Process for stabilization of oil from plant materials

The oil content of bran can be stabilized by the addition of a stabilizing agent to grain prior to, during or after milling of the grain, particularly rice. The treatment improves the yield of edible oil attainable when the oil is extracted from the milled bran.

This invention relates to the stabilization of oil in plant materials 
having a high oil content, and especially to those plant materials which 
also contain high levels of enzymes which degrade oils. More particularly, 
this invention relates to processes for stabilizing oil in legumes and 
grains, and has particular application to the oil in rice bran. 
Oils in plant materials are subject to degradation by enzymes present in 
the plant material. Such breakdown begins immediately upon crushing or 
milling of the plant material, and in certain instances, can lead to 
sufficient degradation of the oil such that it is not economically 
feasible to extract an edible oil from the plant material. Breakdown of 
the oil in the bran of most grains is a particular problem due to the 
relatively high levels of degradative enzymes found in such grains and the 
difficulties faced when attempting to inactivate these enzymes. 
For example, the oil from rice bran, which has a relatively high oil 
content and relatively high levels of oil degrading enzymes, is 
underutilized as a foodsource due to the rapid breakdown of the oil into 
free fatty acids which in turn are oxidized thus leading to rancidity. 
Breakdown of the oil in rice bran begins immediately after milling by 
action of lipases and other degradative enzymes hydrolyzing the oils to 
free fatty acids (FFA). Crude rice bran oil with FFA levels of over 10% 
generally is not economically suitable for edible oil production, and is 
designated as industrial oil for use in the manufacture of soaps and 
similar items. Hydrolysis of the oil occurs rapidly, at a rate of 
approximately 1% per hour for the first several hours after milling. 
Consequently, FFA levels of over 10% can readily accumulate thus 
significantly reducing the value of the oil as a foodstuff. 
Stabilization of oil from cereal brans would increase the economic value of 
the bran and would make it possible to recover the oil for use as a 
foodstuff. Existing methods for stabilizing the oil in cereal bran involve 
cooking of the bran immediately after milling and subjecting milled bran 
to simultaneous high temperature and high pressure treatment such as that 
afforded by extruders used in the food processing industry. However, there 
remains a need for more economical means of stabilizing the oil in brans, 
especially in rice bran, which does not require the use of industrial food 
processing machinery. 
It is an object of this invention to provide a method of stabilizing the 
oil in plant materials having a relatively high oil content and having oil 
degrading enzymes which does not require the use of industrial food 
processing equipment. 
SUMMARY OF THE INVENTION 
It has been found that the oil in plant materials, in particular the oil in 
plant materials having a relatively high concentration of oil and having 
oil degrading enzymes, can be stabilized without the use of industrial 
food processing equipment by the addition of a stabilizing agent either 
prior to or after milling of the plant material. Preferably the 
stabilizing agent is added to the plant material at the time of milling as 
the milling process facilitates efficient mixing of the stabilizing agent 
with the plant material. Preferably the plant material to be stabilized is 
a grain or legume having a relatively high oil content. More preferably 
the plant material to be stabilized is rice bran. Preferred stabilizing 
agents which act through reaction with cations comprise cyanide, sulfide, 
azide, carbon monoxide, ethylenediaminetetraascetic acid (EDTA), 
1,10-phenanthroline, diethyldithiocarbamate (DDC), .alpha., 
.alpha.'-dipyridyl, o-phenanthroline, orthophosphates, pyrophosphates, 
borates, carbonates, 8-hydroxyquinoline, thiourea, 2,3-dimercaptopropanol 
(BAL), mercaptoethanol, imidazole, mercaptoethylamine, and citric acid. 
Preferred stabilizing agents which act through reaction with thiol groups 
comprise chloroacetophenone, chloropicrin, bromobenzyl cyanide, 
fluoripyruvate, maleic acid, N-ethylmaleimide, quinones, acrolein, sorbic 
acid, acetoacetate, methylglyoxal, oxidized glutathione, sulfites, 
tetrathionate, performic acid, hydrogen peroxide, iodine, alloxan, 
porphyrexide, porphyrindin, O-iodobenzoate, ferricyanide, 5,5'-dithiobis 
(2-nitrobenzoic acid), 2-pyridine disulfide, and 4-pyridine disulfide. 
