Processing Soybean into Oil

Soybean oil is widely used oil and is commonly called ‘pooled oil’. Soybean oil is a very healthy food ingredient despite the bad publicity of making people fat.
Soybeans are the world’s largest source of veg- etable protein for both humans and animals. Soybean oil (SBO) has a triglyceride composition rich in monounsaturated (23% oleic acid) and polyunsaturated fatty acids (FA) (57% linoleic acid; 7% linolenic acid). This makes the oil extremely sensitive to oxidative damage and quality deterioration. This process eventually leads to color and flavor reversion, accompanied by the development of beany off-flavors, which in an advanced phase could change into a fishy and painty smell.
Processing of soybean into oil typically consists of three steps in the entire soybean oil plant : They are preparation of soybeans, solvent extraction, soybean oil refining and hydrogenation of soybean oil.

1. Preparation of Soybeans

Soybeans are conveyed from the process bins to the mill by means of belts or mass flow conveyors and bucket elevators. In the mill, the beans may be aspirated again, weighed, cleaned of tramp metal by magnets, and fed into corrugated cracking rolls. The cracking rolls "crack" each bean into four to six particles, which are passed through aspirators to remove the hulls (processed separately after the removal of residual bean chips). These hulls may be combined with the hulls from the grain cleaning step. Next, the cracked beans and bean chips are conveyed to the conditioning area, where they are put either into a rotary steam tubed device or into a stacked cooker and are heated to "condition" them (i. e., make them pliable and keep them hydrated). Conditioning is necessary to permit the flaking of the chips and to prevent their being broken into smaller particles. Finally, the heated, cracked beans are conveyed and fed to smooth, cylindrical rolls that press the particles into smooth "flakes", which vary in thickness from approximately 0.25 to 0.51 millimeters (0.010 to 0.020 inches). Flaking allows the soybean oil cells to be exposed and the oil to be more easily extracted.

2. Soybean Oil Solvent Extraction

The soybean oil extraction process consists of "washing" the oil from the soybean flakes with hexane solvent in a countercurrent extractor. Then the solvent is evaporated (i. e., desolventized) from both the solvent/oil mixture (micella) and the solvent-laden, defatted flakes. The oil is desolventized by exposing the solvent/oil mixture to steam (contact and noncontact). Then the solvent is condensed, separated from the steam condensate, and reused. Residual hexane not condensed is removed with mineral oil scrubbers. The desolventized oil, called "crude" soybean oil, is stored for further processing or loadout.

3. Soybean Oil Refining

Crude oil is typically shipped for refining to establishments engaged in the production of edible vegetable oils, shortening, and margarine. Crude vegetable oils contain small amounts of naturally occurring materials such as proteinaceous material, free fatty acids, and phosphatides. Phosphatides are removed for lecithin recovery or to prepare the crude oil for export. The most common method of refining oil is by reacting it with an alkali solution which neutralizes the free fatty acids and reacts with the phosphatides. These reacted products and the proteinaceous materials are then removed by centrifuge. Following alkali refining, the oil is washed with water to remove residual soap, caused by saponification of small amounts of the triglycerides (oil). Color-producing substances within an oil (i. e., carotenoids, chlorophyll) are removed by a bleaching process, which employs the use of adsorbents such as acid-activated clays. Volatile components are removed by deodorization, which uses steam injection under a high vacuum and temperature. The refined oil is then filtered and stored until used or transported.

4. Hydrogenation of soybean oil (optional)

This process will increase the stability of the soybean oil and will make it less liquid. However, this process will create more saturated fats and will reduce the favourable unsaturated fats. In addition, transfats are produced which can cause hearth problems. Hydrogenated soybean oil is used in foods that require more solid fat such as margarines.