Abstract
This project work studied the refining of crude soya bean oil extracted
from soya bean seed using alkali /caustic refining method. The work
was carried out using phosphoric acid for the degumming
/pretreatment process and sodium hydroxide for the neutralization
/refining of the oil. Certain tests were carried out on both the crude
and refined oil such as saponification value, acidic value, iodine value,
specific gravity and viscosity, the results obtained after the tests
include 42.075, 164.28, 589.05, 0.8944 and 0.038 respectively for
refined oil and 44.88, 109.52, 448.8, 0.8148 and 0.035 respectively
for the crude oil.
1.1 INTRODUCTION
Refining of vegetable oils is essential to ensure removal of
germs, phosphatides and free fatty acids (F.F.A) from the oil, to
impact uniform colour by removal of colouring pigments and to
get rid of unpleasant smell from the oil by removal of odiferous
matter.
Refining is carried out either on batch operation or as
continuous operation. With certain oils even physical refining can
be carried out instead of chemical.
For processing less than thirty tones of oil per 24 hours, and
when oil has F.F.A content of 1 percent or less normally batch
process is recommended. Batch process involves low capital
investments, simplicity of operation and low maintenance,
making refining economically a viable proposition even at
capacity as low as 10 tonnes per 24 hours. (According to Dietary
fats and oils in Human Nutrition. (Rome 1977)).
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Soyabean oil is produced from the seed of the legume called
soja max or calyclue max. The seed has an oil content of about
20%, it is the highest volume vegetable oil produced in the world.
The crude oil is obtained by pressing or solvent extraction
method. The main uses of the oil after refining, bleaching and
deodorization and partial hydrogenation are in the manufacture of
Magrine and shortening. The unhydrogenated oil is also used in
blends with other oil but its tending to revert when exposed to air
or higher temperatures limits its use. (Hand book of industrial
chemistry, Reigel et al, (2003)).
Soyabean oil is also used extensively in the manufacture of
drying oil products.
Crude soyabean oil of good quality has a lighter amber
colour which upon alkali refining is reduced to the light yellow
colour of most vegetable seed oils. Soyabean oil produced from
green or immature beans may contain sufficient chlorophyll to
have a greenish cast but this is not usually very evident until
after the yellow red pigment of the oil have been bleached in
hydrogenation (G.S Breck and S.C Bhatia, 2008).
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The crude oil particularly that obtained by solvent extraction
contains relatively large amount of non-glyceride materials
consisting chiefly of phosphatide. They are removed by water
washing during refining processes. The phosphatides removed by
water washing are converted to soya lecithin. The free fatty acid
content of good crude soyabean oil like that many other
vegetable oil is slightly in excess of 0.5 percent. (Hand book of
Industrial chemistry,Reigel et al (2003)).
1.3 BACKGROUND OF THE STUDY
Crude fats and oils are processed by general scheme shown
below with modifications or exceptions for specific species.
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Fig 1:1-Generation flow sheet for refining and processing fats and
oils (according to G.S Breck and S.C Bhatia, 2008).
CRUDE OIL
Margarine
votator
Shortening stock
Liquid Shortening
votator
Shortenings
Margining stock
Mixing, Chiling
Steam salad and
cooking oils
Emulsifirs
Blended oils
Winterization
Rearrangement Hydrogenation
Thermal Fractionation
Red Refined, Bleached Deodourised Oil
Bleaching physical refining Deodourization
Spent Earth
Soap stock
Activated Earth Bleaching
Alkali
Water Degumming Crude Lecthin
Degummed oil
Alkali refining
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The phospholipids (Lecithins) must be removed to avoid
darkening of the oil during high temperature deodourization and
in deep-fat- frying applications. This removal typically is
accomplished during the alkali refining process or in a separate
water/acidic water degumming step before alkali refining. Crude
soyabean oil has an unusually high (2-3.5 percent) phospholipid
content among oils and often is degummed in a separate
operation to not more than a 300 ppm level (as phosphorus) to
avoid precipitation during shipping and storage. Refine soyabean
oil contains 10ppm or less phospholipid. Degumming is achieved
by mixing crude soyabean oil with water to hydrate the
phospholipids and enable their removal by centrifuge. Critrics and
other acids sometimes are added in a step called supper
degumming to help remove phospholipids that are not hydrated
by water. Degummed soyabean oil or crude oils of other species
are neutralized with sodium hydroxide solution to from sodium
salts of the fatty acids which are removed as soap stock by a
continuous centrifuge. The soapstock also includes remaining
phospholipids, some colour and flavor compound. (Hand book of
industrial chemistry, Reigel et al (2003)).
