2009_Book_FoodChemistry

5.3 Individual Aroma Compounds 367
5.3.1.5 Thiazoles
Thiazole and its derivatives are detected in foods
such as coffee, boiled meat, boiled potatoes,
heated milk and beer. Aroma extract dilution
analyses show that among the compounds I–III in
Table 5.22, 2-acetyl-2-thiazoline (II) contributes
most intensively to the aroma of quick fried beef.
Model experiments showed that cysteamine,
formed by the decarboxylation of cysteine, and
2-oxopropanal are the precursors. It was also
found that higher yields of II are obtained at
pH 7.0 compared to pH 5.0. The intermediates in
the reaction path to thiazoline II (Fig. 5.21) were
identified as the odorless 2-(1-hydroxyethyl)-4,5-
dihydrothiazole (a) and 2-acetylthiazolidine (b),
which are in tautomeric equilibrium, presumably
with 2-(1-hydroxyethylene)thiazolidine (c) asthe
intermediate compound (Fig. 5.21). The intermediates
a and b are oxidized to thiazoline II by
atmospheric oxygen in the presence of catalytic
amounts of heavy metals. It is assumed that the
metal ion, e. g., Cu 2+ , oxidizes the eneaminol c to
a resonance-stabilized radical d in a one-electron
reaction (Fig. 5.22). This radical then traps an
oxygen molecule with the formation of a peroxy
radical (e). H-Abstraction from the eneaminol c
results in the conversion of e to 2-acetyl-2-
thiazolinehydroperoxide (f ), which decomposes
to thiazoline I and H 2 O 2 .H 2 O 2 can oxidize the
metal ion and regenerate it for a new cycle.
In the conversion of the precursor b, only the limitation
of the reaction time to 10 minutes in the
temperature range 50–100 ◦ C results in the highest
yield of thiazolidine II (Fig. 5.23).
This is in accord with the aroma formation during
the frying of beef. The concentration of II in meat,
decreases again if heating continues.
Thiazole IV (Table 5.22) can occur in milk when
it is heated, and is responsible for a “stale” offflavor.
Thiazole V (Table 5.22) is a constituent of
tomato aroma. The aroma of tomato products is
usually enhanced by the addition of 20–50 ppb of
thiazole V (for the biosynthesis of the compound,
see Section 5.3.2.5).
Table 5.22. Thiazoles and thiazolines in food
Name Structure Aroma Odor
quality threshold
(µg/kg,
H 2 O)
Cereal,
popcorn
10
5.3.1.6 Pyrroles, Pyridines
The volatile compounds formed by heating food
include numerous pyrrole and pyridine derivatives.
Of special interest are the N-heterocyclic
compounds with the following structural feature:
2-Acetylthiazole
2-Acetyl-
2-thiazoline
2-Propionyl-
2-thiazoline
2-Isobutylthiazole
Popcorn 1
Popcorn 1
Benzothiazole
Quinoline,
rubber
Green,
tomato,
wine
3
(5.13)
This characteristic feature appears to be required
for a roasted odor. In fact, all the pyrrolines and
pyridines listed in Table 5.23 as well as 2-acetylthiazole,
2-acetylthiazoline (cf. Table 5.22) and
acetylpyrazine (cf. Table 5.23) contain this structural
element and have a roasted or cracker-like
odor. However, the thresholds of these compounds
vary greatly. The lowest values were
found for 2-acetyl-and 2-propionyl-1-pyrroline.
The length of the alkanoyl group also influences
the aroma activity because in the transition
from 2-propionyl- to 2-butanoyl-1-pyrroline, the
roasted note suddenly disappears and the odor
threshold increases by several powers of ten.
2-Acetyl-l-pyrroline (Apy) is responsible for the
typical aroma of the crust of white bread and it