Food Chemistry

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 hydropho-
bic interaction along the acyl residues. Corres-
pondingly, the energy and therefore the tempe-
rature 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 inter-
fered by their terminal methyl groups.
The consequence of less symmetry within
the crystal is that the melting points of even-
numbered acids (Cn) exceed the melting points
of the next higher odd-numbered (Cn+ 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 differ-
ence, 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 dou-
ble bond positions within the molecules are
fairly comparable. Thus, when a cis double
bond is at the end of the carbon chain, the

Fig. 3.2.Arrangement of caproic acid molecules in
crystal (according toMeadet al., 1965). Results of
a X-ray diffraction analysis reveal a strong diffraction
in the plane of carboxyl groups (c) and a weak diffrac-
tion at the methyl terminals (m): d: identity period

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 sur-

passes 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 par-

allels the geometry of the acid molecule. Any de-

viation 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)

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