Aroma enantioselective analysis

This unit describes those methods that can differentiate between enantiomers found in foods that contribute to their taste and aroma. These compounds are volatile odorants that are most easily analyzed using enantioselective high resolution-gas chromatography (HRGC). Other methods exist for the separation and analysis of chiral compounds, which include optical methods, liquid and planar chromatography, and electrophoresis, but for food volatiles, gas chromatography has evolved to the point where it is now the cornerstone for the most comprehensive analysis of volatile compounds. 

Fig. 5.13. Enantioselective gas chromatographic analysis of trans-a-ionone in aroma extracts of different raspberry fruit juice concentrates (according to Werkhoff et al., 1990) a and b samples with nature identical aroma, c natural aroma...

The method frequently applied to determine ee values is the enantioselective gas chromatographic analysis of the aroma substance on a chiral phase, e. g., peralkylated cyclodextrins. This method was used, e. g., to test raspberry fruit juice concentrates for unauthorized aromatization with trans-a-ionone. The gas chromatograms of trans-a-ionone from two different samples are shown in Fig. 5.13. The low excesses of the R-enantiomer of ee = 8% (concentrate A) and ee = 24% (B) can probably be put down to the addition of synthetic trans-a-ionone racemate to the fruit juice concentrate because in the natural aroma (C), the ee value is 92.4%. 

Chiral compounds are frequently foimd among the flavor volatiles of fruits and, like many naturally occurring chiral compounds, one enantiomer usually exists with a greater preponderance when compared with its antipode. Chiral odor compounds may show qualitative and quantitative differences in their odor properties (7). For example, (/ )-(+)-limonene has an orange-like aroma while (5)-(-)-limonene is turpentine-like (5)-(+)-carvone is characteristic of caraway while its enantiomer has a spearmint odor (2). However, other chiral compounds, such as y 6-lactones, show very little enantioselectivity of odor perception (7). The occurrence of chiral flavor compounds in enantiomeric excess provides the analyst with a means of authenticating natural flavorings, essential oils, and other plant extracts. The advent of cyclodextrin-based gas chromatography stationary phases has resulted in considerable activity in the analysis of chiral compounds in flavor extracts of fruits, spices and other plants (i-7). 

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