Oilseed extraction process

Figure 14.10.1a. Flow diagram of oilseed extraction process from seed to crude oil and meal.

The overall extraction process is sometimes subdivided into two general categories according to the main mechanisms responsible for the dissolution stage (/) those operations that occur because of the solubiHty of the solute in or its miscibility with the solvent, eg, oilseed extraction, and (2) extractions where the solvent must react with a constituent of the soHd material in order to produce a compound soluble in the solvent, eg, the extraction of metals from metalliferous ores. In the former case the rate of extraction is most likely to be controUed by diffusion phenomena, but in the latter the kinetics of the reaction producing the solute may play a dominant role. 

Solvent Extraction. Extraction processes, used for separating one substance from another, are commonly employed in the pharmaceutical and food processing industries. Oilseed extraction is the most widely used extraction process on the basis of tons processed. Extraction-grade hexane is the solvent used to extract soybeans, cottonseed, com, peanuts, and other oilseeds to produce edible oils and meal used for animal feed supplements. Tight specifications require a narrow distillation range to minimize solvent losses as well as an extremely low benzene content. The specification also has a composition requirement, which is very unusual for a hydrocarbon, where the different components of the solvent must be present within certain ranges (see Exthaction). 

Introduction of the expander, a high-shear extruder with an interrupted-flight screw, in the mid-1980s revolutionized oilseed extraction practices. Essentially all solvent-extracted cottonseed, and approximately 70 percent of domestic soybean tonnage now are processed... 

Introduction of the expander has enabled extraction plants to handle additional seed species, with purchase of only minimal cleaning and dehulling equipment where needed. Prepress-solvent extraction facilities are being replaced by expander-direct solvent extraction equipment, leaving two basic extraction processes in modern large volume oilseed extraction plants expander-direct solvent extraction, and hard press for applications... 

Solvent extraction. The press cake emerging from a screw press still retains 3 to 15 percent of residual oil. More complete extraction is done by solvent extraction of the residues obtained from mechanical pressing. The greater efficiency obtained in the solvent extraction process encouraged the industry for direct application to oilseeds. In the United States and Europe, continuous extractor units are used in which fresh seed flakes are added continuously and are subjected to a counterflow of solvent by which intimate contact is achieved between the seeds and solvent. The common solvent for edible oil is commercial hexane or heptane, commonly known as petroleum ethers, boiling in the range of 146 to 156°F (63.3 to 68.9°C). After extraction, maximum solvent recovery is necessary for economical operation. The solvent is recovered by distillation and is reused. The extraction oil is mixed with prepress oil for refining. The extracted meals contain less than 1 percent of residual oil. 

Some of the more important environmental regulations that affect oilseed extraction and processing are as follows ... 

Aqueous enzymatic oil extraction is another ecofriendly extraction procedure. It is based on simultaneous isolation of oil and protein from oilseed by dispersing finely ground seed in water and separating the dispersion by centrifugation into oil, solid, and aqueous phases. The presence of certain enzymes during extraction enhances oil recovery by breaking cell walls and oil bodies (22). For peanuts, a multistep aqueous extraction process has been described with a recovery of about 98% (23). More recently, the relatively new technique of enzyme-assisted aqueous extraction has been applied to peanuts with a reported oil recovery of 86-92% (24). 

G. Anderson, Oilseed Extraction and Meal Processing, presented at the AOCS World Conference in Singapore. 

Solvent Extraction with Oilseeds—Extraction of the bleaching earths in a mixture with oilseeds is practiced by some extraction plants with processing capabilities, but the potential problems for this type of recovery may outweigh the savings for example, the mineral content of the meal may be increased beyond the acceptable limits, and the recovered oil may decrease the quality of the new oil extracted. The oxidation products and polymers from the recovered oil could contaminate the fresh oil. 

In solvent extraction crude vegetable oil and other useful materials are dissolved in a solvent to separate them from the insoluble meal. Many solvents have been evaluated for commereial extraction. Commercial hexane has been the main solvent for the oilseed processing industry since the 1940s because of its availability at reasonable eost and its suitable fiinetional characteristics for oil extraction. However, the interest in alternative solvents to hexane has continued and is motivated by one of a eombination of faetors desire for a nonflammable solvent, more efficient solvent, more energy efficient solvent, less hazardous and environmentally friendly solvent, solvent with improved produet quality, and solvents for niche/specialty markets. Today eommereial isohexane (hexane isomers) is replacing commercial hexane in a few oilseed extraction operations and other solvents (e.g., isopropanol, ethanol, acetone, etc.) are also being used for various extraction processes or have been evaluated for use as extracting solvents. ... 

Hexane has been used since the 1940 s as an oilseed-extraction solvent on the determination that it is GRAS and it may also be subject to a prior sanction. However, like many other food-processing substances, there is no U.S. FDA regulation specifically listing hexane as GRAS or prior sanctioned. 

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