2009_Book_FoodChemistry

370 5 Aroma Compounds
(5.14)
(5.15)
amino acids are present in the food and the
Strecker degradation dominates, then the formation
of ATPy predominates. This could explain
the preference for ATPy (430 µg/kg) compared
to Apy (24 µg/kg) in the production of popcorn.
(5.16)
The reaction pathway shown in Fig. 5.25 can be
based on the identification of HOP as an intermediate
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 concentration
ratio of 2-oxopropanal to hydroxy-2-
propanone in food decides whether Apy or ATPy
is preferentially formed from proline. If free
(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-acetylpyridine.
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)