Advancements in Oil and Oilseed Processing - Food and Industrial Bioproducts and Bioprocessing - page 119

single horizontal plane rather than vertically (Becker, 1978). In percolation-type extractors,
flaked seeds are formed into a fixed bed usually 50 cm deep. Under these conditions solvent
channeling or micella flow through the flakes is minimized; consequently complete
extraction is achieved (Bernardini, 1976). The particle size of the seeds needs to be large
enough to allow a good solvent flow rate through the seed bed. Fine particles in the bed block
the solvent flow, causing flooding of the bed and consequently lowering extraction efficiency.
Combination of immersion and percolation extractors has also been used for oil extraction
from oilseed. This process involves partial extraction of the oil in a percolation extractor
(until there is 10-15% residual oil in the seeds) followed by wet flaking of the partially
defatted seed as it comes out of the percolation extractor. This operation is carried out in a
special solvent-tight flaker operating in a solvent vapor-saturated atmosphere. Finally,
flaked seeds are extracted in an immersion-type extractor. The advantage of this process is
that seeds are extracted at a low temperature (maximum 50 °C) and the subsequent meal
desolventizing is carried out lower than 105 °C, producing a meal suitable for protein
concentrates and isolates manufacturing (Bernardini, 1976).

4.3.1.3 Meal and oil desolventizing

After oil extraction, residual solvent in meal (spent seeds) is removed in a desolventizer-
toaster (DT), which may have three sections for pre-desolventizing, desolventizing and
toasting. Meal is heated to a temperature that evaporates solvent, but does not deteriorate the
nutritional value of the meal. A DT consists of trays mounted along a vertical shaft with
sweep arms attached to move the meal around. Meal from a DT may still contain some
moisture, which is removed in a rotary steam tube dryer and then cooled by air before being
sent to storage. Desolventizer toasters are commonly used to produce toasted meal for
ruminant and poultry feeds. In some cases, desolventizing and drying is done in the same
equipment, referred to as a desolventizer-toaster-dryer-cooler (Becker, 1978). The drying is
done by hot air and the cooling is accomplished by blowing ambient air through the meal.
Another system, “Vapor Desolventizer-Deodorizer”, allows time, temperature and moisture
in the system to be adjusted during operation. The solvent in the meal is removed by
superheated vapor in the desolventizer; then meal passes to the deodorizer through a lock
(Becker, 1978). The steam pressure in the deodorizer can be regulated between 0.5 and 2
bars. By regulating steam pressure and minor tunings of the height of the discharge dam, the
operators have the flexibility to adjust meal properties, that is the protein dispersible index
(PDI) over a range of 10 to 90.
Solvent recovery from micella (oil solvent) is carried out in single or dual effect

evaporators. Flashing and low-pressure steam stripping remove hexane from the oil down to
less than 0.15% (w/w). Solvent vapors from distillation and stripping columns are condensed
and recycled to the extractor. After this, crude oil goes through a refining process, which
removes undesirable components in the oil.

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4.3.2 Mechanical oil expression
Mechanical pressing of liquid out of liquid-containing solids is referred to as “expression”.
The conventional theories about oil expression from oilseeds were based on the assumption
that oilseed cell walls were non-porous; hence, they had to be raptured by mechanical and
thermal treatment before substantial oil expression could be achieved (Mattil et al ., 1964 ).
A later theory has suggested that the pores (plasmodesmata) of certain biological cell walls