2009_Book_FoodChemistry

5.3 Individual Aroma Compounds 373
(5.19)
Table 5.25. Formation of aroma active alkyl pyrazines
on heating alanine and 2-oxopropanal a
Pyrazine b
Amount (µg)
2-Ethyl-3,5-dimethyl-(II) 27
2-Ethyl-3,6-dimethyl-(IV) 256
2-Ethyl-5,6-dimethyl- 2.6
2,3-Diethyl-5-methyl-(V) 18
a The mixture of educts (2 mmol each; pH 5.6) was
heated for 7 min to 180 ◦ C.
b Roman numerals refer to Table 5.24.
it is postulated that the Strecker reaction of
alanine and 2-oxopropanal represents the start,
resulting in acetaldehyde, aminoacetone and
2-aminopropanal (cf. Formula 5.19).
The precursor of pyrazine IV, 3,6-dimethyldihydropyrazine,
is formed by the condensation
both of two molecules of aminoacetone as
well as two molecules of 2-aminopropanal
(cf. Formula 5.20). The nucleophilic attack
by dihydropyrazine on the carbonyl group
of acetaldehyde and water elimination yield
pyrazine IV. This mechanism also explains the
formation of pyrazine II if 3,5-dimethyldihydropyrazine,
which is produced by the condensation
of aminoacetone and 2-aminopropanal (cf. Formula
5.21), is assumed to be the intermediate.
The preferential formation of pyrazine IV in
comparison with II can be explained by the
fact that the Strecker reaction produces less
2-aminopropanal than aminoacetone because the
aldehyde group in 2-oxopropanal is more reactive
than the keto group. However, both aminocarbonyl
compounds are required to the same extent
for the synthesis of pyrazine II (cf. Formula 5.21).
The powerfully odorous pyrazines VIII–X
(Table 5.24) appear as metabolic by-products
in some plant foods and microorganisms
(cf. 5.3.2.6). Since they are very stable, they
withstand, e. g., the roasting process in coffee
(cf. 21.1.3.3.7).
5.3.1.8 Amines
Not only aldehydes (cf. 5.3.1.1), but also amines
are formed in the Strecker reaction (cf. 4.2.4.4.7).
The odor thresholds of these amines (examples in
Table 5.26) are pH dependent. The enzymatic decarboxylation
of amino acids produces the same
amines as the Strecker reaction; the precursors are
shown in Table 5.26. Both reactions take place
e. g. in the production of cocoa, but the Strecker
(5.20)
(5.21)