Page 32 - FMT_March

Ingredients

the thioglucosides into the
unstable thiohydroxamic-o-
sulphate which too is unstable
and undergoes spontaneous
degradation to isothiocyanate.

The sulphur volatiles responsible
for the odour of onion and garlic
are not present as such in the
intact vegetable tissue, but are
formed rapidly when the tissue
is ruptured by cutting or chewing,
by the action of an enzyme on a
precursor. The precursors and the
enzyme are present in different
cells of the tissue and come into
number of flavanone glycosides. time, due to the formation of the contact only when the cells are
Among these, hesperidin (orange bitter limonin from its nonbitter ruptured. The precursors are
and lemon) and naringenin precursor (limonin monolactone) cysteine sulphoxide derivatives
(grapefruit) are the most common. by the action of the organic acids which on enzymatic decomposi-
Hesperidin is quite tasteless, present in the juice. This can tion are converted into flavor
whereas naringenin has an be prevented by removing the compounds. The latter then un-
extremely bitter taste. precursor by exposing the fruits to dergo nonenzymatic degrada-
ethylene before juice extraction, tion to more volatile compounds
• Terpenoids: Terpenoids are or by the addition of a specific such as sulphides, disulphides
ubiquitous in plant foods. They enzyme to the juice to degrade and trisulphides.
are the major components of limonin.
citrus oils and contribute to the The characteristic odour of garlic
flavor of citrus fruits. Limonene, • sulphur compounds: Certain is due to allicin, which is formed
a monoterpene hydrocarbon, volatile sulphur – containing from the odourless alliin (S-2
possessing a lemon-like odour compounds possess powerful propenyl cysteine sulpoxide)
constitutes approximately 90 and distinctive odours which by the action of the enzyme
percent of most citrus oils. contribute to both the pleasant allinase. Allicin then undergoes
Naturally occurring oxygenated and unpleasant aroma of many nonenzymatic decomposition to
terpenes (mainly alcohols, foods, e.g., vegetables belonging disulphide and thiosulphinate.
aldehydes and ketones) provide to the genus Allium (onion, The disulphide further decom-
the characteristics flavor of garlic) and Brassica (cabbage, poses into a complex mixture of
individual citrus species, e.g., cauliflower, Brussels sprouts, monosulphide and trisulphide.
neral and geranial of lemons and broccoli).
nootkatone (bicyclic sesquiter- The production of the volatile
pene) of grapefruit. Vegetables of the Brassica family constituent of onion takes place
contain the sulphur compounds similarly. Here the precursor S-1-
In the presence of air or dissolved S–methylcysteine sulphoxide propenyl cysteine sulphoxide is
oxygen terpenes undergo struct- and thioglucosides. On cooking enzymatically cleaved to give
ural changes and hydration, the vegetables, the former is propenyl sulphenic acid which
hence citrus juice concentrates converted into dimethyl sulphide is unstable and undergoes re-
prepared by low-temperature which is partly responsible for arrangement to thiopropanal
vacuum evaporation are superior odour. However, the predominant S-oxide, the lachrymatory factor
in flavor than those processed odour is that of the isothiocyanates in onion.
at high temperatures. Juices of formed from thioglucosides
certain varieties of orange and by enzymatic hydrolysis. This • Other volatile components:
grapefruit become bitter when occurs only after rupturing of A number of other important
kept at room temperature for some the cells when enzymes convert volatile components contribute

Food Marketing & Technology 32 March 2020