More preferred stabilizing agents comprise salts of bisulfate, bisulfite 
and metabisulfite, and such salts comprise sodium, potassium, ammonium, 
calcium, and magnesium. Preferably the stabilizing agent comprises sodium 
bisulfate, and sodium metabisulfite. Most preferably the stabilizing agent 
is sodium metabisulfite.

DETAILED DESCRIPTION OF THE INVENTION 
In order that the invention described herein may be more fully understood, 
the following detailed description is set forth. In the description the 
following terms are used. 
"Stabilization" means the treatment of a plant material to inhibit 
degradation of the plant oil. As a result of stabilization, the level of 
free fatty acids in the oil remains low, less than about 10%, thus 
allowing extraction of oil which is suitable as a foodsource for human 
consumption. 
"Stabilizing agent" means an agent which when applied to the plant material 
either before or after millin, such as a grain either before or after 
milling the bran, acts to inhibit the degradation of the oil in the plant 
material. Preferred stabilizing agents comprise salts of bisulfate, 
bisulfite or metabisulfite. Stabilizing agents may further comprise 
sequestering agents, such as aminopolycarboxylic acids, hydrocarboxylic 
acids, and coordination compounds. Stabilizing agents may also comprise 
wetting agents, such as a surfactant. 
In accordance with this invention, stabilization of the oil in plant 
materials is carried out by the addition of a stabilizing agent to the 
plant materials at the time of processing the plant material, such as by 
crushing or milling. Generally, the stabilizing agent is added to the 
plant material in an amount to inhibit the enzymes which degrade the plant 
oil. For example, up to about 10% stabilizing agent on a dry weight basis 
is added to the plant material prior to processing. As will be appreciated 
by a person of ordinary skill in the art, the quantity of stabilizing 
agent required to stabilize the oil in a particular plant material will 
depend upon the degradative enzymes present in the plant material and on 
the particular stabilizing agent used. For example, for the stabilization 
of cereal grains using one of the preferred stabilizing agents, which are 
salts of bisulfate, bisulfite or metabisulfite, between about 1% and about 
10% of these stabilizing agents on a dry weight basis are added to the 
cereal grain prior to processing. Preferably less than about 5% on a dry 
weight basis of the stabilizing agent is added to the cereal grain prior 
to processing. 
Tests for the activity of degradative enzymes present in the plant material 
after stabilization using the process of the invention have demonstrated 
that at least lipases and peroxidases are inactivated in the stabilized 
plant material. 
The process of the invention is also useful for the preparation of Tocol 
products as described in co-pending application Ser. No. 07/952,615 filed 
Jan. 19, 1993 now abandoned the disclosure of which is hereby incorporated 
by reference. 
The processes of this invention may be used to treat all types of plant 
materials and preferably to treat plant materials having a relatively high 
oil content such as is found in many grains and legumes. The process of 
this invention is particularly suitable for the stabilization of the oil 
in bran from rice. Following stabilization the stabilized oil may be 
extracted from the plant material and recovered therefrom. The oil may 
then be processed using conventional methods such as, desolventization, 
deodorization, degumming, bleaching and refining to yield an edible oil. 
Alternatively, the stabilized plant material can be processed for 
consumption. 
With respect to brans in particular, following milling, enzymes present in 
the unstabilized grain products, such as lipases, peroxidases, catalases, 
polyphenol oxidases and lipoxygenases present in bran, become active. The 
active enzymes degrade the oils releasing free fatty acids (FFA). It is 
preferred that the FFA levels in the oils be less than about 10% and it is 
more preferred that the FFA levels be about 4% or less. FFA levels may be 
monitored using the official AOCS method Ca Sa-40. 
The following examples are set forth to further illustrate certain 
preferred embodiments of the invention, and are not intended to be 
limiting in nature. In the example rice is utilized as the plant material, 
although one of ordinary skill in the art would be aware that the method 
is equally applicable to other plant materials. 
Generally, the milling of rice is accomplished in a two step process in 
which the hulls are removed by shelling of the grain and then the bran is 
separated from the rice kernel by milling. The following is a typical 
protocol for the milling of rice, collecting the milled bran and 
stabilizing the bran oil. 
Rough rice (paddy rice) from a farm is dried in a commercial-type 
continuous flow, non-mixing, heated air dryer. Drying is carried out to 
lower the moisture content of the rice from a level of between about 18 
and 22 percent to a level between about 10 and 13 percent. The dried rice 
is then cleaned by removing dust, stones, seeds and sticks by aspiration 
in a commercial rice cleaning machine, followed by gravity separation in a 
stoner and particle size separation in a disk grader and a drum separator. 
