2009_Book_FoodChemistry

348 5 Aroma Compounds
Solid foods are first extracted, the addition of water
may be required to increase the yield of aroma
substances.
An extraction combined with distillation can be
achieved using an apparatus designed by Likens–
Nickerson (Fig. 5.6).
In this process, low-boiling solvents are usually
used to make subsequent concentration of
the aroma substances easier. Therefore, this
process is carried out at normal pressure or
slightly reduced pressure. The resulting thermal
treatment of the food can lead to reactions
(examples in Table 5.6) that change the aroma
composition. The example in Table 5.8 shows
the extent to which some aroma substances are
released from glycosides during simultaneous
distillation/extraction.
Table 5.9. Relative retention time (t rel )ofsomecompounds
separated by gas chromatography using Porapak
Q as stationary phase (Porapak: styrene divinylbenzene
polymer; T: 55 ◦ C)
Compound t rel Compound t rel
Water 1.0 Methylthiol 2.6
Methanol 2.3 Ethylthiol 20.2
Ethanol 8.1 Dimethylsulfide 19.8
Acetaldehyde 2.5 Formic acid
Propanal 15.8 ethyl ester 6.0
5.2.1.2 Gas Extraction
Volatile compounds can be isolated from a solid
or liquid food sample by purging the sample with
an inert gas (e. g., N 2 ,CO 2 , He) and adsorbing
the volatiles on a porous, granulated polymer
(Tenax GC, Porapak Q, Chromosorb 105), followed
by recovery of the compounds. Water is
retarded to only a negligible extent by these polymers
(Table 5.9). The desorption of volatiles is
usually achieved stepwise in a temperature gradient.
At low temperatures, the traces of water are
removed by elution, while at elevated temperatures,
the volatiles are released and flushed out by
a carrier gas into a cold trap, usually connected to
a gas chromatograph.
5.2.1.3 Headspace Analysis
Fig. 5.6. Apparatus according to Likens and Nickerson
used for simultaneous extraction and distillation of
volatile compounds.
1 Flask with heating bath containing the aqueous sample,
2 flask with heating bath containing the solvent
(e. g. pentane), 3 cooler, 4 condensate separator: extract
is the upper and water the lower phase
Table 5.8. Isolation of odorants from cherry juice –
Comparison of distillation in vacuo (I) with simultaneous
distillation and extraction (II)
Odorant I (µg/1) II
Benzaldehyde 202 5260
Linalool 1.1 188
The headspace analysis procedure is simple: the
food is sealed in a container, then brought to the
desired temperature and left for a while to establish
an equilibrium between volatiles bound to the
food matrix and those present in the vapor phase.
A given volume of the headspace is withdrawn
with a gas syringe and then injected into a gas
chromatograph equipped with a suitable separation
column (static headspace analysis). Since the
water content and an excessively large volume of
the sample substantially reduce the separation efficiency
of gas chromatography, only the major
volatile compounds are indicated by the detector.
The static headspace analysis makes an important
contribution when the positions of the aroma sub-