2009_Book_FoodChemistry

170 3 Lipids
Fig. 3.5. Biosynthesis of unsaturated fatty acids. Synthesis pathways: a, c, g in higher plants; a, c, g and a, c, d, f
in algae; a, b and d, f (main pathway for arachidonic acid) or e, f in mammals
3.3.1 Triacylglycerols (TG)
3.3.1.1 Nomenclature, Classification,
Calorific Value
Glycerol, as a trihydroxylic alcohol, can form
triesters with one, two or three different fatty
acids. In the first case a triester is formed with
three of the same acyl residues (e. g. tripalmitin;
P 3 ). The mixed esters involve two or three different
acyl residues, e. g., dipalmito-olein (P 2 O)
and palmito-oleo-linolein (POL). The rule of this
shorthand designation is that the acid with the
shorter chain or, in the case of an equal number
of carbons in the chain, the chain with fewer double
bonds, is mentioned first. The Z number gives
the possible different triacylglycerols which can
occur in a fat (oil), where n is the number of different
fatty acids identified in that fat (oil):
Z = n3 + n 2
(3.16)
2
For n = 3, the possible number of triglycerols (Z)
is 18. However, such a case where a fat (oil)
contains only three fatty acids is rarely found
in nature. One exception is Borneo tallow
(cf. 14.3.2.2.3), which contains essentially only
16:0, 18:0 and 18:1 (9) fatty acids.
Naturally, the Z value also takes into account
the number of possible positional isomers within
a molecule, for example, by the combination
of POS, PSO and SOP. When only positional
isomers are considered and the rest disregarded,
Z is reduced to Z ′ :
(3.17)
Thus, when n = 3, Z ′ = 10.
A chiral center exists in a triacylglycerol when the
acyl residues in positions 1 and 3 are different:
(3.18)
In addition enantiomers may be produced by
1-monoglycerides, all 1,2-diglycerides and
1,3-diglycerides containing unlike substituents.
In the stereospecific numbering of acyl residues
(prefix sn), the L-glycerol molecule is shown in
the Fischer projection with the secondary HOgroup
pointing to the left. The top carbon is then
denoted C-1. Actually, in a Fischer projection, the
horizontal bonds denote bonds in front and the
vertical bonds those behind the plane of the page:
(3.19)
For example, the nomenclature for a triacylglycerol
which contains P, S and O:
sn-POS = sn-1-Palmito-2-oleo-3-stearin .