Extracting oil from the oil seeds is the primary purpose of an oil extraction mill. Besides the oil, cake made out of the dried seeds is another important commercial product. The whole process of oil extraction is completed mechanically in a series of stage. A well designed and engineered oil mill ensures a higher quality of final products.
Oil milling industry runs hand in glove with the agricultural cultivation of oil producing crop. The process of oil extraction mill starts with the procurement of oil seeds. The quality of seed, primarily determined by the percentage of damaged or unwanted seeds, maturity level has a visible effect on the oil extracted. Quality of seed can be judged by an expert by gauging the density and coloration of the seeds. Once procured proper, storage is the next essential step. It is important to ensure that the seeds are stored in a dry and aerated place to maintain the health of seeds.
Oil seeds procured from farms are bound to contain unwanted impurities like soil, pebbles, leaves, wooden twigs. If not cleared at this stage, they would be processed further with the seeds and get mixed with oil. Besides degrading the quality of the oil produced, it will also harm the machinery. Some hard materials which are not crushed can get stuck in the machinery and damage some of its essential parts. Some impurities like sand and crushed pebbles have a tendency to absorb oil thus lowering the yield. Quality of residual cake, which is another important industrial product, also gets adversely affected by the presence of impurities in the same. Therefore, oil seeds go through a procedure called Precleaning. Depending on the type and concentration of different type of impurities, you can choose from a variety of available precleaning methods like simple hand cleaning machines, stationary seed cleaning, vibrating seed cleaning, rotating seed cleaning and cleaning of seeds by brushing.
To remove the stones that might still be present, oil seeds go to a second process called de-stoning before they are sent for decortication. After ensuring final cleaning of all impurities from the seeds, they now are taken to next step which removes there shell or outer covering also known as husk. This is called decortication. Using rollers and ridges, lined with cutter edges the hard covering is delicately cracked and then removed by further processing. Decortication removes the unwanted part of the seed, which would have only absorbed the valuable oil before getting separated as cake. So this is an important process in the oil extraction. The broken covers / husks are separated from seeds by blowing air.
Seed conditioning: An efficient milling plant does not only quality of the oil but also tries to improve the economic returns from the mill. Seed conditioning is an important step which is essential to maximize extraction of oil without compromising the quality. It has a direct effect on the production volume of oil and overall efficiency of the mill. This step is completed in two parts. Seed Cracking is the first step in which the seed is passed between cold rollers. This ensures effective Cooking of the seed, which is next step. Oil is absorbed in the cells of the seed, inform of fats. Cooking is a scientific process, which coagulates the proteins in the wall of the fat containing cells. In addition, the microscopic oil droplets in the seed are united in the process which makes its extraction easier and quicker. Heating the seed in a steam kettle built on the top of the expeller machine, increases the flow capacity of oil and increases the permeability of the cell walls. Besides, heating also removes any bacteria or mould present in the cell. Cooking is performed in well monitored temperature and moisture conditions, which may be different for different seeds.
Finally the seeds are ready to be expelled. In a fully automatic oil expelling machine, cooked seed is fed through an automatic line into a horizontal chamber. The chamber consists of a horizontal shaft mounted with several worms. The screwing action of the worm pushes the seed further. The chamber is designed to a conical shape so that an internal pressure is built up due to the forward pushing and collection of seed at expulsion end. Oil is released as a result of the squeezing from the pressure. The extracted oil flows through several perforations at this end. The left out seed cake flows out through opening around the cone. In a well-designed expeller machine most of the oil Is squeezed out and a very small amount of it is left out in the cake. Cake is another important commercial product of filtering.
At this stage there are various suspended particles and cake residue in the oil. Oil produced by expelling has a typical odour and colour, which makes it less acceptable in the marketing. Although oil in its crude form is also marketed but its value further increases after it is refined to make it colourless and odourless.It is preferred to have a filtration plant as well in the plant.
Starting the process of filteration, the oil is first pumped and passed through a filtration cloth. This step separates out all the suspended solid particles present in the oil by depositing it on the filter cloth. It makes the oil cleaner and clearer. The oil is collected and sent for storage. Storage capacity in each plant is designed according to the operating capacity and individual needs of oil expelling mill. A higher storage is preferred and suggested to keep the mill operational during a sluggish market.
Although it is not suggested in most of the cases, but still in undeveloped countries people continue to use oil after physical filteration. Oil obtained at this stage is still not purified and contains lot of chemical substances which should be removed before consumption. In india the demands for refined oil has considerably increased but still there is a big market for new establishments in oil refining sector.
Fresh oil cakes are hot and contain superficial moisture. Before it can be stored in gunny bags, it needs to be spread in ventilated and aerated space for at least a day or two. After which it can be sent to market. Oil cakes have primarily utilization as cattle feed. Oil cakes are an essential part of an oil expeller’s commerce. A plant cannot make much profit just by selling oil, hence the commerciality of the mill is determined only after calculating the sales of oil cake.