Agriculture Reference
In-Depth Information
Flavonols
Flavonols have a similar core ring structure to the anthocyanins and follow the
same ring-numbering system to denote where side groups are attached. They
are also normally linked to sugar groups to form glycosides within allium
plants. The most important flavonol in onions and shallots is quercetin, while
in leek and chive kaempferol is more common (see Fig. 8.9).
These flavonols are antioxidants and they can be ingested into the
bloodstream, hence there is much interest in their potential health benefits (see
below). Fenwick and Hanley (1990a) show 14 flavonols that have been found
in edible alliums differing in how many and which sugar groups are linked to
the core quercetin or kaempferol ring, and at which points they are linked.
Glucose is by far the most frequently occurring attached sugar. Flavonols
absorb ultra-violet light and they tend to occur in greater concentration in the
surface cells of plants. Their synthesis is stimulated by light, and they probably
have a role in minimizing cellular damage from ultra-violet light.
Onion bulbs, particularly red and yellow varieties, have very high contents
of quercetin compared with other vegetables (about 350 mg/kg fresh weight).
This is three times their nearest rival, kale, and nine times the next most
abundant rival, French beans, and several orders of magnitude more than
Fig. 8.9.
The structure and names of the flavonol pigments found in onion bulbs
(from Hirota
et al
., 1998).
