2009_Book_FoodChemistry

5.5 Natural and Synthetic Flavorings 397
is rigorously guarded after the “know-how”
is acquired.
5.5.3 Aromas from Precursors
The aroma of food that has to be heated, in which
the impact aroma compounds are generated
by the Maillard reaction, can be improved by
increasing the levels of precursors involved in the
reaction. This is a trend in food aromatization.
Some of the precursors are added directly,
while some precursors are generated within the
food by the preliminary release of the reaction
components required for the Maillard reaction
(cf. 4.2.4.4). This is achieved by adding protein
and polysaccharide hydrolases to food.
In an apolar solvent, e. g., a triacylglycerol, (E)-2-
hexenal decreases much more rapidly than in
a polar medium in which its stability exceeds
that of hexanal (Table 5.43). It oxidizes mainly
to (E)-2-hexenoic acid, with butyric acid, valeric
acid and 2-penten-1-ol being formed as well. The
reaction pathway to the C 6− and C 5− acids is
shown in Formula 5.48.
5.5.4 Stability of Aromas
Aroma substances can undergo changes during
the storage of food. Aldehydes and thiols are especially
sensitive because they are easily oxidized
to acids and disulfides respectively. Moreover,
unsaturated aldehydes are degraded by reactions
which will be discussed using (E)-2-hexenal and
citral as examples. These two aldehydes are important
aromatization agents for leaf green and
citrus notes. (Z)-3-Hexenal, an important contributor
to the aroma of freshly pressed juices,
e. g., orange and grapefruit (cf. 18.1.2.6.3), is considerably
more instable than (E)-2-hexenal (Table
5.43) and, consequently, hardly finds application
in aromatization.
Table 5.43. Half-life periods in the degradation of C 6
and C 7 aldehydes in different solvents at 38 ◦ C a
Aldehyde Water/ Buffer b / Triacetin
Ethanol Ethanol
(8 + 2, v/v) (8 + 2, v/v)
n-Hexanal 100 91 86
(E)-2-Hexenal 256 183 71
(Z)-3-Hexenal 42 36 26
n-Heptanal 79 76 73
(E)-2-Heptenal 175 137 57
(Z)-4-Heptenal 200 174 64
a The half-life period is given in hours.
b Na-citrate buffer of pH 3.5 (0.2mol/l).
(5.48)
At the acidic pH values found in fruit, autoxidation
decreases, (E)-2-hexenal preferentially adds
water with the formation of 3-hydroxy-hexanal.
In addition, the double bond is iso-merized
with the formation of low concentrations of
(Z)-3-hexenal. As a result of its low threshold
value, (Z)-3-hexenal first influences the aroma
to a much greater extent than 3-hydroxyhexanal
which has a very high threshold (cf. 18.1.2.6.3).
Citral is also instable in an acidic medium, e. g.,
lemon juice. At citral equilibrium, which consists
of the stereoisomers geranial and neral in the ratio
of 65:35, neral reacts as shown in Formula: 5.49.
It cyclizes to give the labile p-menth-l-en-3,8-diol
which easily eliminates water, forming various
p-menthadien-8-ols. This is followed by aromatization
with the formation of p-cymene, p-cymen-8-ol,
and α,p-dimethylstyrene. p-Methylacetophenone
is formed from the last mentioned
compound by oxidative cleavage of the
8 -double bond. Together with p-cresol,
p-methylacetophenone contributes apprecia-