2009_Book_FoodChemistry

17.1 Vegetables 791
34–246 µg/kg. The formation involves the
attachment of H 2 S to the aldol condensation
product of propanal and enzymatic reduction of
the carbonyl group.
(17.12)
The key precursor for garlic aroma is S-allyl-Lcysteine
sulfoxide (alliin) which, as in onions,
occurs in garlic bulbs together with S-methyland
S-propyl-compounds. The allyl and propylcompounds
are assumed to be synthesized from
serine and corresponding thiols:
(17.11)
Alkylthiosulfonates (III) are also responsible
for the aroma of raw onions, while
propyl- and propenyl disulfides (IV) and
trisulfides are also supposed to play a role
in the aroma of cooked onions. The aroma
of fried onions is derived from dimethylthiophenes.
Precursors of importance for the aroma of
onions, other than compound I, are S-methyl and
S-propyl-L-cysteine sulfoxide. Precursor I is
biosynthesized from valine and cysteine (cf. reaction
sequence 17.12).
(17.13)
Diallylthiosulfinate (allicin) and diallyldisulfide
are formed from the main component by means
of the enzyme alliinase. Both are character impact
compounds of garlic.
17.1.2.6.7 Watercress (39)
Phenylethylisothiocyanate is responsible for
the aroma of this plant of the mustard fam-