5.6 Relationships Between Structure and Odor 399Table 5.44.Odor thresholds (T) in air of alkanals and
(E)-2-alkenals
Alkanal T (pmol/ (E)-2-Alkenal T (pmol/
stimulant) stimulant)
5:0 125 5:1 1600
6:0 80 6:1 900
7:0 66 7:1 1250
8:0 4 8:1 100
9:0 7 9:1 0. 4
10:0 30 10:1 25
ception with an odor threshold 17.5 times lower
than that of nonanal. Decanal and (E)-2-decenal
have similar intensive odors. In chiral compounds
(cf. 5.2.5 and 5.3.2.4) as well as cis/trans isomers,
e. g., C 6 and C 9 aldehydes with a double bond, the
molecule geometry influences the odor intensity
and quality (Table 5.45).
Except for the pair (E/Z)-6-nonenal, the odor
threshold of the E-isomer exceeds that of the
corresponding Z-isomer. In particular, the values
for (E)- and (Z)-3-hexenal differ greatly. Some
of the aldehydes listed in Table 5.44 and 5.45
are formed by the peroxidation of unsaturated
fatty acids (cf. 3.7.2.1.9). However, they play
a role in aromas only when they are produced
in foods in a concentration higher than their
odor threshold concentration. The aroma active
aldehydes usually include hexanal, which ap-
pears as the main product in the volatile fraction
of peroxidized linoleic acid and, therefore, can
surmount the relatively high odor threshold
(Table 5.44). (E)-2-Nonenal also belongs to this
Table 5.45.Dependence of the odor thresholds (T) of
C 6 –C 9 aldehydes on theposition and geometry of the
double bond
C 6 -Aldehyde T (pmol/ C 9 -Aldehyde T (pmol/
stimulant) stimulant)
(E)-2–6:1 900 (E)-2–9:1 0. 4
(Z)-2–6:1 600 (Z)-2–9:1 0. 014
(E)-3–6:1 > 400 (E)-3–9:1 0. 5
(Z)-3–6:1 1 .4 (Z)-3–9:1 0. 2
(E)-4–6:1 77 (E)-4–9:1 9
(Z)-4–9:1 1. 6
(E)-5–9:1 70
(E)-6–9:1 0. 05
(Z)-6–9:1 1. 3
group. Although it is formed in considerably
lower concentrations than hexanal, it can prevail
in aromas due to its very low odor threshold.
This also applies to (Z)-3-hexenal which is enzy-
matically formed from linolenic acid (cf. 3.7.2.3)
and has a very low odor threshold. Consequently,
it plays a much larger role in aromas, e. g., of
fruit and vegetables, olive oil and fish, than
its quantitatively more dominant companion
substance (E)-2-hexenal.5.6.3 AlkylPyrazines.........................................
The following example illustrates how pro-
nounced the specificity of the sense of smell
can be in cyclic compounds. The relationship
between structure and odor activity was tested
with 80 alkyl pyrazines. A part of the results is
showninFig.5.38.
In the series of mono-, di-, tri- and tetram-
ethylpyrazines P1–P6, trimethylpyrazine (P5)
shows the highest aroma activity. In the transition
from dimethylpyrazines to trimethylpyrazine, the
odor quality changes from nutty to earthy/roasted.
If the methyl group in the ring position 2 of P5
is substituted by an ethyl group, P7 is formed,
which has an odor threshold approximately 6000
times lower and an unchanged odor quality.
If the ethyl group moves to the 3- (P8) or
5-position (P9), the odor threshold increases
substantially. It increases even more if the ethyl
group is substituted by a propyl group (P10–P12).
An ethenyl group in position 2 instead of the
ethyl group gives P13, but the odor threshold
remains as low as with P7. If the ethenyl group
moves round the ring (P14, P15), the threshold
value again increases substantially. The insertion
of a second ethyl group in position 3 of P7
and P13 changes neither the threshold value nor
the odor quality in P16 and P17 respectively.
However, if the methyl group in position 2 of P14
or in position 3 of P15 is replaced by an ethyl
group, the resulting pyrazines P18 and P19
have very high threshold values. A comparison
between P17 and P18 shows that whether the
ethenyl group is in position 2 or 3 of ethenyl-
ethyl-5-methylpyrazines is very important for
the contact of the alkyl pyrazines with the
odor receptor. If the methyl and ethyl group in
P19 exchange positions, P20 is formed and the