Three major methods are used to obtain soybean oil from the soybean itself; a chemical extraction method and two mechanical extraction methods.
In the chemical process of extraction, the soybean oil is separated from the soybean meal and then refined using chemicals including a solvent; the oil is then put through several other refinements. Solvent extraction of oil seeds can be performed by equipment of either percolation-type or immersion-type. One commonly used percolation extractor is the rotary extractor, an enclosed vertical system that moves perforated baskets using a rotary motion around a central vertical shaft. The liquid solvent (hexane) is pumped over a bed of flakes, percolates down through the bed and leaves though the bottom through a perforated plate, mesh screen, or wedge wire screen bar system. At the end of the extraction cycle the flakes are allowed to drain and are dropped into the discharge hopper. Miscella is pumped counter-current to the flow of the flakes. Counterflow is very important to solvent extraction (as well as desolventizing) as it helps to remove oil more efficiently than parallel-flow systems. The miscella becomes richer as the oil is extracted from the soybean flakes.
In the mechanical method of extraction, the oil is pressed from the soybean without using any chemicals. These procedures are hydraulic pressing, expeller pressing and solvent extraction. Hydraulic pressing is one of the oldest pressing methods. This is a batch press procedure that requires hand labor and currently is no longer used for soybeans. Expeller pressing has replaced the hydraulic pressing procedure for extraction of oils. Soybeans are not commonly used in this method either.
Soybeans must be carefully cleaned, dried and dehulled prior to oil extraction. The presence of hulls during oil extraction will result in a lower oil yield due to the absorption of the oil into the particles. Initially, the soybeans are fed into cracking rolls to crack the beans, followed by separation of the hulls with the use of shaker screens and aspiration The cracked beans are exposed to temperatures to approximately 74 - 79 C for 30 - 60 minutes prior to using smooth-surface rollers to flatten into large flakes of uniform thickness. The purpose of the conditioning is to make it easier to extract the oil.
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How is soybean oil refined?
After extraction and concentration, the crude soybean oil contains many oil-insoluble and oil-soluble impurities that must be removed. Oil-insoluble material may be removed through filtration; however, the soluble materials must be removed by implementing several different techniques listed below.
Degumming
Degumming is a process that involves mixing crude soybean oil with 2-3% water, gently agitating for 30 - 60 minutes (being careful to prevent the introduction of air and subsequent oxidation of oil) at a temperature of 70 ƒ C. This hydrates impurities that can be settled, filtered, or centrifuged out from the degummed oil.
Alkali Refining
Continuous automated refining operations are used to remove objectionable impurities that may affect oil quality. Caustic soda is used in refining to remove free fatty acids, phosphatides and gums, colorants, insoluble matter and other material. This mixture is heated centrifuged to separate out the caustic from the refined oil.
Bleaching
Normally, a continuous vacuum bleaching process is employed by adding activated earth to the refined oil to remove color, odor, and other impurities. Shortenings are typically bleached to a white color while margarines, and salad and cooking oils can remain yellow-tinged. The odor, flavor, and oxidative stability of the bleached soybean oil are greatly improved.
Hydrogenation
Hydrogenation of soybean oil increases the melting point and improves stability of the oil. The hydrogenation will give different degrees of hardness for specific products desired. This reaction occurs between high purity hydrogen gas, a catalyst (such as nickel) and the fats and oils resulting in the addition of hydrogen to unsaturated bonds causing saturation. Hydrogenation occurs in a vacuum-pressure vessel containing oil into which the hydrogen gas is dispersed as fine bubbles while heating the mixture and agitating. When the desired hydrogenation is obtained, the mixture is cooled; the catalyst is filtered out to obtain a solution that is completely clear. The partially hydrogenated oils remain liquid and the more highly hydrogenated soybean oil is hardened.
Deodorization
Deodorization is a high temperature, high vacuum, steam-distillation process that is necessary for the removal of volatile flavor and odor compounds to transform the oil into a bland-tasting clear liquid desirable to consumers. The removal of the free fatty acids will also increase stability of the oil. The oil is sparged with steam at high temperature and vacuum that also prevents the introduction of oxygen and thus oxidation from occurring during the deodorization process.
Winterization
Soybean oil use for application as cooking oil must be winterized to meet requirements of the American Oil Chemists Society cold test. Winterization is a process performed on partially hydrogenated soybean oil that involves the chilling of the oil to allow the solid portion to crystallize and the subsequent filtration of the two phases .Winterization is a form of fractionation or the removal of solids at selected temperatures. It involves the removal of a small quantity of crystallized material from edible oils by filtration to avoid clouding of the liquid fractions at refrigeration temperatures. Oil is chilled slowly to about 6 C during a 24-hr period. Cooling is stopped and the oil/crystal mixture is allowed to stand for 6 to 8 hrs.
Dewaxing
Methods such as dewaxing and solvent fractionation are also used to clarify oils. Dewaxing squeezes or presses liquid oil from solid fat by hydraulic pressure to produce hard butters and specialty fats. Solvent fractionation involves the crystallization of desired fractions from a mixture of triglycerides dissolved in a suitable solvent. Fractions may be selectively crystallized at different temperatures, separated and the solvent removed for a final product of specific triglyceride or fatty acid composition.
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