Food Chemistry

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 hy-
drophobic tails protrude into the gaseous phase.
The solubility of the acids increases with decreas-
ing 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 ben-
zine, is not suitable for free fatty acids.
The solubility of fatty acids increases with an in-
crease 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 uti-
lized 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 frac-
tional crystallization is limited since, for example,
stearic acid is substantially more soluble in ace-
tone containing oleic acid than in pure acetone.
This mutual effect on solubility has not been con-
sideredinFig.3.3.

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 inacetone (according to
Meadet al., 1965)

Conjugated fatty acids absorb light at various

wavelenths depending on the length of conjuga-

tion 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 ChemicalProperties

3.2.3.1 Methylation of Carboxyl Groups.

The carboxyl group of a fatty acid must be de-

polarized by methylation in order to facilitate gas

chromatographic separation or separation by frac-

tional distillation. Reaction with diazomethane is

preferred for analytical purposes. Diazomethane

is formed by alkaline hydrolysis of N-nitroso-N-

methyl-p-toluene sulfonamide.

The gaseous CH 2 N 2 released by hydrolysis is

swept by a stream of nitrogen into a receiver con-

taining 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)

Fig. 3.4.Electron excitation spectra of conjugated fatty

acids (according toPardun, 1976). 1 9,11-isolinoleic

acid, 2α-eleostearic acid, 3 parinaric acid