Advancements in Oil and Oilseed Processing - Food and Industrial Bioproducts and Bioprocessing - page 114

be segregated from traditional crops to preserve their identity. Otherwise enhanced quality
will be lost. Furthermore, consumer acceptance of genetically modified crops is still a
controversial issue and labeling of bioengineered crops has been mandated in some
international markets. Therefore, separate handling systems for bioengineered and traditional
products may need to be created.
4.2.2 Preparation of seeds for oil extraction
Unit operations for preparation of seeds for oil extraction vary slightly depending on the
physical properties and oil content. However, most oilseeds go through cleaning, drying,
de-hulling, size reduction, flaking and cooking processes.
Oilseeds need to be cleaned to remove plant stems, sticks, leaves and foreign material
before storage. Such material may decompose and generate heat in the stored seed mass,
diminishing oil and meal quality. Foreign materials in seeds are typically separated out by a
combination of rotating or vibrating coarse screens, reels and aspiration. This process is
commonly referred to as scalping. Sand and dirt are also removed by fine screening.
Magnetic devices, plate, drum or electromagnets, installed over conveyor belts remove
metal contaminants from seeds. Some oilseeds, such as peanuts, may contain stones which
are similar in size to the seeds. Hence they cannot be separated by screening. Seeds are
separated from stones by gravity in destoners.
The moisture content of oilseeds often has to be reduced to minimize degradation in
storage and to improve the effectiveness of downstream processing. For example, soybeans
are often received at 13% moisture or higher and need to be dried to 10% moisture to
facilitate efficient hull removal. Depending on the regulations of the country in which
oilseeds are traded, seeds with high moisture contents may be accepted with some financial
penalty (Sutherland and Ghaly, 1982). High moisture seeds are dried with heated air either
on the farm or at the elevators. Fixed-bed-batch (both parallel and radial flow), recirculating-
batch and continuous-flow dryers are used for drying oilseeds. The design of oilseed dryers
requires basic knowledge of drying parameters, including resistance of seeds to air flow,
moisture sorption properties and drying rates, and also safe drying temperatures. Because of
the high oil content in oilseeds, safe drying air temperatures and fire risk are very important
factors that have to be taken into account during dryer design. A study carried out with
sunflower and rapeseed (16% initial moisture content) showed that the maximum inlet
drying air temperature to preserve germination was 60 °C, which was also the maximum
safe drying air temperature in a fluidized-bed dryer (Sutherland and Ghaly, 1982). It was
also indicated in the latter study that higher inlet air temperatures could be used provided
that seed temperature at any point within the dryer does not exceed the stated safe value.
Inlet air temperatures higher than maximum safe drying temperature was not recommended
for fixed-bed batch dryers, as seeds near the air inlet could approach the inlet air temperature,
but for recirculating batch and continuous-flow dryers a slight increase in inlet air temperature
could be allowable.
The amount of hull on oilseeds varies significantly. The percentage of hulls for cotton
seed, sunflower seeds and soybean are 36, 24 and 8%, respectively (Clark et al ., 1980 ;
Pierce, 1970). Hulls and shells of oilseeds do not contain a significant amount of oil (less
than 1%). Most oilseeds need to be separated from their outer husk or shell prior to oil
extraction. De-hulling or shelling or decorticating increases oil production efficiency and
capacity of the extraction equipment, and reduces wear in the extractor as the husks are
abrasive. If not removed, hulls reduce the total oil yield by absorbing or retaining oil in the