Food Chemistry

348 5 Aroma Compounds

Solid foods are first extracted, the addition of wa-
ter may be required to increase the yield of aroma
substances.
An extraction combined with distillation can be
achieved using an apparatus designed byLikens–
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.

Fig. 5.6.Apparatus according toLikensandNicker-
sonused for simultaneous extraction and distillation of
volatile compounds.
1 Flask with heating bath containing the aqueous sam-
ple, 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 simultan-
eous distillation and extraction(II)

Odorant I (μg/1) II

Benzaldehyde 202 5260
Linalool 1. 1 188

Table 5.9.Relative retention time (trel)ofsomecom-

pounds separated by gas chromatography using Pora-

pak Q as stationary phase (Porapak: styrene divinylben-

zene polymer; T: 55◦C)

Compound trel Compound trel

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 GasExtraction..........................................

Volatile compounds can beisolated from a solid

or liquid food sample by purging the sample with

an inert gas (e. g., N 2 ,CO 2 , He) and adsorb-

ing the volatiles on a porous, granulated polymer

(Tenax GC, Porapak Q, Chromosorb 105), fol-

lowed by recovery of the compounds. Water is

retarded to only a negligible extent by these poly-

mers (Table 5.9). The desorption of volatiles is

usually achieved stepwise in a temperature gradi-

ent. At low temperatures, the traces of water are

removed by elution, while at elevated tempera-

tures, 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 HeadspaceAnalysis

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 estab-

lish 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 separa-

tion column (static headspace analysis). Since the

water content and an excessively large volume of

the sample substantially reduce the separation ef-

ficiency 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-