Food Chemistry

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 inter-
mediatesaandbare oxidized to thiazoline II by
atmospheric oxygen in the presence of catalytic
amounts of heavy metals. It is assumed that the

Table 5.22.Thiazoles and thiazolines in food

Name Structure Aroma Odor
quality threshold
(μg/kg,
H 2 O)

2-Acetyl-
thiazole

Cereal,

popcorn

10

2-Acetyl-
2-thiazoline

Popcorn 1

2-Propionyl-
2-thiazoline

Popcorn 1

Benzo-
thiazole

Quino-

line,

rubber

2-Isobutyl-
thiazole

Green,

tomato,

wine

3

metal ion, e. g., Cu^2 +, oxidizes the eneaminolcto

a resonance-stabilized radicaldin 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 eneaminolc

results in the conversion ofe 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 precursorb, only the lim-

itation of the reaction time to 10 minutes in the

temperature range 50–100◦C results in the high-

est 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” off-

flavor. 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).

5.3.1.6 Pyrroles,Pyridines

The volatile compounds formed by heating food

include numerous pyrrole and pyridine deriva-

tives. Of special interest are the N-heterocyclic

compounds with the following structural feature:

(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-acetyl-

thiazole, 2-acetylthiazoline (cf. Table 5.22) and

acetylpyrazine (cf. Table 5.23) contain this struc-

tural element and have a roasted or cracker-like

odor. However, the thresholds of these com-

pounds 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