2009_Book_FoodChemistry

168 3 Lipids
proceeds under mild conditions without formation
of by-products. Further possibilities for
methylation include: esterification in the presence
of excess methanol and a Lewis acid (BF 3 )as
a catalyst; or the reaction of a fatty acid silver
salt with methyl iodide:
R − COOAg + CH 3 l −→ R − COOCH 3 + Agl,
(3.10)
3.2.3.2 Reactions of Unsaturated Fatty Acids
A number of reactions which are known for
olefinic hydrocarbons play an important role in
the analysis and processing of lipids containing
unsaturated fatty acids.
3.2.3.2.1 Halogen Addition Reactions
The number of double bonds present in an oil or
fat can be determined through their iodine number
(cf. 14.5.2.1). The fat or oil is treated with
a halogen reagent which reacts only with the
double bonds. Substitution reactions generating
hydrogen halides must be avoided. IBr in an inert
solvent, such as glacial acetic acid, is a suitable
reagent:
During this reaction, an equilibrium is established
between the isolene and the conjugated
forms of the fatty acid, the equilibrium state
being dependent on the reaction conditions.
This isomerization is used analytically since
it provides a way to simultaneously determine
linoleic, linolenic and arachidonic acids in
a fatty acid mixture. The corresponding conjugated
diene, triene and tetraene systems of
these fatty acids have a maximum absorbance
at distinct wavelengths (cf. Fig. 3.4). The
assay conditions can be selected to isomerize
only the naturally occurring cis double
bonds and to ignore the trans fatty acids
formed, for instance, during oil hardening
(cf. 14.4.2).
3.2.3.2.3 Formation of a π-Complex
with Ag + Ions
Unsaturated fatty acids or their triacylglycerols,
as well as unsaturated aldehydes obtained
through autoxidation of lipids (cf. 3.7.2.1.5), can
be separated by “argentation chromatography”.
The separation is based on the number, position
and configuration of the double bonds present.
The separation mechanism involves interaction
of the π-electrons of the double bond with Ag +
ions, forming a reversible π-complex of variable
stability:
(3.11)
The number of double bonds is calculated
by titrating the unreacted IBr reagent with
thiosulfate.
3.2.3.2.2 Transformation of Isolene-Type Fatty
Acids to Conjugated Fatty Acids
Allyl systems are labile and are readily converted
to a conjugated double bond system in the presence
of a base (KOH or K-tertbutylate):
(3.12)
(3.13)
The complex stability increases with increasing
number of double bonds. This means a fatty
acid with two cis double bonds will not migrate
as far as a fatty acid with one double bond on
a thin-layer plate impregnated with a silver
salt. The R f values increase for the series 18:2
(9, 12) < 18 :1 (9) < 18:0. Furthermore, fatty
acids with isolated double bonds form a stronger
Ag + complex than those with conjugated bonds.
Also, the complex is stronger with a cis- than
with a trans-configuration. The complex is also
more stable, the further the double bond is
from the end of the chain. Finally, a separation
of nonconjugated from conjugated fatty acids
and of isomers that differ only in their double
bond configuration is possible by argentation
chromatography.