Advancements in Oil and Oilseed Processing - Food and Industrial Bioproducts and Bioprocessing - page 129

from the extraction process. Next the crude miscella is pumped to a reaction vessel where
lye is added and mixed thoroughly until the impurities in the crude oil precipitate in the soap
phase. Then the light colored refined miscella is separated from the dark and gummy
soapstock by using an explosion-proof centrifuge. The light yellow miscella is pumped to a
stripper to recover hexane. Leaving the stripper at 104 °C, the refined oil passes to a pressure
leaf-type filter to remove the last traces of soap and any impurities before cooling and
entering the storage tank. During miscella refining FFAs and PLs are also removed along
with gossypol. The major advantage of miscella refining over conventional refining is that
crude miscella is refined immediately after solvent extraction before the solvent stripping
has set the color (Hendrix, 1984).

4.4.2.3 Steam refining

Steam refining, also referred to as physical refining, is the removal of fatty acids and other
volatiles by superheated steam at 200-270 °C at low pressure after preliminary degumming
and bleaching steps. Physical refining, which has been adapted by oil refining industry as a
viable alternative to the caustic/chemical refining process, is based on the higher volatility
of FFAs than TAGs at high temperatures and low pressures. During the process, volatile
compounds including FFAs are volatilized and neutral oil droplets are entrained within the
stripping steam. The final FFA content in the refined oil can be reduced to 0.005% when
physical refining is used. Utilization of a packed column with 4-5 m height and 250 m 2 of
packing surface area/m 3 was suggested for steam refining of palm oil (about 30% FFA
content) after degumming (Bernardini, 1993). The advantages of the steam refining process
include: elimination of objectionable soapstock production, consequently reducing oil
losses, production of higher quality FFA and simplified operations. However, during high
temperature processing heat labile oil components might be destroyed. It is important to
note that phosphorous content of oil has a significant effect on physical refining. Oils with
high non-hydratable PLs are not suitable for physical refining. This process is energy
intensive because of the high temperatures involved.

4.4.2.4 Selective removal of FFAs

Selective removal of FFAs from oil by microorganisms, specifically the Pseudomonas strain
(BG1), has been reported. (Cho et al ., 1990). BG1 assimilates lauric, myristic, palmitic,
stearic, and oleic acids as carbon sources without secreting extracellular lipases. However,
short chain fatty acids with less than 12 carbon atoms and linoleic acid are not utilized and
sometimes inhibited the growth of BG1. Solubility of fatty acids in water had a significant
effect on the rate of FFA removal. Maximum biomass was obtained when BG1 was grown
on oleic acid. Even though butyric, valeric, caproic, caprylic and capric acids all have higher
solubility in water than oleic acid, they were not utilized by BG1, probably due to the
toxicity of short chain fatty acids to microorganisms. Although removal of FFAs by
microorganisms is an interesting approach, this technique remains at the research level.
Although it is not widely used, selective solvent extraction is practiced by small
operations to neutralize very high FFA content oils. Selective solvent or liquid-liquid
extraction of FFAs from TAGs is based on the differences in solubility of FFA and TAG
in various organic solvents. Phase separation and selective extraction of FFAs (0-50%) in
peanut oil could be achieved by use of aqueous isopropyl alcohol (IPA) (75% and 80%).
Phase separation was found to be depended on the fatty acid content in the mixture and