Effect of curing and cooking on flavonols and anthocyanins in traditional varieties of onion bulbs

The stability of the major flavonol glucosides and anthocyanins was studied in two regional varieties of Portuguese onion (a white variety “branca da Póvoa” and a red variety “vermelha da Póvoa”). White and red onions from 2007 and 2008 harvests were subjected to field curing with and without light, but the red cultivar from 2008 was also subjected to typical domestic processing, including chopping and different cooking treatments. Field curing resulted in increases in quercetin content compared to levels at lifting, especially important for all white bulbs (33–40% increase). Flavonol and anthocyanin levels in onions cured in the dark were similar to those obtained in bulbs cured in the light. The treatments chopping followed by refrigerated storage, oven roasting and frying, did practically not contribute to modify the total levels of flavonols. Moderate microwave cooking did not affect to the flavonol content, but intense microwave treatment cause flavonol losses of 16% and 18% for quercetin 3,4′-diglucoside (QdG) and quercetin 4′-glucoside (QmG), respectively. Boiling onions for 30 min leaded losses of quercetin glycosides, which leached to the boiling water without being degraded at 37% and 29% for QdG and QmG, respectively. Boiling for 60 min had more severe effects, since it caused the degradation of quercetin derivatives at 53% and 44% for QdG and QmG, respectively. For anthocyanins, the severity of the cooking treatments was in the following order: frying > boiling > roasting (microwave roasting > oven roasting).