2009_Book_FoodChemistry

5.3 Individual Aroma Compounds 371
Fig. 5.24. Formation of 2-acetyl-1-pyrroline (according to Hofmann and Schieberle, 1998)
Fig. 5.25. Formation of 2-acetyltetrahydropyridine (according to Hofmann and Schieberle, 1998)
5.3.1.7 Pyrazines
A large number of volatile pyrazines are formed
on heating food. Seventy compounds are known
alone in the group of alkyl pyrazines consisting
only of the elements C, H and N. In dilution
analyses, e. g., of coffee, bread crust, fried meat
and cocoa liquor, only the first six compounds
in Table 5.24 were detected; pyrazine II and V
reached the highest FD factors.
According to gas chromatographic–olfactometric
studies, pyrazines II, III, V and VI (Table 5.24)
have the lowest odor thresholds (0.07 pmol/l air)
that have ever been measured for alkyl pyrazines
(cf. 5.6.3). Of these four pyrazines, II and V are
produced in food in higher concentrations than III
and VI (cf. example coffee, 21.1.3.3.7). As a result
of this favorable ratio of concentration to odor
threshold, the aroma activities of II and V exceed
those of the other alkyl pyrazines.
Although the odor thresholds of pyrazines I
and IV are much higher than those of pyrazines II,
III, V and VI (Table 5.24), they are still detected
in dilution analyses because they are formed
in much higher concentrations on heating food
and, consequently, can partially compensate for
their “aroma weakness”. 2-Oxopropanal and
alanine are the precursors of pyrazines II, IV
and V as well as 2-ethyl-5,6-dimethylpyrazine,
which is odorless in the concentrations present in
food. In accord with the formation of pyrazine
in food, pyrazine IV is the main compound in
model experiments (Table 5.25), followed by II
and V. To explain the formation of II and IV,