2009_Book_FoodChemistry

5.2 Aroma Analysis 349
stances in the chromatogram are determined by
olfactometry (cf. 5.2.2.2).
More material is obtained by dynamic headspace
analysis or by solid phase microextraction
(SPME). In the dynamic procedure the
headspace volatiles are flushed out, adsorbed
and thus concentrated in a polymer, as outlined
in 5.2.1.2. However, it is difficult to obtain
a representative sample by this flushing procedure,
a sample that would match the original
headspace composition. A model system assay
(Fig. 5.7) might clarify the problems. Samples (e)
and (f) were obtained by adsorption on different
polymers. They are different from each other
and differ from sample (b), which was obtained
directly for headspace analysis. The results might
agree to a greater extent by varying the gas flushing
parameters (gas flow, time), but substantial
differences would still remain. A comparison of
samples (a) and (g) in Fig. 5.7 shows that the
results obtained by the distillation-extraction
procedure give a relatively good representation
of the composition of the starting solution, with
the exception of ethanol. However, the formation
of artifacts is critical (cf. 5.2.1.1).
SPME is based on the partitioning of compounds
between a sample and a coated fiber immersed in
it. The odorants are first adsorbed onto the fiber
(e. g. nonpolar polydimethylsilo-xane or polar
polyacrylate) immersed in a liquid food, a food
extract or in the headspace above a food sample
for a certain period of time. After adsorption
is completed, the compounds are thermally
desorbed into a GC injector block for further
analysis.
Particularly in food applications headspace SPME
is preferred to avoid possible contamination of
the headspace system by non-volatile food components.
Also SPME analysis is quite sensitive
to experimental conditions. In addition to the
stationary phase, sample, volume concentration
of odorants, sample matrix and uniformity as
well as temperature and time of the adsorption
and desorption processes influence the yield.
In quantitative SPME analysis these influences
are eliminated by the use of labelled internal
standards (cf. 5.2.6.1).
5.2.2 Sensory Relevance
Fig. 5.7. A comparison of some methods used for
aroma compound isolation (according to Jennings and
Filsoof, 1977).
a a Ethanol, b 2-pentanone, c heptane, d pentanol,
e hexanol, f hexyl formate, g 2-octanone, h d-limonene,
i heptyl acetate and k γ-heptalactone. b Headspace
analysis of aroma mixture a. c From aroma mixture
10 µl is dissolved in 100 ml water and the headspace
is analyzed. d As in c but the water is saturated with
80% NaCl. e As in c but purged with nitrogen and
trapped by Porapak Q. f As in c but purged with nitrogen
and trapped by Tenax GC. g As in e but distilled
and extracted (cf. Fig. 5.6)
In many earlier studies on the composition of aromas,
each volatile compound was regarded as an
aroma substance. Although lists with hundreds
of compounds were obtained for many foods, it
was still unclear which of the volatiles were really
significant as odorants and to what extent important
odorants occurring in very low concentrations
were detected.
The studies meanwhile concentrate on those compounds
which significantly contribute to aroma.
The positions of these compounds in the gas chromatogram
are detected with the help of dilution
analyses. Here, both of the following methods
based on the aroma value concept (cf. 5.1.4) find
application.