Processing Oilseeds

Once harvested, oilseeds must be transported to processing plants and may be stored before being processed. Preventing a reduction in oilseed quality during transportation and storage is a major problem. Although weather conditions throughout the growing season affect the quality of oilseeds, rain at harvesting causes particular problems. Wet seed must be dried and cooled before storage to prevent mold growth and free fatty acid formation; the latter is caused by enzymatic hydrolysis of the triglycerides. Wet seed is typically dried and cooled by passing air through the seed piles in the storage houses. Oil is removed in two ways: mechanically pressing it from the seeds and soaking the seeds in a solvent, usually hexane, that dissolves the oil. Because of its efficiency, this latter process, called solvent extraction, is used in almost all commercial operations (15). A combination of these two processes, prepress solvent extraction, is sometimes used for high oil content oilseeds.

Figure 2 is a simplified schematic of the oilseed solvent extraction operation. First the dried seeds are cleaned, to remove stones, metals, and other objects that would damage the processing equipment. They are then dehulled and flaked. The flakes, typically about 0.025 cm thick, provide a more efficient extraction medium. The flakes are either prepressed or sent directly to the extractor. Extracted flakes are heated in a desolventizer to vaporize the solvent, which is recycled to the extractor. The meal from the desolventizer can be used as is or processed further. The oil-solvent mixture coming from the extractor, called miscella, is sent to an evaporator, where the solvent is driven off and recycled. The crude oil is sent to refining to be treated with caustic to remove most of the nontriglyceride components. The oil is then bleached and deodorized to give it a light color and bland odor. If it is to be used as a salad oil, it is also winterized. In the winterization process the oil is cooled to a low temperature and any crystallized material is removed from the liquid oil, which is now a suitable salad oil.

Table 1. Composition of Various Oilseeds and Characteristics of Their Oils

Coconut" Corn6

Moisture, % 47 16

Number of seeds g 0.0005 3


Cottonseed0 Olive Palm'' Kernel

Seed composition

36 17 70 40

Peanut Rape Soybean Sunflower

49 40 20 47

26 23 36 23

Saponification number

Iodine number

Refractive index Specific gravity


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