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Extraction & refining of vegetable oils
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Saturday, 16 September, 2006, 08 : 00 AM [IST]
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Dr K D Yadav
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s depends on the raw material. The common raw materials for vegetable oils are oil seeds and oil fruits delivering pulp oils.
Oil seeds are relatively stable and can be stored for a longer period without quality deterioration; while oil bearing fruits (e g palm fruits), bran (e g rice bran) deteriorate in oil quality very fast due to enzymatic reactions and thus need to be processed immediately and preferably close to the place of harvest.
Pulp oils are thus produced close to the production site of the oil fruit. Palm fruits are cooked (thus sterilised) and the fruits are then separated from the bunch stalk. Oil is then extracted by mechanical pressing and the must is centrifuged to obtain oil. The press cake (residue) is dried and used as burning material (fuel for processing) or as cattle feed.
The following table shows the typical percentage oil present in different oil bearing raw materials and yield from the different best oil seeds on their expelling (mechanical extraction).
The oil can be obtained from its source material by two steps, especially in seeds containing more than 20% oil. Firstly, by mechanical pressing in oil mills to get expeller grade oils and the residue having about 6-8% oil content as oil cake. Secondly, by solvent extraction of the oil cake to get solvent grade oil and residue as de-oiled cake (DOC) having oil
content less than 1%. The expeller grade oils are of two types i.e., cold pressed oils or normal expeller oils.Cold pressed oils receive minimal or no heat treatment during their mechanical pressing and thus have minimal deterioration in their natural oil quality (e g Kachi Ghani, virgin oils etc). Whereas, normal expeller oils receive some heat treatment prior to expelling for better oil recovery but having less impurities than solvent oils and more suitable raw materials for refined oils/filtered oils.
The solvent grade oils contain more impurities and are darker in colour, thus not an ideal raw material for refined oils. When solvent grade oil is used for the production of RBD oils, the same must be declared on the labels.
Soya bean oil, rice bran oil are the oils obtained by solvent extraction route, as their source material has oil content less than 20% and difficult to mechanical pressing. Cotton seed oil cake is not subjected to solvent extraction process mainly due to the presence of gossypol in it.
Oil cake from oil mill is taken to the solvent plant for further removal of oil. In the solvent plant the typical operations involve cake braking and its solvent extraction using food grade solvent hexane. The cake passes through the extractor on perforated rotary bed and gets subject to counter current extraction of oil from it using hexane to yield oil in the form of miscella (solvent containing oil). The miscella so obtained is subjected to filtration to remove fines and further evaporation of solvent for oil recovery. The recovered
solvent is recycled for fresh extraction of cake. The solvent containing cake coming out of the extraction chamber passes through DTDC (desolventiser-toaster-drier-cooler), wherein it gets de solventised to make it free from residual solvent and deoiled meal or cake (DOC) containing less than 1% oil is obtained.
Refining of oil
The crude or raw oil obtained contains various impurities depending upon the processing method adopted for its extraction from the source raw material. Based on the raw oil quality, the refining process and the treatments required during the processing to obtain refined oils differ.
The filtered oils obtained by mechanical pressing (cold pressed/normal pressing) are the most natural forms of oils and are not subjected to any refining treatment except polish filtration to achieve clarity & shine and are suitable for direct consumption if natural smell/odour/taste is not the issue.
However, to obtain refined oils which are light in colour; are absolutely odourless and bland in taste; all the impurities in raw oils like moisture, insolubles, FFA (free fatty acids), colour, odour, gums, waxes etc are removed during various steps of refining.
The oil may be subjected to chemical refining or physical refining process depending on its initial colour and free fatty acids content. For oils having high initial ffa (> 3%) physical refining is normally the method of choice. In physical refining the ffa is removed by steam distillation as against chemical refining wherein ffa is removed by reacting the oil with caustic lye solution to convert the ffa into soap and separating it from neutral oil by centrifugal separation.
Thus in chemical refining the raw oil is subjected to various steps of refining like degumming (water or acid), alkali neutralisation and washing, bleaching with activated earth and its separation, deodorisation so as to separate the impurities like gums (phospholipids), free fatty acids, colour bodies (plant pigments) and odouriferous compounds etc from oil respectively and yield finally the refined bleached and deodorised oils (RBD oils). RBD oils are light in colour, absolutely odourless and are bland in taste. Oils like sunflower oil, corn oil, rice bran oil, sesame oil are also subjected to dewaxing treatment after their bleaching stage since they contain waxes which may cause haziness or turbidity in the finished oil especially at lower ambient storage temperatures.
Dewaxing step removes these waxes and absolutely transparent and sparkling clear RBD oils are obtained.In physical refining the raw oil is normally subjected to water/acid degumming (using citric and/phosphoric acid , bleaching with activated earth followed by deacidification cum deodorization to remove impurities like gums, plant pigments, free fatty acids and odouring compounds respectively and finally yield RBD oils.
-The author is general manager - technical, Kamani Oil Industries
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