2009_Book_FoodChemistry

5.5 Natural and Synthetic Flavorings 395
5.5.1.5 Synthetic Natural Aroma Compounds
In spite of the fact that a great number of
food aroma compounds have been identified,
economic factors have resulted in only a limited
number of them being synthesized on a commercial
scale. Synthesis starts with a natural
compound available in large amounts at the right
cost, or with a basic chemical. Several examples
will be considered below.
A most important aroma compound worldwide,
vanillin, is obtained primarily by alkaline
hydrolysis of lignin (sulfite waste of the wood
pulp industry), which yields coniferyl alcohol. It
is converted to vanillin by oxidative cleavage:
Table 5.41. Site-specific 13 C isotopic analysis of vanillin
from different sources
Source R (%) a in R(%) a total
CHO Benzene ring OCH 3
Vanilla 1.074 1.113 1.061 1.101
Lignin 1.062 1.102 1.066 1.093
Guaiacol 1.067 1.102 1.026 1.089
a R( 13 C/ 12 C) was determined by site-specific natural
isotope fractionation NMR (SNF-NMR). Standard deviation:
0.003–0.007.
Thymol is obtained by alkylation and is then further
hydrogenated into racemic menthol:
(5.45)
A distinction can be made between natural and
synthetic vanillin by using quantitative 13 Canalysis
(cf. 18.4.3). The values in Table 5.41 show that
the 13 C distribution in the molecule is more meaningful
than the 13 C content of the entire molecule.
The most important source of citral, used in large
amounts in food processing, is the steamdistilled
oil of lemongrass (Cymbopogon flexuosus). Citral
actually consists of two geometrical isomers:
geranial (I) and neral (II). They are isolated from
the oil in the form of bisulfite adducts:
(5.47)
A more expensive processing step then follows,
in which the racemic form is separated and
1(–)-menthol is recovered. The d-optical isomer
substantially decreases the quality of the aroma
(cf. 5.3.2.4).
The purity requirement imposed on synthetic
aroma substances is very high. The purification
steps usually used are not only needed to meet
the stringent legal requirements (i. e. beyond
any doubt safe and harmless to health), but also
to remove undesirable contaminating aroma
compounds. For example, menthol has a phenolic
off-flavor note even in the presence of only 0.01%
thymol as an impurity. This is not surprising
since the odor threshold value of thymol is lower
than that of 1(−)-menthol by a factor of 450.
5.5.1.6 Synthetic Aroma Compounds
(5.46)
The aroma compound menthol is primarily synthesized
from petrochemically obtained m-cresol.
Some synthetic flavorings which do not occur in
food materials are compiled in Table 5.42. Except
for ethyl vanillin, they are of little importance in
the aromatization of foods.