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The soap stock can be dried if refining is done adjacent to an
extraction plant or acidified again to remove fatty acids and sold
to the olechemical industry. The oil is then water washed and
centrifuge one or two times to remove residual soaps.
According to GS Breck and S.C Bhatia, a total degumming
process for removing essentially all the phosphatide from
soyabean oil using first an acid and then an alkali and two
centrifuges has shown higher yields than conventional refining.
This process however, does not remove prooxidant metals
efficiently and for this reason has not found commercial
acceptance in the united state.
G.S Breck and S.C Bhatia have stated that Dijkstra has described
a novel process where the washing water is recycled to the oil
feed and use to dilute concentrated alkali. This process does not
generate an aqueous effluent and can be used for both acid and
alkali refining, thus allowing refiners to change gradually from
alkali refining to physical refining. Neutralization of soyabean oil
with alkali solution assures elimination of free fatty acids without
notable change in the phosphatide content. The phosphatidic
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concentration obtained from oil previously neutralized in the
miscella was of higher quality than the phosphatidic concentration
obtained from the oil of the starting miscella. Aqueous ammonia
has the advantage of being safe for the environment because the
deacidification agent can be repeated or reused. Oils especially
soyabean oil with low degree of oxidation can be fully deacidified
only with the help of the ammonia. The same effect can
frequently be achieved by a preliminary desliming with 5 percent
formic or citric acid. Deodourization at 2100
c of oils that have
been deacidified with ammonia and washed with water yield
bland and pale edible oils having good storability (G.S Breck and
S.C Bhatia).
List and Erickson state that of all the unit processing operations,
refining has the most significant effect on oil quality measured by
colour, oxidative stability and storage properties.
If soyabean oil is not properly refined, subsequent processing
operation such as bleaching, hydrogenation and deodourization
will be impared so that finished products will not fail to meet
quality standards. Also, poor refining will reduce the yield of
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natural oil, thereby lowering manufacturing profits. (JAOCS, Vol.
60).
According to G.S Breck and S.C Bhatia, caustic refining
removes free fatty acid to 0.01-0.03percent level and remove
virtually all the phosphatides. Crude soyabean oil contains trace
amount (several part per million (ppm)) of prooxidant metals
such as iron and copper. Caustic refining usually removes 90-95
percent of these metals. However, it should be emphasized that
even though caustic refining reduces metallic contamination to
low levels, residual iron and copper still remain strong
prooxidants in refined oils and must be taken in to account during
storage and handling. At a constant percentage of water, the
total amount of caustic used influences colour removal ie the
more caustic used, the lower the colour of the refined oil.
List and Erickson reported that plots of residual iron versus
residual phosphorus content of deodourized oil showed that iron
increases at phosphorus content below about 1ppm, reaches a
constant value of about 2-20ppm phosphorus, then beings to
increase. Thus, the decreased oxidative to stability at phosphorus
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content above 20ppm can be explained by the sufficiently high
iron content (ie greater than 0.2 ppm) which exerts a strong
prooxidant effect. Similarly, decreased stability at phosphorus
content below 2ppm can also be explained because of the
increased iron content. At the same time, it should also be
pointed out that the traditional method for calculating the amount
of refining lye is based on the free fatty acid content and
therefore gives no indication of conditions leading to optimum
phosphorus removal. Phosphatide content generally exceed that
free fatty acids in crude soyabean oil by a factor of about 6. In
refining process control, crude oil is usually educated for refining
cost by the American oil chemist?s society (AOCS)
chromatographic method. (JAOCS, vol 60).
1.3 STATEMENT OF THE PROBLEM
In the market today, most vegetable oils solidify at a low
temperature of less than 250
c. This work is to process and refine
edible and quality soyabean oil that will not undergo solidification
at a low temperature.
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1.4 OBJECTIVES OF THE STUDY
The objective of refining and processing fats and oils include:
? Removal of free fatty acids, phospholipids (gums) colour and
off-flavour/odour compounds and toxic substances to
produce light- coloured and bland products with long shelf
lives.
? Obtaining a mixture of the triacyl-glycerols with the desired
solid content profiles over the range of product use.
? Preparation and storage of semi-solid products with desired
textures.
1.5 SCOPE OF THE STUDY
The crude oil extracted from soyabean needs further treatment to
convert it to a bland, stable, nutrition products that is used to
manufacture margarine, shortening, salad and cooking oil,
mayoniaise, food products, Olechemicals.
This study entails the process of producing good quality oil
through caustic/alkali refining process which is going to be
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compared with other good quality products in the market like
grand product etc.