Advancements in Oil and Oilseed Processing - Food and Industrial Bioproducts and Bioprocessing - page 117

one year of operation because ruminants fed on trichloroethylene extracted meal died of
“bloody nose disease”. The exact mechanism of trichloroethylene extracted meal toxicity
was never identified but it was hypothesized that trichloroethylene reacted with sulfhydryl
groups of the amino acid cysteine, because S-(trans-dichloro-vinyl)-L-cysteine produced
the same symptoms when fed to animals (Seto et al ., 1958 ).
Ethanol has also been utilized for oil extraction. Oil solubility in ethanol varies with
temperature and water content. Soybean oil is completely miscible with absolute ethanol
above 70 °C (Johnson and Lusas, 1983). As ethanol concentration decreases and water
content increases, oil solubility is significantly reduced in the mixture. The higher cost and
latent heat of vaporization are the major disadvantages of ethanol as a solvent for oilseed
extraction. Recent developments in bioethanol production may reduce the cost of ethanol,
making it a viable alternative to hexane. Solvent mixtures can also be used to extract oil.
Hexane/alcohol azeotropes have been used for extraction of residual lipids from hexane-
extracted meals to improve flavor and odor, specifically from soybean and peanuts. “Grassy”
and “beany” flavors in oilseeds are associated with the presence of phosphatides, which can
be easily extracted with hexane/alcohol mixtures. Similarly, hexane/alcohol azeotropes,
specifically hexane/methanol, are very effective in extracting aflatoxin from meal.
Dichloromethane or methylene chloride is an excellent solvent for oil extraction because
of its low boiling point (39.8 °C), which makes desolventization of oil and meal easy.
Furthermore, it is non-flammable and has low specific heat, latent heat of vaporization and
low solubility of water. Utilization of dichloromethane for oil extraction was first
demonstrated in the 1940s but the process was not economically feasible at the time because
of the relatively high cost of dichloromethane. In 1986, the feasibility of cottonseed oil
extraction by using dichloromethane was demonstrated at a pilot scale study (Johnson et al .,
1986). Residual oil content in the meal was lower than typically achieved with hexane
extraction. Cottonseed meal produced during the process was suitable for use in poultry feed
formulations, because gossypols present in cottonseed were extracted with oil and removed
from meal. No residual aflatoxin was detected in alkali-refined oil.
Acetone, isopropyl alcohol, methyl and ethyl acetate esters, ethylene glycol monomethyl
ether (methyl cellosolve), ethylene glycol monoethyl ether (ethylene cellosolve) and amines
have also been examined to determine their suitability for oil extraction from oilseeds
(Johnson and Lusas, 1983) but hexane remains to the choice of solvent for large oilseed
extraction operations today.

4.3.1.2 Extraction process

There are three major steps in a traditional solvent extraction system: oil extraction, oil
desolventizing and meal toasting. In general, residual oil in solvent-extracted oil would be
less than 1%. Oil extraction is a diffusion controlled process that involves diffusion of
solvent into a solid matrix to solubilize oil then diffusion of the oil-solvent mixture into a
liquid phase. Mathematical models have been developed to describe the solvent extraction
process (Abraham et al ., 1988 ; Becker, 1978 ; Mattil et al ., 1964). It has been shown that the
oil extraction rate for hexane extraction of flaked soybeans is proportional to the 3.97 power
of the flake thickness and 3.5 power of the residual oil (Mattil et al ., 1964 ). In general, oil
solubility in a solvent increases with increasing extraction temperature. High temperature
has also a positive effect on viscosity and diffusivity of oil. Viscosity decreases while
diffusivity increases as the extraction temperature increases, resulting in shorter extraction
times. Energy required for solvent recovery decreases when higher operating temperature is