Page 398 - Biosystems Engineering

Extraction, Refining, and Stabilization of Edible Oils 375

processes such as liquid–liquid extraction with 98 percent methanol
3
and without the aid of stripping steam have been used to a limited
extent to deacidify expensive specialty oils with high initial acidity.

13.4 Bleaching and Deodorization

Bleaching Edible Oil Refined oil can be further bleached to an almost
colorless state using activated earth. Fuller’s earth is prepared by
treating natural earth with HCl, which consists of a large number of
small pores where the coloring matter of oil is absorbed, and then
dried. The oil is mixed with activated earth in a tank fitted with agita-
tors and heated to a moderate temperature. The oil is then clarified by
filtration. In addition to fuller’s earth, activated carbon, kieselguhr, or
diatomaceous earth may be used. The colored compound is also
removed in certain cases either by oxidation or reduction to colorless
derivatives or by decomposition with concentrated H SO .
2 4
The quality of edible oils depends on the removal of trace metals,
free fatty acids, gummy materials, coloring, and various organic mat-
ter that promote oxidation and produce objectionable compounds
that are detrimental to oil flavor and stability. Different types of
adsorbents 9,10 are used to remove coloring materials and other impu-
rities in oils.
The term ”bleaching“ refers to the process of removing color-
producing substances and for further purifying the fat or oil. Normally,
bleaching is accomplished after the oil has been refined.
Chlorophyll is readily extracted along with the oil from oil seeds.
Chlorophyll imparts a greenish color to crude oil. In conventional
refining techniques, the chlorophyll is converted to the pigment pheo-
phytin, which makes oil dark and contributes to an off flavor. It may
also promote oxidation of the oil and thus reduce its storage stability.
Removal of chlorophyll is essential; however, removal of other
colored pigments is not particularly important because the pigments
are effectively removed during either hydrogenation or deodoriza-
tion due to the heat-bleaching effect during those two processes. 11
There are indications that chlorophyll and its derivatives may be
present in the oil—not in true solution but as a suspension of very
small particles, probably colloidal in size. During the last decade,
new bleaching agents have been developed that are more effective in
removing chlorophyll than conventional bleaching clays. The indus-
try is carefully evaluating whether the increased costs of these agents
is offset by the savings in clay requirements and reduced oil losses. 12
The usual method of bleaching is by adsorption of the color-
producing substances on an adsorbent material. Acid-activated
22
bleaching earth or clay as bentonite or alumina-pillared (Al-pillared)
acid-activated clay is the adsorbent material that has been used most
12
extensively. This substance consists primarily of hydrated aluminum