2009_Book_FoodChemistry

166 3 Lipids
in Fig. 3.2. The dimer molecular arrangement
is thereby retained. The principal reflections
of the X-ray beam are from the planes (c) of
high electron density in which the carboxyl
groups are situated. The length of the fatty
acid molecule can be determined from the
“main reflection” site intervals (distance d in
Fig. 3.2). For stearic acid (18:0), this distance is
2.45 nm.
The crystalline lattice is stabilized by hydrophobic
interaction along the acyl residues. Correspondingly,
the energy and therefore the temperature
required to melt the crystal increase with
an increased number of carbons in the chain.
Odd-numbered as well as unsaturated fatty acids
can not be uniformly packed into a crystalline
lattice as can the saturated and even-numbered
acids. The odd-numbered acids are slightly interfered
by their terminal methyl groups.
The consequence of less symmetry within
the crystal is that the melting points of evennumbered
acids (C n ) exceed the melting points
of the next higher odd-numbered (C n+1 ) fatty
acids (cf. Table 3.6).
The molecular arrangement in the crystalline
lattice of unsaturated fatty acids is not strongly
influenced by trans double bonds, but is strongly
influenced by cis double bonds. This difference,
due to steric interference as mentioned
above, is reflected in a decrease in melting
points in the fatty acid series 18:0, 18:1 (tr9)
and 18:1 (9). However, this ranking should
be considered as reliable only when the double
bond positions within the molecules are
fairly comparable. Thus, when a cis double
bond is at the end of the carbon chain, the
deviation from the form of a straight extended
acid is not as large as in oleic acid. Hence,
the melting point of such an acid is higher.
The melting point of cis-2-octadecenoic acid
is in agreement with this rule; it even surpasses
the 9-trans isomer of the same acid
(Table 3.11).
The melting point decreases with an increasing
number of isolated cis-double bonds (Table 3.11).
This behavior can be explained by the changes
in the geometry of the molecules, as can be seen
when comparing the geometric structures of oleic
and arachidonic acid.
3.2.2.3 Urea Adducts
When urea crystallizes, channels with a diameter
of 0.8–1.2 nm are formed within its crystals and
can accomodate long-chain hydrocarbons. The
stability of such urea adducts of fatty acids parallels
the geometry of the acid molecule. Any deviation
from a straight-chain arrangement brings
about weakening of the adduct. A tendency to
form inclusion compounds decreases in the series
18:0 > 18:1 (9) > 18:2 (9, 12).
A substitution on the acyl chain prevents adduct
formation. Thus, it is possible to separate
branched or oxidized fatty acids or their methyl
esters from the corresponding straightchain
compounds on the basis of the formation of urea
adducts. This principle is used as a method for
preparative-scale enrichment and separation of
branched or oxidized acids from a mixture of
fatty acids.
Table 3.11. The effect of number, configuration and
double bond position on melting points of fatty acids
Melting
Fatty acid point ( ◦ C)
Fig. 3.2. Arrangement of caproic acid molecules in
crystal (according to Mead et al., 1965). Results of
a X-ray diffraction analysis reveal a strong diffraction
in the plane of carboxyl groups (c) and a weak diffraction
at the methyl terminals (m): d: identity period
18:0 Stearic acid 69
18:1 (tr9) Elaidic acid 46
18:1 (2) cis-2-Octadecenoic acid 51
18:1 (9) Oleic acid 13.4
18:2 (9, 12) Linoleic acid −5
18:2 (tr9, tr12) Linolelaidic acid 28
18:3 (9, 12, 15) α-Linolenic acid −11
20:0 Arachidic acid 75.4
20:4 (5,8,11,14) Arachidonic acid −49.5