5.2 Aroma Analysis 349stances in the chromatogram are determined by
olfactometry (cf. 5.2.2.2).
More material is obtained by dynamic head-
space analysis or by solid phase microextrac-
tion (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 proce-
dure, 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
Fig. 5.7.A comparison of some methods used for
aroma compound isolation (according toJenningsand
Filsoof, 1977).
aaEthanol,b2-pentanone,cheptane,dpentanol,
ehexanol,fhexyl formate,g2-octanone,hd-limonene,
iheptyl acetate andkγ-heptalactone.bHeadspace
analysis of aroma mixturea.cFrom aroma mixture
10 μl is dissolved in 100 ml water and the headspace
is analyzed.dAs incbut the water is saturated with
80% NaCl.eAs incbut purged with nitrogen and
trapped by Porapak Q.fAs incbut purged with ni-
trogen and trapped by Tenax GC.gAs inebut distilled
and extracted (cf. Fig. 5.6)
directly for headspace analysis. The results might
agree to a greater extent by varying the gas flush-
ing 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 com-
ponents. 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 SensoryRelevance
In many earlier studies on the composition of aro-
mas, 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 re-
ally significant as odorants and to what extent im-
portant odorants occurring in very low concentra-
tions were detected.
The studies meanwhile concentrate on those com-
pounds which significantly contribute to aroma.
The positions of these compounds in the gas chro-
matogram 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.