Agriculture Reference
In-Depth Information
Allicin, other thiosulfinates and the lachrymatory factor of onion are all
highly reactive and can undergo further rearrangement and combination with
themselves, their reaction products and with sulfenic acids to produce, among
others:
O
↑
1.
Thiosulfinates
R-S-S-R
O
↑
2.
Thiosulfonates
R-S-S-R
O
3.
Monosulfides
R-S-R
4.
Disulfides
R-S-S-R
5.
Trisulfides
R-S-S-S-R
can be the same or any pair of R groups listed above that
exist as ACSO precursors in an allium species.
More complex structures, including S and C atoms in heterocyclic rings,
have also been identified. The actual composition of the reaction mixture
depends on the initial concentration and ratio of the R groups forming
sulfenic acids, the temperature - hence the difference in flavour of fresh and
cooked alliums - the polarity of solvents surrounding the reactants and the
pH. The first technique used to extract garlic volatiles was steam distillation,
and under these conditions diallyl disulfide (DADS) accumulates. This method
is still widely used to produce garlic extracts for encapsulation. More than 80
volatile compounds have been identified in fresh and steam-distilled extracts
of alliums. Notable compounds include, from garlic, ajoene (see Fig. 8.4),
which has a stronger anti-thrombotic effect than aspirin and which forms
from allicin in ethanolic solutions; and, from onions, cepaenes of similar
structure to ajoene (see Fig. 8.4), and these too have potent anti-thrombotic
properties.
Here, R and R
The biosynthesis of flavour precursors
Sulfur enters the plant as sulfate from the soil solution, and after being
absorbed by a high sulfate-affinity transporter protein it moves in the vascular
tissue to the leaves and there undergoes sequential enzymic reduction and
transfer reactions to be incorporated in the amino acid cysteine. Cysteine may
then combine with glutamic acid and then glycine to form glutathione. There
