Natural aroma chemicals for use in foods and beverages - Natural Food Additives, Ingredients and Flavourings - page 224

[10.54] [10.55]

It has been known for many years that the alpha-ionone found in raspberry extract
does approach homochirality, with an enantiomeric excess of at least 99.9%
R -isomer [10.54] (by headspace analysis). However, in a simplifi cation which
rivals Orwell's 'two legs good, four legs bad', this has been converted into '100%
R -isomer = natural, <100% R -isomer = synthetic'. This wretched dictum is even
being used to promote chiral gas chromatography (GC) systems, to enable people
to 'prove' whether their raspberry fl avours are natural or not. Yet it is not based on
scientifi c fact. Both isomers are found in nature; work by Haarman and Reimer
(Werkhoff et al. 1991) on a variety of foodstuffs found that while the R -isomer
predominates, the S -isomer [10.55] is found at the level of 5% in Darjeeling tea (by
solvent extraction), 3% in vanilla pods and 5% in carrots (both by headspace
analysis). Boronia absolute was found to have 9% S -isomer. Even in raspberry
there was some S -isomer, if only 0.1%! In addition, work by Firmenich and the
University of Hamburg found that the S -isomer, not the R -isomer, was the main
isomer in black tea (Konig et al. 1989). This is a particular problem, as the ionones
can be obtained by air oxidation of carotenes; the elevated temperature and
purifi cation by distillation leads to loss of chirality as the chiral centre has an active
proton alpha - to two double bonds, one of which is also conjugated to a carbonyl
group. This loss of chirality is shown below in this case via a carbanion mechanism;
as a result, the alpha -ionone thus obtained has little enantiomeric excess, typically
R:S = 60:40.
We have seen above that both enantiomers are found in nature. The techniques
used fulfi l all the criteria of the European Union's Regulation (EC) No. 1334/2008
and the US 21 CFR 101.22, yet due to the 'two legs good, four legs bad', the
present paradigm of naturalness in the ionones, there is resistance to use what is
potentially a very important natural aroma chemical. This absurdity has even
spilled over into issues with beta -ionone [10.56], which itself is not and cannot be
chiral! Distillation of beta -ionone leads to formation of alpha -ionone, an
energetically unfavourable transformation as this takes a double bond out of
conjugation, but since the alpha -form has the lower boiling point, the equilibrium
is always shifted in its favour as the distillation proceeds. Hence, distilled beta -
ionone always contain traces of alpha-ionone, and since beta -ionone is achiral,
the resulting alpha -ionone has zero enantiomeric excess leading, once again, into
accusations of 'un-naturalness' when the material is analysed on a chiral GC
column!

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