Food Chemistry

658 14 Edible Fats and Oils

(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 hydrogena-
tion of an oil is the pattern of linoleic acid
isomersformed. The two isomers formed dur-
ing 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. 010. 0
18:0 4. 24. 2
18:1(9) 26. 030. 4
18:1a 05. 5
18:2(9,12) 52. 542. 5
18:2(conjugated)b 00. 7
18:2c 05. 2
18:3(9,12,15) 7. 30. 7
aThis 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).
bIt consists of various conjugated fatty acids.
cIsolinoleic and isolinolelaidic acids.

14.4.2.3 The Process

The hydrogen required can be obtained by elec-

trolysis of dilute aqueous KOH, through water-

to-gas conversion:

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 prod-

ucts, H 2 S and CO, have to be completely re-

moved.

Oil hydrogenation is performed in an auto-

clave 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 consist-

ency of the end-product. Selective hydrogenation

of double bonds is favored by a high concen-

tration of catalyst (which, depending on the Ni

activity, is 200–800 g Ni/t fat), a high tempera-

ture and low pressure of hydrogen gas. After hy-

drogenation, 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 mat-

ter 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-