The husks are then removed using a rubber roller sheller. Paddy (husks or 
hulls) were removed using a paddy separator for the first pass, followed 
by using a paddy separator for the second and third passes. 
When stabilization of the bran is desired, the stabilizing agent is then 
applied to the brown rice either prior to milling the bran from the rice 
kernel in a friction mill or to the milled bran as it is aspirated from 
the rice kernels to yield polished rice. Preferably, the stabilizing agent 
is added to the brown rice prior to milling the bran. The stabilized raw 
bran is then conveyed to a filter/sifter to remove residual broken rice. 
The stabilized bran can now be utilized as a foodstuff without further 
processing, and can be stored, or packaged for shipping as desired. 
Alternatively, the oil can be extracted from the stabilized bran, and the 
extracted oil is suitable for human consumption. 
As is apparent from the above description, the process according to the 
invention does not require the use of industrial food processing equipment 
and can be readily utilized with virtually any milling procedure. 
As a specific example of stabilizing rice bran, sodium metabisulfite is 
added to brown rice to between about 1-10% of the dry weight of the bran. 
The sodium metabisulfite stabilizing agent is preferably prepared as a 20% 
solution for addition to the rice prior to milling the bran. Most 
preferably, the 20% solution of sodium metabisulfite includes 1000 ppm of 
the surfactant sodium dioctylsulfosuccinate as a wetting agent. The rice 
is then milled in a friction mill and the stabilized bran with the 
stabilizing agent is aspirated off and conveyed to a filter/sifter as 
above. 
To improve extraction of oil from the rice bran, the stabilized bran may be 
fed directly into the feed hopper of an expander cooker. Water and steam 
are added through injection ports in the barrel of the expander to raise 
the moisture content of the bran to about 18-24%. Flow of the bran through 
the expander cooker is controlled by a discharge die plate. The moisture 
level is maintained during cooking and the temperature is held between 
about 90.degree. C. and 135.degree. C. for between about 15 and 90 
seconds. During cooking some constituents of the bran are gelatinized into 
a fluid paste which binds the particles together. The bran is expanded as 
it exits the discharge die plate due to the sudden decrease in pressure 
which causes the liquid water to vaporize, and the bran forms a porous 
compact pellet. Vaporization of water causes breakage within the cells and 
a porous, compact bran pellet is formed which is ideally suited for 
extraction by solvent migration percolation. Prior to extraction the 
porous compact bran is cooled to approximately 130.degree. F. 
If the oil from 1--100 g of bran is to be extracted, the stabilized bran is 
immersed in hexane in a ratio by weight of about two to one. The hexane is 
generally heated to about 60.degree. C. using a steam table incorporated 
into an explosion proof vented hood, but other solvents and other 
temperatures may also be employed. The hexane/oil micella is removed from 
the bran by filtration. About 5-6 washings are necessary to bring the oil 
content of the bran to less than one percent. The defatted bran and the 
hexane/oil micella are both desolventized under gentle heating with steam. 
If the oil from 100-500 lbs. or more of stabilized bran is to be extracted, 
it is more practical to use the following protocol. The stabilized bran is 
fed into a counter-current extractor at a flow rate of about 111 lbs/hour. 
Fresh hexane is introduced at a rate of around 312 lbs/hr. The fresh 
solvent temperature is maintained at about 50.degree. C., while the 
extractor temperature is maintained at around 52.degree. C. The residence 
time in the extractor is typically around 45 minutes. The product is a 
defatted bran with an oil content of less than one percent. The hexane/oil 
micella exiting the discharge of the extractor is filtered through a plate 
and frame filter press. The filtered micella is then pumped to a steam 
heated still where the hexane is evaporated and collected by a condenser 
for reuse. 
Following extraction and desolventization, crude rice bran oil is typically 
degummed, dewaxed, bleached and physically refined using steam 
distillation. Degumming is carried out by a two stage addition under 
agitation of 2% water by weight and then 0.15% phosphoric acid (85% 
reagent grade) by weight. The temperature is held at about 82.degree. C. 
to 88.degree. C. for 10 minutes. Then the sludge containing the gums is 
removed via ultracentrifugation. (See U.S. Pat. No. 4,049,686). The 
degummed bran is cooled to about 5.degree. C. to 8.degree. C. and held for 
24 hours. The dewaxed oils form a layer above the waxes which can be 
decanted using a vacuum pump. Bleaching is carried out according to the 
official AOCS method 6c 8a-52. Physical refining is carried out in a glass 
deodorizer at about 250.degree. C. and around 3 mm Hg for about 2 hours. 
