2009_Book_FoodChemistry

14.4 Processing of Fats and Oils 655
lized in marine oil fractionation, e. g., in the production
of vitamin A concentrates.
14.4.1.4 Bleaching
In order to remove the plant pigments (chlorophyll,
carotenoids) and autoxidation products, the
fat or oil is stirred for 30 min in the presence of
Al-silicates (bleaching or Fuller’s earth) in a vacuum
at 90 ◦ C. The silicate has to be activated prior
to use – a suspension in water is treated with hydrochloric
acid, followed by thorough washing
with water, then drying. The amount of silicate
used is 0.5–2% of the fat (oil) weight. It is often
used together with 0.1–0.4% activated charcoal.
The bleached oil is removed from the adsorbent
by filtration. The oil retained by the absorbent
can be recovered by hexane extraction and recycled
into the refining process. The residual alkali
soaps, gums, part of the unsaponifiable matter and
the heavy metal ions are also removed during the
bleaching process.
After bleaching, some oils or fats which contain
polyunsaturated fatty acids show an increase in
absorbance at 270 nm. This is due to decomposition
of hydroperoxides, formed by autoxidation,
into oxo-dienes (I) and fatty acids with three double
bonds (II):
Table 14.14. Removal of chloro pigments in the refining
of rapeseed oil (values in mg/kg)
Amounts after
Raw oil Deacidifi- Bleach- Deodorication
ing zation
Chlorophyll A 2.62 0.89 0.028 0.007
Chlorophyll B 2.92 0.08 0.059 0.023
Pheophytin A 35.6 31.5 0.235 0.108
Pheophytin B 4.99 6.85 0.071 0.036
Sum 46.1 39.3 0.393 0.174
Fig. 14.5. Oxidation of soybean oil on exposure
to room light (according to Usuki et al., 1984).
(1), (2), (3), and (4) contain 39, 233, 425, and
623 µg/kg respectively of the mixture chlorophyll
A/chlorophyll B/pheophytin A/pheophytin B
(1:3:10:3)
As shown in Table 14.14 with rapeseed oil, most
of the chlorophylls and their degradation products
are removed during bleaching. However, the
rest, which is in the range of 70–1200 µg/kg in refined
plant oils, could still accelerate photooxidation
(Fig. 14.5). As a result of their high stability
on exposure to light, the pheophytins are stronger
photooxidants than the chlorophylls. As shown
in Table 14.14, the pheophytins are the predominanting
pigments.
14.4.1.5 Deodorization
(14.4)
Deodorization is essentially vacuum steam distillation
(190–230 ◦ C, 0.5–10 mbar). The volatile
compounds, together with undesirable odorants
present in the fat or oil, are separated in this re-