Advancements in Oil and Oilseed Processing - Food and Industrial Bioproducts and Bioprocessing - page 115

pressed cake. Furthermore, wax and color compounds present in the hulls are extracted
along with oil. These compounds are not desirable in edible oils and need to be removed
during the refining process. De-hulling reduces fiber and increases protein content of the
meal. Knife, disk and impact type de-hullers are widely used. An impact de-huller, which is
commonly used for sunflower seeds, consists of a rotating blade that drives seeds onto a
hard wall outside the diameter of the blades. The force of impact causes the hulls to break.
The seeds are then separated using shaking conveyor belts, multiple sifting screens or
vacuum (MilIer et al ., 1986). De-hulling efficiency is often measured by the residual fiber
content in the meal and the residual oil content in the hulls. The industry standards for oil
and fiber contents of soybean meal are: minimum 0.5% oil and maximum 3.3-3.5% fiber for
(minimum) 47.5-49% protein meal, and maximum 7% fiber and minimum 0.5% oil in 44%
protein meal (12% moisture content) (National Oilseed Processors Association, 2010).
4.2.3 Size reduction and flaking
Most oilseeds are reduced in size to facilitate hull removal, heating, drying and flaking
prior to oil extraction. Canola, rapeseed and corn germ do not require size reduction as they
are already sufficiently small. Cracking mills are used for seed size reduction. High
capacity cracking mills can process up to 1000 metric tons (tonnes) per day of oilseeds. In
general, oilseeds are flaked prior to solvent extraction. A flaking mill has two large diameter
rolls turning in opposite direction and forced together by hydraulic cylinders. As the seeds
are pulled through the flaking mill, they are stretched and flattened. Unfortunately, literature
on the mechanical aspects of oilseed flaking, specifically technical information on
deformations and forces that seeds experience as they move through the flaking rolls, is
very limited (Levine, 1993). Such information is very important for designing flaking rolls
for oilseeds. Flaking ruptures seed cellular structure and reduces the distance that solvent
has to travel to contact oil in the cells. Typical flake thickness is in the range of 0.01-0.015
inch or 0.25-0.37 mm (Singha et al ., 1999). Flaking of oilseeds also increases surface area
for increased contact between solvent and seed during the solvent extraction process. Oil
from the cracked or flaked seeds should be extracted as quickly as possible (within 24 h) to
minimize meal and oil quality deterioration. Flaking mills which can process 300-500
tonnes of seeds per day are available.
4.2.4 Cooking/Tempering
Oilseeds are cooked or tempered to denature proteins, release oil from the cells and inactivate
enzymes. For example, rapeseed contains the enzyme myrosinase. This enzyme catalyzes
hydrolysis of glucosinolates, which are naturally present in rapeseed. During the hydrolysis
process undesirable compounds such as isothiocyanates and nitriles form. These compounds
are soluble in oil and lower the quality of oil. Rapeseed is cooked in multistage cookers to
keep the glucosinolates intact and inactivate the myrosinase (Christian, 2010). Rapeseed is
preheated to 20-50 °C in less than five minutes and contacted with direct steam at 120 °C.
The cooking temperature for canola is lower (less than 100 °C) than that of conventional
rapeseed varieties because of the lower glucosinolate content of canola (Gunstone, 2004).
Cooking also gives seeds elasticity for efficient pressing. Oil needs to be extracted from
cooked seeds immediately. Cooking at high temperature is not necessary for sunflower
seeds (Pierce, 1970). Cracked and de-hulled soybeans are conditioned or tempered by
increasing the temperature to 65°C and adjusting the moisture by using live steam.