2009_Book_FoodChemistry

3.2 Fatty Acids 167
3.2.2.4 Solubility
Long-chain fatty acids are practically insoluble in
water; instead, they form a floating film on the
water surface. The polar carboxyl groups in this
film are oriented toward the water, while the hydrophobic
tails protrude into the gaseous phase.
The solubility of the acids increases with decreasing
carbon number; butyric acid is completely
soluble in water.
Ethyl ether is the best solvent for stearic acid and
other saturated long-chain fatty acids since it is
sufficiently polar to attract the carboxyl groups.
A truly nonpolar solvent, such as petroleum benzine,
is not suitable for free fatty acids.
The solubility of fatty acids increases with an increase
in the number of cis double bonds. This is
illustrated in Fig. 3.3 with acetone as a solvent.
The observed differences in solubility can be utilized
for separation of saturated from unsaturated
fatty acids. The mixture of acids is dissolved at
room temperature and cooled stepwise to −80 ◦ C.
However, the separation efficiency of such a fractional
crystallization is limited since, for example,
stearic acid is substantially more soluble in acetone
containing oleic acid than in pure acetone.
This mutual effect on solubility has not been consideredinFig.3.3.
Conjugated fatty acids absorb light at various
wavelenths depending on the length of conjugation
and configuration of the double bond system.
Figure 3.4 illustrates such behavior for several
fatty acids. See 3.2.3.2.2 for the conversion of
an isolene-type fatty acid into a conjugated fatty
acid.
3.2.3 Chemical Properties
3.2.3.1 Methylation of Carboxyl Groups
The carboxyl group of a fatty acid must be depolarized
by methylation in order to facilitate gas
chromatographic separation or separation by fractional
distillation. Reaction with diazomethane is
preferred for analytical purposes. Diazomethane
is formed by alkaline hydrolysis of N-nitroso-Nmethyl-p-toluene
sulfonamide.
The gaseous CH 2 N 2 released by hydrolysis is
swept by a stream of nitrogen into a receiver containing
the fatty acid solution in ether-methanol
(9:1 v/v). The reaction:
R − COOH + CH 2 N 2 −→ R − COOCH 3 + N 2
(3.9)
3.2.2.5 UV-Absorption
All unsaturated fatty acids which contain an
isolated cis double bond absorb UV light at
a wavelength close to 190 nm. Thus, the acids
can not be distinguished spectrophotometrically.
Fig. 3.3. Fatty acid solubility in acetone (according to
Mead et al., 1965)
Fig. 3.4. Electron excitation spectra of conjugated fatty
acids (according to Pardun, 1976). 1 9,11-isolinoleic
acid, 2 α-eleostearic acid, 3 parinaric acid