2009_Book_FoodChemistry

3.8 Unsaponifiable Constituents 227
the structures and properties of which have been
thoroughly elucidated in the past decade or two,
thus reflecting the advance in the analytical chemistry
of fats and oils.
Studies aimed at elucidating the constituents, and
their structures, of unsaponifiable matter are motivated
by a desire to find compounds which can
serve as a reliable indicator for the identity of a fat
or an oil.
The steroid skeleton is made up of four condensed
rings;A,B,CandD.Thefirstthreeareinthe
chair conformation, whereas ring D is usually planar.
While rings B and C, and C and D are fused in
a trans-conformation, rings A and B can be fused
in a trans- or in a cis-conformation.
3.8.1 Hydrocarbons
All edible oils contain hydrocarbons with an even
or an odd C-number (C 11 to C 35 ). Olive, rice
and fish oils are particularly rich in this class of
compounds. The main hydrocarbon constituent
of olive oil (1–7 g/kg) and rice oil (3.3g/kg) is
a linear triterpene (C 30 ), squalene:
(3.100)
This compound is used as an analytical indicator
for olive oil (cf. Table 14.24).
Squalene is present in a substantially higher
concentration in fish liver oil. For example, shark
liver oil has up to 30% squalene, and 7% pristane
(2,6,10,14-tetramethylpentadecane) and some
phytane (3,7,11,15-tetramethylhexadecane).
3.8.2 Steroids
The unsaponifiable part of edible fats contains
a series of cyclic triterpenes which have structures
related to that of steroids. Quantitatively, the
3β-hydroxysteroids are the sterols which are to
the fore. Especially diverse is the sterol spectrum
of plant fats which contain not only desmethyl,
but also 4-methyl and 4,4-dimethyl sterols.
3.8.2.1 Structure, Nomenclature
(3.101)
Conformational isomers introduced by fusing
rings A and B in cholest-5-ene-3-β-ol (cholesterol;
cf. Formula 3.101) are not possible since
the C-5 position has a double bond.
By convention, the steric arrangement of substituents
and H-atoms is related to the angular
methyl group attached at C-10. When the plane
containing the four rings is assumed to be the
plane of this page, the substituent at C-10, by definition,
is above the plane; all substituents below
the plane are denoted by dashed or dotted lines.
They are said to be α-oriented and have a transconformation.
Substituents above the plane are
termed β-oriented and are shown by solid line
bonds and, in relation to the angular C-10 methyl
group, are of cis-conformation.
In cholesterol (cf. Formula 3.101) the HO-group,
the angular methyl group at C-13, the side chain
on C-17 and the H-atom on C-8 are β-oriented
(cis), whereas the H-atoms at C-9, C-14 and C-17
are α-oriented (trans). These steroids that are not
methylated at position C-4 are also denoted as
desmethyl steroids.
3.8.2.2 Steroids of Animal Food
3.8.2.2.1 Cholesterol
Cholesterol (cf. Formula 3.101) is obtained
biosynthetically from squalene (see a textbook of
biochemistry). It is the main steroid of mammals
and occurs in lipids in free form or esterified with
saturated and unsaturated fatty acids. The content
of cholesterol in some foods is illustrated by the
data in Table 3.49.
Autoxidation of cholesterol, which is accelerated
manyfold by 18:2 and 18:3 fatty acid