Food Chemistry

3.3 Acylglycerols 169

3.2.3.2.4 Hydrogenation..........................................

In the presence of a suitable catalyst,
e. g. Ni, hydrogen can be added to the
double bond of an acyl lipid. This het-
erogeneous catalytic hydrogenation occurs
stereo selectively as a cis-addition. Catalyst-
induced isomerization from an isolene-type
fatty acid to a conjugated fatty acid oc-
curs with fatty acids with several double
bonds:

(3.14)

Since diene fatty acids form a more stable
complex with a catalyst than do monoene
fatty acids, the former are preferentially hy-
drogenated. Since nature is not an abundant
source of the solid fats which are required in
food processing, the partial and selective hy-
drogenation, just referred to, plays an important
role in the industrial processing of fats and oils
(cf. 14.4.2).

3.2.4 BiosynthesisofUnsaturatedFattyAcids

The biosynthetic precursors of unsaturated fatty
acids are saturated fatty acids in an activated
form (cf. a biochemistry textbook). These are aer-
obiocally and stereospecifically dehydrogenated
by dehydrogenase action in plant as well as an-
imal tissues. A flavoprotein and ferredoxin are in-

(3.15)

volved in plants in the electron transport system

which uses oxygen as a terminal electron acceptor

(cf. Reaction 3.15).

To obtain polyunsaturated fatty acids, the dou-

ble bonds are introduced by a stepwise process.

A fundamental difference exists between mam-

mals and plants. In the former, oleic acid syn-

thesis is possible, and, also, additional double

bonds can be inserted towards the carboxyl end of

the fatty acid molecule. For example,γ-linolenic

acid can be formed from the essential fatty acid

linoleic acid and, also, arachidonic acid (Fig. 3.5)

can be formed by chain elongation ofγ-linolenic

acid. In a diet deficient in linoleic acid, oleic

acid is dehydrogenated to isolinoleic acid and its

derivatives (Fig. 3.5), but none of these acquire

the physiological function of an essential acid

such as linoleic acid.

Plants can introduce double bonds into fatty

acids in both directions: towards the termi-

nal CH 3 -group or towards the carboxyl end.

Oleic acid (oleoyl-CoA ester or β-oleoyl-

phosphatidylcholine) is thus dehydrogenated to

linoleic and then to linolenic acid. In addition

synthesis of the latter can be achieved by another

pathway involving stepwise dehydrogenation

of lauric acid with chain elongation reactions

involving C 2 units (Fig. 3.5).

3.3 Acylglycerols...........................................

Acylglycerols (or acylglycerides) comprise the

mono-, di- or triesters of glycerol with fatty acids

(Table 3.1). They are designated as neutral lipids.

Edible oils or fats consist nearly completely of tri-

acylglycerols.