Food Chemistry

5.3 Individual Aroma Compounds 371

Fig. 5.24.Formation of 2-acetyl-1-pyrroline (according toHofmannandSchieberle, 1998)

Fig. 5.25.Formation of 2-acetyltetrahydropyridine (according toHofmannandSchieberle, 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/lair)
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 re-

sult 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,