Food Chemistry

14.4 Processing of Fats and Oils 657

bean oil, hydrogenated selectively to increase

its stability against oxidation.

• Fats that melt close to 30◦C and have a plas-
  tic or spreadable consistency at room tempera-
  ture.

Fully or partially hydrogenated oils are important
raw materials for margarine manufacturing and
serve as deep-frying fats (cf. 14.4.8).

14.4.2.2 Catalysts

The principle of the heterogeneous catalytic hy-
drogenation of unsaturated acylglycerols was out-
lined under 3.2.3.2.4. The most widely used cata-
lyst is carrier-bound nickel. Raney nickel, cop-
per, and noble metals serve special purposes. The
choice of catalyst is made according to:

• Reaction specificity.


• Extent of trans-isomer formation


• Duration of activity and cost.

To determine the specificity of a catalyst, the reac-
tion rates for each individual hydrogenation step
must be determined. Simplified, there are three
reaction rate constants (k) involved (AG = acyl-
glycerol):

Triene-AR

k 3

−→Diene-AR

k 3

−→Monoene-AR

⏐

⏐

⏐

k^1

AR-acylresidue Saturated-AR

(14.5)

The catalytic reactions considered here require
that k 3 >k 2 >k 1. The following equations deter-
mine the specificity “S”:

S 32 =

k 3

k 2

;S 21 =

k 2

k 1

;S 31 =

k 3

k 1

(14.6)

That means, the greater the value of “S”, the
faster the hydrogenation at this step. Therefore,
specificity (or selectivity) is proportional to the
value of “S”. For the three catalysts mentioned,
Table 14.16 shows that the hydrogenation of di-
ene→monoene by Ni 3 S 2 and the hydrogenation
of triene→monoene by copper become acceler-
ated with marked specificity. Copper is particu-
larly suitable for decreasing the linolenic acid

Table 14.16.Properties of hydrogenation catalysts

Selectivity trans-Fatty acids

Catalyst S 32 S 21 (weight-%)a

Nickel-contact 2–3 40 40

Ni 3 S 2 -contact 1–2 75 90

Copper-contact 10–12 50 10

atrans-Fatty acids as monoenoic acids total content is

calculated as elaidic acid.

content in soybean and rapeseed oils. However,

copper catalysts are not sufficiently economical,

since they can not be used more than twice. Their

complete removal, which is necessary because

this is a prooxidatively active metal, is relatively

tedious.

Although noble metals are up to 100 times more

effective than nickel catalysts, they are not popu-

lar because of their high costs. It is of great ad-

vantage that the nickel catalyst can be used re-

peatedly for up to 50 times under the following

conditions: the plant oil must be deacidified, freed

of gum ingredients and contain no sulfur com-

pounds (cf. rapeseed oil, 14.3.2.2.5). The favor-

able ratio of duration of activity to cost places the

nickel catalysts ahead with advantages not readily

surpassed by any other catalyst. For the produc-

tion of nickel-carrier catalysts, kieselguhr or zeo-

lite is impregnated with nickel hydroxide, which

is precipitated out of a solution of nickel nitrate

with sodium hydroxide or carbonate. After dry-

ing, nickel hydroxide is reduced to nickel with

hydrogen at 350–500◦C.

For the production of carrier-free nickel catalysts,

nickel formate is suspended in a fat and then de-

composed:

Ni(HCOO) 2 −→

( 200 − 250 ◦C)

Ni+2CO 2 +H 2 (14.7)

The catalysts obtained with a Ni content of 22–

25% are pyrophoric. For this reason it is embed-

ded in fat and handled and marketed in this form.

To evaluate the catalysts, calculation programs

were developed for the determination of the ac-

tual selectivity of a catalyst based on the fatty acid

composition of the starting material and of the hy-

drogenated product.

During hydrogenation, linolenic acid yields,

among others, isolinoleic and isooleic acids (cf.

Reaction 14.8).