Food Chemistry

5.3 Individual Aroma Compounds 369

Fig. 5.23.Dependence
on time and tempera-
ture of the formation
of 2-acetyl-2-thiazoline
from 2-(l-hydroxyethyl)-
3,5-dihydrothiazole (ac-
cording toHofmannand
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)

0. 1 White-bread crust, rice,

cooked meat, popcorn

2-Propionyl-
1-pyrroline

0. 1 Popcorn, heated meat

2-Acetyltetra-
hydropyridine
(ATPy)

1. 6 White-bread crust,

popcorn

2-Acetylpyridine 19 White-bread crust

hydroxyacetone phosphate (cf. Formula 5.16),
whichisinvolvedintheStreckerdegradation
(cf. Formula 5.14). Another source of 2-oxopro-
panal 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 se-
quence 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

Streckerdegradation of amino acids, e. g., proline

(cf. Formula 5.14), is in the enolized form the re-

action 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

Schiffreaction to a 6-ring results in ATPy.