Biomedical Engineering Reference
In-Depth Information
48.3 °C and pH 4.9. The process produced rapeseed oil with 3.1 ppm phosphorous content.
The process was then scaled up to 400 tonnes oil/day production. Degumming process effi-
ciency was significantly affected by pH at the large scale. When the pH was maintained
between 4.6 and 5.1, phosphorous content in the degummed rapeseed oil could be reduced
to less than 10 ppm.
In general, enzymatic degumming minimizes oil retained in the gum, resulting in improved
oil yields. The enzymatic hydrolysis reaction typically requires 5-6 hours and utilizes less
water than water and acid degumming processes. Reductions in enzyme production costs and
enhanced enzyme activity and stability could make enzymatic degumming process very
attractive for oil refining industry in the near future.
4.4.2 Deacidification/Refining
Good quality oil contains over 95% neutral lipids, triacylglycerides (TAGs). FFA content of
commercial crude oils is usually about 1-3% while high quality crude oils can have only
0.5% or less FFAs. However, palm, olive, rice bran and some specialty oils, such as wheat
germ and rice bran oils, may contain 20% or more FFAs. As an industry rule, the FFA content
of refined edible oils should be less than 0.05%. Although most of the long chain FFAs do not
significantly impair the taste of the oil, the short chain FFAs may have a soapy and rancid
flavor. Furthermore, FFAs accelerate oxidation reactions, consequently reducing the oxidative
stability of the oils. Hence, removal of FFAs is essential to obtain high quality oil.
4.4.2.1
Chemical refining
Crude oils are traditionally deacidified or refined by chemical methods. During chemical
refining a heavy soapstock (sodium or potassium salts of fatty acids) is formed. Soapstock is
separated from refined oil by gravity settling, filtration or centrifugation. Sodium hydroxide,
also referred to as caustic or lye, is widely used for chemical oil refining. The proper strength
and amount of lye is critical for achieving high FFA removal with minimal TAG loss and
degradation. The presence of color and surface-active compounds in oil interfere with the
reaction of FFAs with lye (Mattil et al ., 1964). The degummed oil at 30-40 °C is mixed with
the required amount of lye and pumped through a high-shear mixer. The mixing time is
usually 5-10 min. The oil is then heated to 75 °C and centrifuged to remove soapstock. Soda
ash or sodium carbonate can also be used to remove FFAs from crude oil. However, carbon
dioxide released during refining causes foaming. In addition, entrainment of gas in the
soapstock prevents proper settling. The deacidification with alkali refining process has a
number of drawbacks: (1) high oil losses due to oil entrainment in soapstock, (2) production
of low value soapstock, (3) high water requirement to wash oil after caustic treatment which
leads high waste water volume and disposal costs.
4.4.2.2
Miscella refining
In cottonseed, gossypol, a complex polyphenolic compound, contributes to oil toxicity and
dark color and is regarded as an undesirable component. During cottonseed processing,
gossypol must be removed to produce edible oil and animal feed. Gossypol in crude
cottonseed oil is typically removed in miscella (mixture of oil
hexane) before oil
desolventation in the extraction plant. The crude oil-hexane mixture (45-65% oil : 35-55%
hexane) is filtered to remove any meal, scale or insoluble impurities that may be carried
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