BIOCHEMISTRY, HEALTH BENEFITS AND FOOD SCIENCE OF ALLIUMS - Onions and other Vegetable Alliums - page 371

FLAVONOIDS
Two types of flavonoids are of particular interest in alliums; these are the
anthocyanins responsible for the colour in red-skinned onion cultivars and the
flavonols, which give a yellowish hue to onion flesh and are important
precursors for yellow and brown skin pigments (see Chapters 3, 'Genetics' and
7, 'The Curing of Onion Skins').
Anthocyanins
Anthocyanins are red, purple or blue pigments found in many flowers and
fruits. They all have the same basic central chemical ring structure, with the
different groups attached to that ring causing their colour to vary (Salisbury
and Ross, 1991). Anthocyanins have sugar groups (usually glucose) attached
to certain positions on the core ring (see Fig. 8.8).
This core ring - minus attached sugar groups, i.e. after hydrolysis - is
termed an anthocyanidin. The predominant anthocyanidin in red onions is
cyanidin. There are also some reports of traces of peonidin and pelargonidin.
The glucosyl group attached at position 3 in onion cyanidins may itself have
various side groups attached. Figure 8.8 shows the predominant configurations
found in European and North American red cultivars (Fossen et al. , 1996;
Donner et al. , 1997). Anthocyanins constitute 0.11-0.22% of the dry weight of
bulbs of red North American onion cultivars (Donner et al. , 1997). Cyanidin is
also the predominant anthocyanidin in red shallots and A. wakegi (Arifin et al. ,
1999) and in top onion, A. altaicum and chives (Fossen et al. , 1996).
Fig. 8.8. The main anthocyanin pigments found in the bulbs of three red onion
cultivars (glc, glucose) (from Fossen et al ., 1996).