Refining and Bleaching of oils

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REFINING AND BLEACHING
OF OIL
Introduction
Oils are esters of glycerol and fatty acids. They are
extracted from various animal and vegetative sources.
The crude oil so obtained comprise of many undesired
components like phosphatides, free fatty acids pigments,
odour, flavours, waxes, heavy metals and pesticides. Thus
they need to be processed to get desired quality product.
Processing of Oils
The processing of crude oil, so obtained after extraction,
includes the following steps:-
1. Refining– This includes degumming and neutralization
and is done to remove gums, soaps and free fatty acids.
2. Bleaching– Also called Adsorptive Bleaching, it is done
to remove coloured components.
3. Hydrogenation– Also called Hardening, it is done for
raising the melting point and improving quality, taste
and odour of oils.
4. Deodorization– It is mainly steam distillation carried
out at low pressure and high temperature to remove
volatile odour causing components.

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Refining of Oils
Crude oil as received from extraction plant, contain several
non – triglyceride components which must be removed.
This is done by refining.
In refining, chemical and physical processes are combined
to remove these undesirable components.
There are two main routes of refining:-
a. Chemical Refining or Alkaline Refining
b. Physical Refining or Steam Refining
Process of Refining
The steps involved in oil refining are:-
1. DEGUMMING
It is the first step in refining and is done for the removal
of gums, i.e., hydratable and non – hydratable
phosphatides.
Hydratable phospholipids and metals are removed
easily with water. While the non – hydratable
phospholipids are converted to hydratable ones by
addition of small amounts of concentrated phosphoric
acid or citric acid.
For degumming, the oil is heated at 60° - 70° C, then
water and coagulating agents are added. Finally, the
gums are separated by centrifugation.
2. NEUTRALIZING
Neutralizing is done for the removal of free fatty acids.
Neutralization is done in 2 ways:-
A. Alkali Refining: In alkali refining method, free fatty
acids are neutralized with an excess of 0.1 % NaOH

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solution and the mixture is heated at about 75° C to
break any emulsions formed. This leads to the
formation of soaps (foots) which are removed by
centrifugation.
B. Steam Refining: In steam refining, fatty acids are
removed by steam distillation. This process is also
called Stripping. It involves treatment with sparging
steam under high vacuum so that the fatty acids are
removed by distillation. Maximum temperature of
240° to 250° C is sufficient to reduce the free fatty
acid content to a level of about 0.05 to 0.1%.
Neutralizing also remove contaminants like aflotoxin
and organophosphorous pesticides.
Bleaching of Oils
Bleaching is the second step in oil processing. It is done to
remove coloured components from oils.
The oils are bleached with an adsorbent called Bleaching
Agent. These are surfactant powdery materials, like –
 Natural bleaching earths – Fuller’s Earth, clay.
 Acid activated bleaching earths.
 Activated charcoal.
 Synthetic amorphous silica compounds.
The finely powdered adsorbent is added to heated oil,
preferably in the absence of air (to avoid oxidation) and
good agitation. The coloured components are adsorbed by
bleaching agent and the product is then filtered and made
free from bleaching agent.

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According to the process of bleaching, it could be dry or
wet –
1. DRY BLEACHING
During dry bleaching, oil is heated and mixed with
bleaching agent. It is carried out at about 70 torr
pressure, this greatly reduces humidity in oils.
2. WET BLEACHING
In this process, water is added to oil while it is in
contact with bleaching agent in bleaching reactor.
Presence of small amounts of water results in more
efficient use of bleaching agent. Water is introduced in
the form of citric acid solution or by taking wet oil.
After bleaching, oil has to be dried before filtration.
Importance of Refining and Bleaching
of Oils
 Refining and bleaching is necessary to remove
suspended, dispersed and dissolved impurities and
colours.
 Refining and bleaching produces an edible oil with
characteristics that consumer desires such as bland
flavour and odour, clean appearance, light colour,
stability to oxidation and suitability for frying.
 It removes components like gums, soaps, pigments (for
example, carotinoids and chlorophylls) which act as
contaminants.
 It also removes other pollutants like trace metals,
pesticides, polycyclic aromatic hydrocarbons, etc.

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References:-
1. Shreve’s Chemical Process Industries; George T. Austin
2. Introduction to Chemical Engineering; S.K. Ghosal,
S.K. Sanyal, S. Datta.
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