Food Lipids: Chemistry, Nutrition, and Biotechnology

with hot oil (80–110�C) for 15–30 minutes to form a slurry. Mixing enhances oil
contact with the adsorbent. The pigments are adsorbed onto the surfaces of various
clays or earths (some may be activated by treatment with acid), even sometimes
activated carbon, and the solids are removed by filtration. Activated earths are made
from certain bentonites, specifically montmorillonite. Acid activation is believed to
be achieved by replacing aluminum ions in the clay structure [52] with hydrogen
ions by treating with sulfuric acid; excess acid is removed with water, and the activated
earth is dried and milled [38]. The hard-to-bleach oils are normally done so
with acid-activated clays.
Bleaching power seems to be a function of the clay’s bound acidity, and clays
with high total acidity and a reasonable level of acidity are preferred. If a bleaching
clay is washed completely free of residual acid, bleaching power is greatly reduced.
Clay from which water has been removed gives better results than clays containing
adsorbed water; but if the earth is dried to less than 10% moisture, its internal
structure will collapse, reducing surface area and thus adsorptive power. This partly
explains why heating the oil/clay slurry is important, to remove water that is adsorbed
in the clay lattices.
About 0.2–2% bleaching clay is usually used, the precise amount depending
upon the amounts of pigments present. At the low end, 0.2–0.4% is used for soybean
oil, while rice bran oil requires considerably more (3–5%). In addition to removing
pigments and residual soap, bleaching takes out trace metals and some oxidation
products. It is important to remove as completely as possible any residual soap
(typically