2009_Book_FoodChemistry

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 heterogeneous
catalytic hydrogenation occurs
stereo selectively as a cis-addition. Catalystinduced
isomerization from an isolene-type
fatty acid to a conjugated fatty acid occurs
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 hydrogenated.
Since nature is not an abundant
source of the solid fats which are required in
food processing, the partial and selective hydrogenation,
just referred to, plays an important
role in the industrial processing of fats and oils
(cf. 14.4.2).
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 double
bonds are introduced by a stepwise process.
A fundamental difference exists between mammals
and plants. In the former, oleic acid synthesis
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 of γ-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 terminal
CH 3 -group or towards the carboxyl end.
Oleic acid (oleoyl-CoA ester or β-oleoylphosphatidylcholine)
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).
3.2.4 Biosynthesis of Unsaturated Fatty Acids
The biosynthetic precursors of unsaturated fatty
acids are saturated fatty acids in an activated
form (cf. a biochemistry textbook). These are aerobiocally
and stereospecifically dehydrogenated
by dehydrogenase action in plant as well as animal
tissues. A flavoprotein and ferredoxin are in-
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 triacylglycerols.
(3.15)