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| Extractor | Ram Press |
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| Power Ghani | Plate Press |
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| Mini 40 | Traditional Animal Powered Ghani |
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Issue 11 - June July 1999 Principles of Oil Extraction, Part 1From the Small Farm Resource page on the netThe extraction of oil from an oil bearing commodity involves some or all of the following steps, depending on the particular commodity to be processed, the scale of operation and the technical options available to the processor. DecorticationThe first stage of the process sometimes requires the separation of the oil bearing part of the plant be it a nut, fruit or seed. The process is generally referred to as decortication or dehulling. Seeds with thin testa similar to rapeseed or sesame seed can be processed without decortication. BreakingIn most cases the oil bearing material is then broken into smaller pieces by pounding in a pestle and mortar or by manual or motorised grinding. When motorised oil expellers are used the decorticated or undecorticated material can often be fed directly to the expeller. CookingThe next step involves heating the oil bearing material, sometimes with the addition of a little water to assist in the rupture of oil bearing cells and in the liberation of oil. At small scale this is carried out in a pan over a fire but mechanised heaters or kettles are available and used in commercial plants small scale heater-Cecoco. Figure 1 - Small Scale Heater (scorcher) Cecoco ExtractionThe oil bearing material is now ready for oil extraction. Five basic technical options exist; hot water floatation, ghanis, manual presses, powered expellers and solvent extraction. Traditional Methods1.Hot water floatation is probably the simplest method and is still used in many rural areas. The ground material is placed in boiling water and simmered for several hours. On removing from the fire and cooling the oil floats to the surface and is skimmed off. In general the oil is then heated in a shallow pan to drive off the last traces of water. This improve" the keeping quality of the oil as water has a catalytic role in the development of rancidity in oils. The extraction efficiency is generally low, and problems often occur with the formation of oil-water emulsions which makes the final separation difficult. In some cases salt is used to break such emulsions. 2.In many countries a large rotating pestle and mortar system known as a ghani is used for oil extraction. Ghanis are powered by animals or motors (power-ghanis) although sometimes human power is used. The mortar is firmly fixed in the ground and as the pestle rotates oil is released by friction and pressure and runs out of a small aperture at the base of the mortar. A typical one bullock ghani can process 40kg of material/day. In the case of power-ghanis either the pestle or mortar is fixed, the other rotating. Power ghanis usually are operated in pairs and have a typical capacity of 100 kg/day. The extraction efficiency is generally greater than animal units. 3.Other traditional methods are still used to extract oil from oil bearing materials. Such systems include the use of heavy stones, wedges, levers and twisted ropes to apply pressure to the material and so squeeze out oil. They are inefficient, of low capacity and labour intensive. Manual PressesTraditional presses are now increasingly being replaced by better engineered, more efficient mechanical presses. Many different types of mechanical press are in use but they fall into two basic types, plate presses and ram presses. In the first type a plate or piston is forced into a perforated cylinder containing the oil bearing material by means of a worm. In some cases hydraulic jacks have been used, care is needed to make sure there is no leakage of hydraulic fluid that might contaminate the edible oil. In a ram press a piston forces the oilseed forward in a perforated cage fitted with an adjustable choke at the outlet, which controls the pressure. Ram presses provide a greater shearing action than simple screw presses and have been found to be considerably more efficient for some raw materials. It is important that when selecting a particular type of press its suitability for the raw material to be processed is determined.
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Index 11From the EditorLettersNewsQueensland Bushfood AssociationA word on Buying SeedYarrawarra Aboriginal CorpWhat's fruiting?Native Herb Forum 1.Ethnoecological Research.Illawarra plum.Methods of Growing BushtuckerBushfoods and Farm ForestryBushfood Artist.Backhousia citriodora.FEATURE: Davidson Plum.Principles of Oil Extraction. JQueensland Conference.Solanum centrale association.Somewhat Useful Pages.The Value Adders: Greg Trevena and Fudge A'fareBook Review.Red Ochre GrillFamous Palates |
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| Extractor | Ram Press |
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| Power Ghani | Plate Press |
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| Mini 40 | Traditional Animal Powered Ghani |
Powered PressesTo obtain greater throughputs and extraction efficiencies it becomes necessary to use powered devices and the oil expeller is the most common of these. A whole range of expellers are available with capacities ranging from a few kg/hr up to tons/hr. They all work on the same basic principle. The raw material, which may have been previously heated to aid in the release of oil, is fed continuously to the expeller where it is fed by the wormshaft into a horizontal cylinder. A controllable pressure is built up in the cylinder by means of an adjustable choke at the cylinder exit. The internal pressure ruptures oil cells in the material and oil flows out through perforations in the cylinder cage. Some care has to be taken when selecting an expeller for a particular commodity. Many have been designed for particular applications, in terms of internal pressure, amount of shearing action etc. Certain types tend to be more adjustable and hence less product specific than others. Small scale expellers c 40 kg/hrNone of the above extraction systems are able to remove all of the oil from a material. In most small scale rural situations this is of little or no importance as the cake, that remains after the oil has been removed, finds uses in local dishes, in the manufacture of secondary products or for animal feed. Some raw materials however do not release oil by simple expelling; the most notable being rice bran. In order to remove oil from commodities that do not respond to expelling or to extract the final traces of oil after expelling it is necessary to use solvent extraction. Solvent extraction is a high technology process that has to be carried out at comparatively large scale. Capital costs are high. Essentially the process is one of continuous counter-current extraction with the raw material flowing in one direction against a solvent; usually hexane. After oil extraction the solvent passes to a recovery plant where the solvent is stripped off under vacuum. The crude oil then passes on for refining. Due to the large scale involved it would seem unlikely that solvent extraction would find much application in minor oil product processing. |
