370 5 Aroma Compounds
(5.14)(5.15)(5.16)The reaction pathway shown in Fig. 5.25 can be
based on the identification of HOP as an inter-
mediate in the formation of ATPy and on a model
experiment in which 2-methyl-1-pyrroline was
used instead of 1-pyrroline. 2-Acetyl-3-methyl-
3,4,5,6-tetrahydropyridine (cf. Formula 5.17)
was produced, i. e., a displacement of the methyl
group from position 2 in the 5-ring of the starting
compound to position 3 in the 6-ring of the
product. This shift can only be explained by the
ring enlargement mechanism (Fig. 5.25).
A comparison of the reaction paths in Fig. 5.24
and Fig. 5.25 allows the conclusion that the con-
centration ratio of 2-oxopropanal to hydroxy-2-
propanone in food decides whether Apy or ATPy
is preferentially formed from proline. If free
amino acids are present in the food and the
Streckerdegradation dominates, then the forma-
tion of ATPy predominates. This could explain
the preference for ATPy (430 μg/kg) compared
to Apy (24 μg/kg) in the production of popcorn.(5.17)
Although the odor threshold increases by about
a factor of 10, the popcorn-like aroma note
remains on oxidation of ATPy to 2-acetyl-
pyridine. Substantially greater effects on the
aroma are obtained by the oxidation of APy to
2-acetylpyrrole, which has an odor threshold that
is more than 5 powers of ten higher and no longer
smells roasted.
2-Pentylpyridine contributes to the smell of
roasting lamb fat (greasy, suety odor; threshold:
0 .12 μg/kg water); it produces an aroma defect in
soybean products (cf. 16.3.1.1). The precursors
identified were ammonia from the pyrolysis of
asparagine and glutamine and 2,4-decadienal:(5.18)