5.3 Individual Aroma Compounds 387Some terpenes are readily oxidized during food
storage. Examples of aroma defects resulting
from oxidation are provided in Table 5.5 and
Section 22.1.1.1.
5.3.2.5 Volatile Sulfur Compounds...............................
The aroma of many vegetables is due to volatile
sulfur compounds obtained by a variety of
enzymatic reactions. Examples are the vegetables
of the plant familiesBrassicaceaandLiliaceae;
their aroma is formed by decomposition of
glucosinolates or S-alkyl-cysteine-sulfoxides
(cf. 17.1.2.6.7).
2-Isobutylthiazole (compound V, Table 5.22) con-
tributes to tomato aroma (cf. 17.1.2.6.13). It is
probably obtained as a product of the secondary
metabolism of leucine and cysteine (cf. postu-
lated Reaction 5.30).
(5.30)Isobutyric acid is the precursor of asparagus
acid (1,2-dithiolane-4-carboxylic acid) found in
asparagus. It is dehydrogenated to give methyl-
acrylic acid which then adds on an unknown
S-containing nucleophile (see Formula 5.31).
During cooking, asparagus acid is oxidatively
decarboxylated to a 1,2-dithiocyclopentene (see
Formula 5.32), which contributes to the aroma of
asparagus.(5.31)Volatile sulfur compounds formed in wine and
beer production originate from methionine and
are by-products of the microorganism’s meta-
bolism. The compounds formed are methional (I),
methionol (II) and acetic acid-3-(methylthio)-
propyl ester (III, cf. Reaction 5.33).(5.32)(5.33)Tertiary thiols (Table 5.35) are some of the most
intensive aroma substances. They have a fruity
odor at the very low concentrations in which they
occur in foods. With increasing concentration,
they smell of cat urine and are calledcatty
odorants.Tertiary thiols have been detected in
some fruits, olive oil, wine (Scheurebe)and
roasted coffee (Table 5.35). They make important
contributions to the aroma and are possibly
formed by the addition of hydrogen sulfide to
metabolites of isoprene metabolism. In beer,