Oil extraction :Extraction of oil from the prepared oilseeds can be done by one of the following methods:
(i) traditional method: ghani,
(ii) Improved methods:
(a) mechanical pressing (hydraulic pressing),
(b) screw pressing (expeller pressing),
(c) prepress solvent extraction, and
(d) direct solvent extraction.
(i) Ghani: A ghani consists of a wooden mortar and pestle. The mortar is fixed to the ground, and the pestle is located in the mortar, where the raw material is crushed by friction and pressure. An animal is required to move the pestle and as this continues the oil is pressed out, runs through a hole at the bottom of the mortar, and the residue (cake) is then scooped out. Ghanis are limited in that two animals are required, since any one animal will tire after 3-4 hours. Motorized ghanis are becoming increasingly popular and are fast replacing animal-powered equipment. Mustard oil, groundnut oil and sesame oil are traditionally produced by this method.
(a) Hydraulic pressing: Prepared raw materials to be crushed are placed in heavy perforated or slotted metal cages (12 to 16 Nos.) and a metal plunger is used to press out the oil. The maximum pressure applied is 2000 psi for a period of 20-50 minutes. The oil content of the press cake may vary from 5 to 8% depending on the raw material.
(b) Expeller pressing ( screw pressing): The raw material is continuously fed to the expeller, which grinds, crushes, and presses out the oil as it passes through the machine. Oil flows through the perforations in the casing and is collected underneath. The residue, or oilcake, is pushed out of the end of the unit Most small expellers are power-driven. The oil content of the press cake after second pressing may vary from 5 to 8%.
(c) Prepress solvent extraction: In this process, the oil from the raw material is expressed in a screw press by single pressing and then the residual oil from the press cake is solvent-extracted by using food grade hexane followed by desolventization of the material. The protein quality in the meal obtained by this method is not adversely affected.
(d) Direct solvent extraction: This process is used for the oilseeds having low fat content e.g., soybean. The process consists of the following steps:
(i) cooking of the material in steam and flaking,
(ii) solvent extraction of the flakes using food grade hexane,
(iii) desolventizing the meal.