Handbook of Solvents - George Wypych - ChemTech - Ventech!

938 Phillip J. Wakelyn, Peter J. Wan
for up to 120 min. Overcooking lowers the nutritional quality of the meal and can darken
both the oil and meal. Poor-quality seeds with high levels of free fatty acids cannot be
cooked for as long a period as high-quality seeds because of darkening. Darker oil requires
additional refining to achieve a certain bleach color.
Expanders: Sometimes low shear extruders called expanders are used. This equipment
has the capability to process both low- and high-oil content materials. The meats are fed into
an extruder after dehulling, flaking, and cooking and are heated as they are conveyed by a
screw press through the extruder barrel. The meats are under considerable pressure and temperature
when they reach the exit of the extruder. The change in pressure as the material
leaves the extruder causes it to expand and the oil cells are ruptured, releasing the oil, which
is rapidly reabsorbed. The expanded “collets” produced are then cooled and extracted with
solvent.
14.10.3.2 Oil extraction
Prepress solvent extraction: In this process the oil-bearing material are first mildly pressed
mechanically by means of a continuous screw press operation to reduce the oil by half to
two-thirds of its original level before solvent extraction to remove the remaining oil in the
pre-pressed cake. Pressing follow by solvent extraction is more commonly used when high
oil content materials (e.g., canola/rapeseed, flaxseed, corn germ) are processed.
Direct solvent extraction: This process involves the use of a nonpolar solvent, usually
hexane, to dissolve the oil without removing proteins and other non-oil soluble compounds.
Solvent extraction yields about 11.5% more oil than does the screw press method, and less
oil remains in the meal. The cooked flakes or collets (if expanders are used) are mixed with
hexane in a batch or continuous operation. The hexane vapor pressure limits the practical
operating temperature of the extraction and its contents to about 50-55 o C. The resulting
miscella (oil-solvent mixture) and the marc (solvent laiden collets) are heated to evaporate
the solvent, which is collected and reused. The oil is freed from the miscella, by using a series
of stills, stripping columns, and associated condensers. The hexane-free oil (i.e., crude
oil) is cooled and filtered before leaving the solvent-extraction plant for storage or further
treatment. This is the crude oil normally traded in the commodity market. Occasional overheating
of the oil-solvent miscella will cause irreversible color changes in the oil.
14.10.3.3 Processing crude oil
Most crude edible oils, obtained from oil-bearing materials, consist primarily of triglycerides
(triacylglycerols). The triglycerides (approximately 95% of the crude oil) are the constituents
recovered for use as neutral oil in the manufacture of finished products. The
remaining nontriglyceride portion contains variable amounts of other lipophilic
compounds, such as free fatty acids (FFA), nonfatty materials generally classified as
“gums,” phospholipids (phosphatides), tocopherols, color pigments, trace metals, sterols,
meal, oxidized materials, waxes, moisture, and dirt. Most of these minor lipid components
are detrimental to finished product color, flavor, and smoking stability, and so must be removed
from the neutral oil by a processing/purification process. The object of the processing/purification
steps is to remove the objectionable impurities while minimizing possible
damage to the neutral oil and tocopherols and loss of oil during such processing.
Lecithin and cephalin are common phosphatides found in edible oils. Soybean, canola/rapeseed,
corn, and cottonseed are the major oils that contain significant quantities of
phosphatides. Alkaline treatment used for FFA reduction is also capable of removing most