The level of free fatty acids (FFA) in the extracted oil is determined 
using the AOCS Official Method Ca Sa-40. Briefly, based upon the FFA range 
expected for the oil, a sample of oil having the appropriate weight is 
mixed with the appropriate amount of neutralized alcohol and then titrated 
with the standard sodium hydroxide while shaking until the appearance of 
the first permanent pink color, having the same intensity as that of the 
neutralized alcohol prior to addition to the oil sample, persists for 30 
seconds. The neutralized alcohol solution consists of 95% alcohol 
containing 1% phenolphthalein which has been neutralized with alkali until 
a faint but permanent pink color persists. Specifically, to measure FFA in 
the range of 0.2-1%, about 28.2 g of sample oil is weighed, mixed with 50 
ml of neutralized alcohol and then titrated with 0.1 N alkali, such as 
sodium hydroxide, until the appearance of the permanent pink color. Using 
oleic as an example, the %FFA is determined by multiplying the molecular 
weight of oleic with the normality of the alkali titrant and the volume of 
alkali titrant, which is then divided by the grams of sample and 
multiplied by 100. 
The results of the FFA analysis in oil extracted from bran, which had been 
stabilized as described above or was left untreated, was determined over a 
period of several days and is displayed in Table 1. The stabilizing agent 
used in this example was 10% sodium metabisulfite on a dry weight basis. 
Similar results have been obtained using lower percentages of sodium 
metabisulfite and sodium bisulfate on a dry weight basis as the 
stabilizing agent. Our tests for the presence of degradative enzymes in 
the stabilized bran oil indicated that at least the lipases and 
peroxidases had been inactivated by treatment of the bran with the 
stabilizing agent. 
In Table 1, Total oil means the percent of oil on a weight basis extracted 
from the bran. FFAIO means the percent of free fatty acid present in the 
extracted oil as measured by the AOCS Official Method Ca Sa-40. FFAIS 
means the percent of free fatty acid in sample, and represents the percent 
of FFA that would be present in unextracted bran. The amount of FFAIS is 
calculated based upon the FFAIO and the Total Oil in the sample as is 
established in the art. 
TABLE 1 
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Treated with 
Untreated Control Stabilizing Agent 
Test Day 
Total Oil 
FFAIO FFAIS Total Oil 
FFAIO FFAIS 
______________________________________ 
1 21.58 2.88 0.65 18.92 2.33 0.38 
2 21.63 9.43 1.95 19.31 2.59 0.58 
3 19.94 16.18 3.19 18.51 3.06 0.56 
4 21.17 16.32 3.39 19.04 3.13 0.57 
5 20.00 19.77 3.95 18.14 3.31 0.60 
6 19.58 20.42 4.00 18.63 3.14 0.58 
7 20.69 28.59 5.92 18.29 4.01 0.73 
8 19.71 31.47 6.20 18.08 4.19 0.76 
9 20.68 32.89 6.80 18.39 4.48 0.82 
10 21.04 31.11 6.52 21.13 4.62 0.85 
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As demonstrated in Table 1: (i) the total oil extracted from the treated 
and untreated samples is comparable, thus allowing direct comparison 
between the results of the tests for FFA levels; (ii) the FFAIO level in 
the treated sample remains low for the duration of the 10 day test period 
whereas the FFAIO level in the untreated control rises quickly to greater 
than 10%, thus demonstrating that the treated oil has been stabilized 
which significantly increases its economic value; and (iii) the FFAIS 
level in the treated sample remains low for the duration of the 10 day 
test period whereas the FFAIS level in the untreated control quickly 
rises, thus demonstrating that the stabilizing agent is capable of 
stabilizing the oil in the bran as well as the extracted oil. 
A person having ordinary skill in the art would be aware that the described 
example may be altered to provide other embodiments which utilize the 
process of this invention. Therefore, it will be appreciated that the 
scope of this invention is to be defined by the appended claims, rather 
than by the specific embodiments which have been presented by way of 
example.