2009_Book_FoodChemistry

658 14 Edible Fats and Oils
14.4.2.3 The Process
The hydrogen required can be obtained by electrolysis
of dilute aqueous KOH, through waterto-gas
conversion:
(14.8)
The diversity of the reaction products present in
partially hydrogenated fat is increased further
by the positional- and stereoisomers of the
double bonds. Hydrogenation of soybean oil
in the presence of a copper catalyst gives, for
example, a number of trans-monoene fatty acids
(Table 14.17). The extent of isomerization is
affected, among other factors, by the type of
catalyst used in hydrogenation. Efforts are being
made to reduce the formation of trans-fatty acids
during hydrogenation because these acids have a
detrimental effect on the composition of the blood
lipids. Also, the presence of a hydrogenated fat in
a mixture is easily detected via the identification
of trans-fatty acids, e. g., by IR spectroscopy or
gas chromatography (cf. 14.5.2.3).
A further drawback of the partial hydrogenation
of an oil is the pattern of linoleic acid
isomersformed. The two isomers formed during
hydrogenation, linolelaidic acid 18:2 (9
trans, 12 cis) and 18:2 (9 cis, 12 trans) are, unlike
linoleic acid, not essential fatty acids (cf. 3.2.1.2).
Table 14.17. Fatty acid composition of a soya oil before
and after hydrogenation with a copper catalyst
Hydrogenation
Fatty acid before after
(weight-%) (weight-%)
16:0 10.0 10.0
18:0 4.2 4.2
18:1(9) 26.0 30.4
18:1 a 0 5.5
18:2(9,12) 52.5 42.5
18:2(conjugated) b 0 0.7
18:2 c 0 5.2
18:3(9,12,15) 7.3 0.7
a This fraction contains eight trans fatty acids: 18:1
(7 tr)–18:1 (14 tr); major components are 18:1 (10 tr)
and 18:1(11 tr).
b It consists of various conjugated fatty acids.
c Isolinoleic and isolinolelaidic acids.
H 2 O + C → H 2 + CO
CO + H 2 O → H 2 + CO 2 (14.9)
or by the decomposition of natural gas with
steam:
CH 3 (CH 2 ) × CH 3 + H 2 O → H 2 + CO
CO + H 2 O → H 2 + CO 2 (14.10)
in the latter two processes, the poisonous by products,
H 2 S and CO, have to be completely removed.
Oil hydrogenation is performed in an autoclave
equipped with a stirrer under hydrogen
gas pressure of 1–5 bar and a temperature of
150–220 ◦ C. More recent developments aim at
a continuous process, e. g., in fixed-bed reactors.
A newer hydrogenation process uses a recycling
hydrogenation unit equipped with a spraying
nozzle, external heat exchanger and recycling
pump.
The process conditions have a significant effect
on the composition and therefore on the consistency
of the end-product. Selective hydrogenation
of double bonds is favored by a high concentration
of catalyst (which, depending on the Ni
activity, is 200–800 g Ni/t fat), a high temperature
and low pressure of hydrogen gas. After hydrogenation,
the fat is filtered, then deacidified,
bleached and deodorized during further refining
(cf. 14.4.1.3–14.4.1.5).
Some constituents of the unsaponifiable matter
are also affected by the hydrogenation
process. Carotenoids, including vitamin A,
are hydrogenated extensively. Some of the
chlorine-containing pesticide contaminants are
hydrogenated. Sterols, under the usual operating
conditions, are not affected. The ratios and levels
of tocopherols are essentially unchanged.
14.4.3 Interesterification
Natural fats and oils are subjected to extensive
interesterification during processing. This in-