Headspace GC Olfactometry

In the recovery of samples for AEDA, highly volatile odorants are lost or are covered by the solvent peak in gas chromatography, e. g., methanethiol and acetaldehyde. For this reason, in addition to AEDA, a sample is drawn from the gas space above the food, injected into the gas chromatograph, transported by the carrier gas stream into a cold trap and concentrated there, as shown in Fig. 5.9. After quick evaporation, the sample is flushed into a capillary column by the carrier gas and chromatographed. At the end of the capUlary, the experimenter sniffs the carrier gas stream and determines the positions of the chromatogram at which the odorants appear. The gas chromatogram is simultaneously monitored by a detector. 

To carry out a dilution analysis, the volume of the headspace sample is reduced stepwise until no odorant is detectable by gas chromatogra-phy/olfactometry. In our example with French files (Fig. 5.10), e.g., the odors of methanethiol, methylpropanal and dimethyltrisulfide were detectable in the sixth dilution, but only methanethiol was detectable in the seventh. The eighth dilution was odorless. Further experiments showed that methanethiol does in fact belong to the key odorants of French fries. 

In GC/olfactometry, odor thresholds are considerably lower than in solution because the aroma substances are subjectied to sensory assessment in a completely vaporized state. The examples given in Table 5.10 show how great the differences can be when compared to solutions of the aroma substances in water. 

When an aroma concentrate contains phenols, organic acids or bases, preliminary separation of these compounds from neutral volatiles by extraction with alkah or acids is advantageous. 

The acidic, basic and neutral fractions are individually analyzed. The neutral fraction by itself consists of so many compounds that in most cases not even a gas chromatographic column with the highest resolving power is able to separate them into individual peaks. Thus, separation of the neutral fraction is advisable and is usually achieved by liquid chromatography, or preparative gas or high performance liquid chromatography. A preliminary separation of aroma extracts is achieved 

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The potent volatiles from freshly prepared French fries were evaluated by aroma extraction dilution and headspace GC-olfactometry (Table 12.2). The most potent odorants included methional, 2-ethyl-3,6-dimethylpyrazine, 2,3-diethyl-5-methylpyrazine, trans,trans-2,4-dec (iiem, dimethyltrisulfide, and... 

The concentration of the very aroma active (F,Z)-2,6-nonadienol (cf. 10.3.6) in the head space is below the detection limit. However, this odorant can be detected by headspace GC-olfactometry (cf. 5.2.2.2). The results in Table 5.36 show that this alcohol as well as (Z)-3-hexenol no longer contribute to the aroma in the 20% fat emulsion. In the emulsion with 1% of fat, (F,Z)-2,6-nonadienol, allyl isothiocyanate and allyl thiocyanate predominate and produce the green, mustard-like aroma (Table 5.36). 

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