Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.10 Food industry 939
of the phosphatides from these crude oils. Tocopherols are important minor constituents of
vegetable oils, which are natural antioxidant that retard the development of rancidity.
Refining, bleaching, and deodorization are the steps that are necessary if the oil is to be
used in food applications. Oil that has only gone through these three steps is called “RBD”
oil. Figure 14.10.2 illustrates the processing pathways.
Refining: Refining involves the removal of nonglyceride materials (phospholipids,
color, and trace materials) and FFA. The goal is to produce a high-quality refined oil with
the highest yield of purified triglycerides. Refining is by far the most important step in processing.
An improperly refined oil will present problems in bleaching and deodorization
and reduce quality.
Some solvent-extracted crude oils, including soybean or canola/rapeseed, contain approximately
2-3% gums, which are mainly phosphatides and require degumming. The principal
phosphatides are lecithin and cephalin. Gums can cause problems through higher then
necessary refining losses, or by settling out in storage tanks. The degumming operation exploits
the affinity of most phosphatides for water, converting them to hydrated gums that are
insoluble in oil and readily separated by centrifugal action. Lecithin can be recovered and
concentrated from the gums in a separate solvent extraction process, usually with acetone.
Either water-degummed oil or crude oil can be treated with sodium hydroxide solution
to saponify free fatty acids that are subsequently removed as soapstock by a primary refining
centrifuge. Conventional alkali refining is by far the most widespread method of edible
oil refining. The success of the alkali refining operation is the coordination of five prime
factors: (1) use of the proper amount of reagent (sodium hydroxide), (2) proper mixing, (3)
proper temperature control, (4) proper residual contact time, and (5) efficient separation.
Oil is alkali-refined by the addition of sodium hydroxide solution at a level sufficient
to neutralize the FFA content of the oil. An excess of sodium hydroxide is required to reduce
the color of the refined oil and to ensure the completion of the saponification reaction and to
remove other trace elements. The amount and strength of the sodium hydroxide solution
needed to neutralize the FFA is dependent on the amount of both FFAs and phosphatides
present in the crude oil. Water-soluble soaps are formed in the primary reaction between the
sodium hydroxide and FFAs. The hydratable phosphatides react with the caustic forming
oil-insoluble hydrates. The caustic used in alkali refining is normally diluted to about
8-14% NaOH, although higher concentrations are occasionally used to reduce color. The
proper amount of NaOH solution added to the oil will produce an adequately refined oil
with the minimum of triglyceride oil loss. The amount of NaOH solution (neutralizing dose
plus excess) is determined by experience and adjusted according to laboratory results.
After the NaOH solution is injected, it is mixed for 6-10 minutes to ensure thorough
contact. The treated oil is then heated to assist in breaking of the emulsion prior to separation
of the soapstock from oil in continuous centrifuges.
Any soap remaining, after the primary soapstock separation, is removed through continuous
hot water washings. In this step, water is added at 10-15% at a temperature sufficient
to prevent emulsification, generally 82-90.5ºC (180-195ºF). The oil is again separated
from the soapy phase in water wash separators and drier prior to bleaching.
Bleaching: The oil is further purified by “bleaching”, which removes color bodies and
trace metals as well as entrained soaps, and products of oxidation that are adsorbed onto the
surface of bleaching agents or adsorbents. Types of adsorbents most commonly used in-