Food Chemistry

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.

(17.11)

Alkylthiosulfonates (III) are also respon-
sible 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 dimethyl-
thiophenes.
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. re-
action sequence 17.12).

(17.12)

The key precursor for garlic aroma is S-allyl-L-

cysteine sulfoxide (alliin) which, as in onions,

occurs in garlic bulbs together with S-methyl-

and S-propyl-compounds. The allyl and propyl-

compounds are assumed to be synthesized from

serine and corresponding thiols:

(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-