2009_Book_FoodChemistry

5.3 Individual Aroma Compounds 369
Fig. 5.23. Dependence
on time and temperature
of the formation
of 2-acetyl-2-thiazoline
from 2-(l-hydroxyethyl)-
3,5-dihydrothiazole (according
to Hofmann and
Schieberle, 1996)
Table 5.23. Pyrrole and pyridine derivatives with a roasted aroma
Name Structure Odor threshold Occurrence
(µg/kg, water)
2-Acetyl-1-
pyrroline (APy)
2-Propionyl-
1-pyrroline
0.1 White-bread crust, rice,
cooked meat, popcorn
0.1 Popcorn, heated meat
2-Acetyltetrahydropyridine
(ATPy)
1.6 White-bread crust,
popcorn
2-Acetylpyridine 19 White-bread crust
hydroxyacetone phosphate (cf. Formula 5.16),
which is involved in the Strecker degradation
(cf. Formula 5.14). Another source of 2-oxopropanal
is the retroaldol condensation of 3-deoxy-
1,2-dicarbonyl compounds in the course of the
Maillard reaction (cf. 4.2.4.4.2). The reaction
route which can explain the formation of Apy
is based on an investigation of the model
1-pyrroline/2-oxopropanal and on labelling
experiments. They show that in the reaction
of proline with [ 13 C] 6 -glucose under roasting
conditions, two 13 C atoms are inserted into the
Apy molecule. As a start in the reaction sequence
to Apy, it is assumed that 2-oxopropanal
(cf. 4.2.4.3.2), which is formed in the degradation
of glucose, is present as a hydrate and participates
in a nucleophilic attack on 1-pyrroline (Fig. 5.24).
The resulting 2-(1,2-dioxopropyl)pyrrolidine is
sensitive to oxygen and, consequently, rapidly
oxidizes to 2-(1,2-dioxopropyl)pyrroline. After
hydration, decarboxylation takes place in accord
with the labelling experiment. This is followed
by rearrangement and oxidation to Apy.
Hydroxy-2-propanone, which is formed by the
Strecker degradation of amino acids, e. g., proline
(cf. Formula 5.14), is in the enolized form the reaction
partner of 1-pyrroline in the formation of
ATPy (Fig. 5.25). The aldol addition of the
two educts gives 2-(1-hydroxy-2-oxopropyl)-
pyrrolidine (HOP) which undergoes ring opening
to yield 5,6-dioxoheptylamine. The subsequent
Schiff reaction to a 6-ring results in ATPy.