Open access publications in the SLU publication database

Abstract

Flavonoids are polyphenolic substances that are common in many fruits and vegetables and may have positive protective effects against e.g. cardiovascular disease and some forms of cancer. These effects have mainly been attributed to their antioxidative properties. One of the most common flavonoids in food plants is quercetin, of which yellow onion (Allium cepa L.) is one of our main dietary sources. The effects of cultivars, nitrogen fertiliser level, lifting time, curing, cultivation system and storage conditions on the quercetin content in yellow onion are investigated in this thesis. Analyses were performed using reversed-phase HPLC after extraction of raw onion samples in ethanol. The main determinant of quercetin level was found to be the amount of global solar radiation during the end of the bulbing phase in August. Annual variations in quercetin content were higher than any treatment effect within each year. Cultivar differences in quercetin levels were significant but inconsistent. Early lifting, when 50% of the onions had fallen leaves, resulted in onions with better keeping quality and equal quercetin content compared with onions with later lifting times. Minimised split application of nitrogen fertilisers reduced the risk of mineral nutrient leaching without lowering the yield or quercetin content. Field curing generally resulted in significant increases in quercetin content compared with levels at lifting. The role of onion size for quercetin content was inconsistent but of minor importance. Quercetin levels were stable during cold storage at constant temperature. No significant differences were found between inorganically fertilised and organically fertilised onions of the same cultivar or between directly sown and transplanted onions. Quercetin content and soil water content within the field were not correlated.