Food Chemistry

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 com-
mercial 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 world-
wide, 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:

(5.45)

A distinction can be made between natural and
synthetic vanillin by using quantitative^13 Canaly-
sis (cf. 18.4.3). The values in Table 5.41 show that
the^13 C distribution in the molecule is more mean-
ingful 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). Cit-
ral actually consists of two geometrical isomers:
geranial (I) and neral (II). They are isolated from
the oil in the form of bisulfite adducts:

(5.46)

The aroma compound menthol is primarily syn-
thesized from petrochemically obtained m-cresol.

Table 5.41.Site-specific^13 C isotopic analysis of van-

illin from different sources

Source R (%)ain R(%)atotal

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

aR( (^13) C/ (^12) C) was determined by site-specific natural
isotope fractionation NMR (SNF-NMR). Standard de-
viation: 0.003–0.007.
Thymol is obtained by alkylation and is then fur-
ther hydrogenated into racemic menthol:
(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

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.