Food Chemistry

3.2 Fatty Acids 165

3.2.2 PhysicalProperties

3.2.2.1 Carboxyl Group.........................................

Carboxylic acids have a great tendency to form
dimers which are stabilized by hydrogen bonds:

(3.4)

The binding energy of the acid dimer dis-
solved in hexane is 38 kJ/mole. Also, the fatty
acid molecules are arranged as dimers in the
crystalline lattice (cf. Fig. 3.2).
The acidic character of the carboxyl group is

based on proton dissociation and on the formation
of the resonance-stabilized carboxylate anion:

(3.5)

The pKsvalues for the C 2 –C 9 short-chain acid
homologues range from 04.75–4.95. The pKs
of 7.9 for linoleic acid deviates considerably
from this range. This unexpected and anomalous
behavior, which has not yet been clarified, is
clearly illustrated in the titration curves for
propionic, caprylic and linoleic acids recorded
under identical conditions (Fig. 3.1).

3.2.2.2 Crystalline Structure, Melting Points.......................

Melting properties of fats depend on the arrange-
ment of the acyl residues in the crystal lattice in
addition to other factors attributed solely to the
structure of triglycerides.
Calculations of the energy content of the carbon
chain conformation have revealed that at room
temperature 75% of the C–C bonds of a saturated
fatty acid are present in a fully staggeredzigzag
or “trans” conformation and only 25% in the en-
ergetically slightly less favorable skew conforma-
tion.
The unsaturated fatty acids, because of their dou-
ble bonds, are not free to rotate and hence have
rigid kinks along the carbon chain skeleton. How-
ever, a molecule is less bent by a trans than by

Fig. 3.1.Fatty acid titration curves (according toBild

et al., 1977). Aqueous solutions (0.1mol/l) of Na-salts

of propionic (1), caprylic (2) and linoleic acids (3) were

titrated with 0.1mol/lHCl

a cis double bond. Thus, this cis-configuration in

oleic acid causes a bending of about 40◦:

(3.6)

The corresponding elaidic acid, with a trans-

configuration, has a slightly shortened C-chain,

but is still similar to the linear form of stearic

acid:

(3.7)

The extent of molecular crumpling is also in-

creased by an increase in the number of cis double

bonds. Thus, the four cis double bonds in arachi-

donic acid increase the deviation from a straight

line to 165◦:

(3.8)

When fatty acids crystallize, the saturated acids

are oriented as depicted by the simplified pattern