Processing of oil seeds, Crop Process Engineering | Crop Process Engineering Notes - Agricultural Engg - Agricultural Engineering PDF Download

Introduction
India is fourth oilseed producing country in the next only to USA, China and Brazil. Many varieties of oilseeds, the major oilseeds are soybean, cottonseed, groundnut, Sunflower, Rapeseed, Sesame seed, Copra, Castor seed and Palm Kernels. India occupies the place of pride as the world’s largest producer of Groundnuts, Sesame seeds, Linseeds and Castor seeds. Ending on the period of cultivation, the oilseeds are classified as “Kharif crop” and “Rabi crop”.
The oils and fats are composed of mixtures of glycerides of various fatty acids. The fats and oils are broadly classified in to edible and non edible. Groundnut, soybean, mustard are some of the sources of the edible oil. The edible oil is main source of fat taken in daily meals and is used for cooking purposes and salad dressings. Oils are also used in the soap industry, paint, varnishes and plasticizers industry. The mechanical expression and solvent extraction methods are employed for the manufacture of oil from the oil seeds.

7.1 Raw material preparation
Oilseed and nut should be properly dried before storage, and cleaned to remove sand, dust, leaves and other contaminants. All raw materials should be sorted to remove stones and moudly nuts. Some moulds, especially in the case of groundnuts, can cause aflatoxin poisoning. When storage is necessary, this should be in weather proof, ventilated rooms which are protected against birds, insects and rodents. Some raw materials (for example groundnuts, sunflower seeds) need dehusking (or decorticating). Decortication is important to give high yields of oil and reduce the bulk of material to processed. However, expellers normally require a proportion of fibrous material in order to work and, particularly with groundnuts; some husk is normally added to allow oil to escape more freely from the press. Coconut is dehusked and split manually by skilled operators. Most oilseeds (copra, palm kernels and groundnuts) need grinding in mills before oil extraction to increase the yields of oil. All oil-bearing materials need to have correct moisture content to maximize the oil yields. 

7.2 Oil Extraction methods
(a) Mechanical expression
During the process of mechanical expression, the oil seeds are compressed in various types of compression devices/equipment. Expression is the process of mechanically pressing liquid out of liquid containing solids. Screw press, roll presses, collapsible plate are some examples of wide range of equipment used for expression of liquid.
(i) Hydraulic press: The hydraulic press is considered of a series of horizontal corrugated iron plates.  These plates are separated by 4 – 14 premoulded oil seed cakes. Pressing is completed in two stages. In first stage, the oil seeds are pressed at about 5 MPa for 15-20 min and then pressure of 28 MPa is applied for 5-10 min to complete the expression process. The recovery of the oil varied depending upon the sizes and seed being pressed. But, the at commercial level, the hydraulic press is replaced by screw type presses.
(ii) Screw press: A screw press has a horizontal main shaft. The screw assembly is formed integrally with this shaft. The screw rotates within a cage or barrel. The barrel is made of case hardened, tool steel bars or rings to allow drainage of the oil as the pressure on the feed material is increased. At the discharge end, a movable choke or cone controls the operating pressure. It is achieved by changing the width of annular space through which the oil cake passes. The choke is adjusted by a hand wheel on the opposite end of the screw. The configuration of screw is such that the volume displacement at the feed end of the press is considerably greater than at the discharge end. As a result of such configuration, as the material is conveyed from feed end to discharge end, it is subjected to increasing pressure. As pressure increases, the material is compressed and oil is expelled through the spacers between the cage lining bars.
(iii) Ram press: A long pivoted lever moves a piston backwards and forewords inside a cylindrical cage constructed from a metal bars spaced to allow the passage of oil. At one end of the piston’s stroke, it opens an entry port from the seed hopper so that seed enters the press cage. When the piston is moved forward, the entry post is closed and the oilseed is compressed in the cage. As a result, oil is expelled from the oilseed the emerges through the gaps in the cage. Compressed seed is pushed out through the gaps in the cage. Compressed seed is pushed out through circular gap at the end of the cage.
(b) Oil Extraction
Extraction is a process of separating a liquid from a solid system with the use of a solvent. Extraction is also a process of diffusion with the help of low boiling point solvent. This process gives a higher recovery of oil and a drier cake than expression. Solvent extraction is capable of removing nearly all of the available oil from oilseed meal.  This extraction process provides meal of better preservation qualities and with higher protein qualities.