Natural aroma chemicals for use in foods and beverages - Natural Food Additives, Ingredients and Flavourings - page 222

The above work has focused on the so-called 'bulk' 13 C analysis. By looking at
fragmentation pathways, it has been possible to identify the delta 13 C of individual
sites within a molecule; for example, the different 13 C 'signatures' within the
vanillin molecule have been elucidated (Dennis et al. 1998).
Other isotopes have been studied as markers. Hydrogen/deuterium (H/D)
ratios can be used, for example with the 'green' molecules trans -2-hexenol and
trans -2-hexenal (Hor et al. 2001), though they can be affected by other 'non-
biological' reactions such as 'acid' protons, including proton exchange at active
sites such as those alpha- to a carbonyl group. The 16 O/ 18 O ratio has also been
studied and found to be of interest.
Site-specifi c natural isotope fractionation nuclear magnetic
resonance (SNIF-NMR)
This wonderfully appropriately named technique, sites-specifi c natural isotope
fractionation nuclear magnetic resonance, makes use of the tendency towards
differential use of isotopes in biological systems (Mosandl 2007). Theoretically a
number of nuclei can be investigated, but a combination of natural abundance and
nuclear properties make the δ H/D values the most practical. This method can be
used to make detailed investigation of δ H/D values at many (theoretically all)
sites even in a complex molecule; but several factors combine to make this method
more of a research tool than a day-to-day quality instrument:
1 A pure sample is generally needed, precluding the analysis of fl avours and
foodstuffs.
2 The technique requires specialised understanding to both carry out and interpret
the results.
3 The instrumentation (and running costs) is extremely expensive; at the
time of writing (spring 2011) the world's largest NMR spectrometer is
the CNRS instrument at Lyon in France costing in the region of EUR
11 million.
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10.7.2 False fl ags: chirality and isomer ratios
This is one of the most misunderstood, yet potentially most valuable, areas
concerning natural aroma chemicals. The fi rst thing we must eliminate is the idea
that geometric isomers, cis-trans (or E-Z) ratios, provide any information about
the naturalness, or otherwise, of a material. The different molecular geometry of
such isomers means their intermolecular forces are signifi cantly different; in lay
terms, they're different shapes and stick together differently, Hence they have
different boiling points, and since the vast majority of aroma chemicals are
isolated and/or purifi ed by distillation, the cis and trans isomers can be separated,
at least to some extent. Hence such ratios tell us about how the distillation has
been carried out, not whether a material is natural or not. Examples of natural
aroma chemicals with geometric isomers include citral [10.17], 5-methyl-2-
phenyl-2-hexenal [10.28] and trans -2- trans -4-decadienal [10.22]; in each case