Advancements in Oil and Oilseed Processing - Food and Industrial Bioproducts and Bioprocessing - page 124

because of the dissolved CO 2 in oil, consequently releasing the oil. The oil yield is expected
to increase because of the presence of free oil in seed and lower viscosity of CO 2 -saturated
oil (Venter et al ., 2006). However, during the actual operation, when the pressure is released
at the nozzle of the press, CO 2 turns to the gas phase and oil is entrained in the meal
(Dijkstra, 2009 ).
Although the industrial applications of SC-CO 2 technology have been growing steadily,
currently most of the applications involve high value products. The high investment cost of
SCF systems remains to be a drawback for this technology. However, cost saving can be
realized from simplified separations and the lower solvent cost. The non-toxicity of SC-CO 2
and significantly reduced waste streams from the process makes this technology unique in
terms of meeting increasingly stringent health and environmental regulations.

4.4 OIL REFINING

Crude oil obtained by both solvent extraction and mechanical pressing contains both desirable
and undesirable compounds. Triacylglycerides (TAGs) (neutral lipids) and health beneficial
compounds such as tocopherols and phytosterols are the desirable compounds in the oil. Free
fatty acids (FFAs), phospholipids (PLs), also referred to as gums, and lipid oxidation products
are removed during oil refining because of their adverse effects on oil quality. There are
several unit operations in a crude oil refining operation. Degumming, deacidification/refining,
bleaching, deodorization and winterization are commonly used for edible oil production.

4.4.1 Degumming
Phospholipids are essential components of oilseed cell structure. They are oil soluble and
extracted along with oil. PLs are not desirable in oil because they settle out of the oil during
shipping and storage and have adverse effects on the color and flavor of oil. The presence of
PLs in oil may create problems during processing, for example frying, because of their
emulsifying properties (they reduce interfacial tension between immiscible liquids, i.e.
water/oil). There are two types of PLs, hydratable and non-hydratable. PLs containing the
functional groups choline and ethanolamine have higher affinity for water than that of the
PLs with acids, acid salts (calcium, magnesium, and iron) and inositol groups. Phosphatidic
acid and the salts of phosphatic acid are commonly known as “non-hydratable phospholipids”.
The amount of non-hydratable PLs in crude oil varies significantly depending on the quality
and type of the seed and the oilseed handling and processing conditions.

4.4.1.1 Water and acid degumming

Degumming refers to the unit operation during which PLs are removed from crude oil.
Several reviews on oil degumming have already been published (Xu and Diosady, 2004;
Gupta, 2008 ; Mattil et al ., 1964). Oil degumming is usually carried out at the crushing or
extraction plant. Hydratable PLs can be removed from the oil by water degumming. Hot
deionized water (at 65-80 °C) or steam is injected into the warm oil. The amount of water/
steam added depends on the amount of hydratable PLs present in the oil. As a rule of thumb,
about 2% water is added to oil and mixed for 30-60 min during a batch operation. Continuous
degumming processes utilize an on-line mixer for mixing oil and the residence time is
usually 10-15 min. During this process PLs absorb water and lose their lipophilic