3t 0.4 0.3
C 18:3c 1.2 6.0
C 20:0 0.1 0.4
Total Trans 13.0 0.7
(%/wt. )
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Certain modifications and improvements will occur to those skilled in the art
upon a reading of the foregoing description. It should be understood that all
such
modifications and improvements have been deleted herein for the sake of
conciseness
and readability but are properly within the scope of the following claims.
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