Vegetable oil is a product made from oily raw material by extrusion and extraction processes. A combination of these methods is used slightly less often: the first stage is extrusion, and the second one is extraction.
The extrusion process involves pressing of oily raw material from pre-treated sunflower seeds. Extraction is based on diffusion and involves recovery of oily material using extraction hexane.
Oil extraction is preceded by cleaning the seeds from various impurities, peeling the seeds and grinding, which ends with obtaining seed meal.
Thereafter the meal is subjected to hydrothermal treatment, which involves humidifying and heating in scrubbers or steaming. The final product of such hydrothermal treatment is pulp. It is much easier to get oil from pulp.
Pressed oil almost always contains various impurities, which significantly reduce the quality of the product and their presence is undesirable. Vegetable oil must be filtered.
Filtering can be of mechanical type, when suspended particles of impurities are removed from vegetable oils. Usually this is done by settling or filtration through special cotton cloth in filter presses. It is also possible to use centrifuges.
In addition to mechanical filtration, refining of sunflower oil also includes hydration, winterization, bleaching, deodorization and polishing.
Hydration is a method for removing phospholipids, protein and mucous substances. It is never used independently, but in combination with settling. Initially oil is treated with a small amount of hot water, and then precipitated impurities are removed by settling.
Winterization is necessary to remove waxes and wax-like substances from vegetable oils.
Neutralization is the process of oil treatment with a water solution of sodium hydroxide. This process is based on the ability of free fatty acids to react with alkali to form water solutions of soap, also referred to as soap stock. The latter are oil-insoluble substances that are deposited on the bottom and can be easily removed from the product.
Adsorptive refining is also called bleaching. The second name arises from the nature of substances removed during oil processing – fat-soluble pigments (chlorophylls, carotenoids, etc.).
Oil bleaching occurs under vacuum at a temperature below 75-80ºC. Bentonite bleaching powder is added to the treated product and the mixture is stirred for 20 to 30 minutes. This time is usually sufficient for adsorption of coloring substances from the oil. After this, the oil requires sedimentation and filtration on filter presses.
Deodorization is a distillation process for removing odorous substances from the oil; these substances include low molecular weight fatty acids, aldehydes, ketones, and other products that affect the taste and smell of oil. Deodorization is also necessary to remove polycyclic hydrocarbons, toxic products and pesticides. The process itself takes place in vacuum by blowing superheated water steam through oil.
Complete refining of vegetable oil is not always necessary. But filtration is always required. When grinding sunflower seeds it is practically impossible to obtain raw material completely free of impurities.
During oil filtration, great attention should be paid to technical operations associated with mechanical mixing of the components. Conventional equipment used for this purpose is unable to provide the necessary contact between components, resulting in excess expenditure of raw materials and inability to provide the necessary stability and quality of refining. A good alternative is the use of the so-called magnetic mill. The specifics of their operation (the combined effect of a vortex layer and a magnetic field of the inductor) can intensify the process of mixing of the components, thus creating the necessary conditions for effective flow of oil degumming process. You can find more detail on tests conducted in the article “Purification of vegetable oils using AVS”.