Food Chemistry

5.5 Natural and Synthetic Flavorings 397

is rigorously guarded after the “know-how”
is acquired.

5.5.3 AromasfromPrecursors..................................

The aroma of food that has to be heated, in which
the impact aroma compounds are generated
by theMaillardreaction, 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 theMaillardreaction
(cf. 4.2.4.4). This is achieved by adding protein
and polysaccharide hydrolases to food.

5.5.4 Stability of Aromas

Aroma substances can undergo changes during
the storage of food. Aldehydes and thiols are es-
pecially sensitive because they are easily oxidized
to acids and disulfides respectively. Moreover,
unsaturated aldehydes aredegraded by reactions
which will be discussed using (E)-2-hexenal and
citral as examples. These two aldehydes are im-
portant aromatization agents for leaf green and
citrus notes. (Z)-3-Hexenal, an important con-
tributor to the aroma of freshly pressed juices,
e. g., orange and grapefruit (cf. 18.1.2.6.3), is con-
siderably more instable than (E)-2-hexenal (Ta-
ble 5.43) and, consequently, hardly finds applica-
tion in aromatization.

Table 5.43.Half-life periods in the degradation of C 6
and C 7 aldehydes in different solvents at 38◦Ca

Aldehyde Water/ Bufferb/ 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
aThe half-life period is given in hours.
bNa-citrate buffer of pH 3.5(0.2mol/l).

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.

(5.48)

At the acidic pH values found in fruit, autoxida-

tion 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 aroma-

tization with the formation of p-cymene, p-cy-

men-8-ol, andα,p-dimethylstyrene. p-Methyl-

acetophenone is formed from the last men-

tioned compound by oxidative cleavage of the



8

-double bond. Together with p-cresol,

p-methylacetophenone contributes apprecia-