Representative varieties and genotypes of different colored onions were compared to determine the extent of differences in the distribution of quercetin and quercetin glycosides. The dry skins, outer rings, and inner rings were separated and extracted with ethanol to obtain quercetin glycosides that were then hydrolyzed to free quercetin, or aglycone. Free quercetin was used as the standard for quantification by reverse phase high performance liquid chromatography (HPLC).
Significant difference (P=0.05) in total quercetin content was observed between dry skin and inner rings (edible parts). A decrease in total quercetin content was observed from the dry skin to inner rings. The highest total quercetin content was observed in the dry skins of Red bone variety (30.74 g/kg dry weight) while Contessa variety contain the least amount (0.082 g/kg dry weight). Total quercetin content in outer scales (1-2 scales) in Kadavan variety is the highest (481 mg/kg fresh weight); however, trace amounts are observed in Contessa. Inner rings (5-6 and 7-10 scales) contain less amount of total quercetin in all the varieties.
Outer scales of all the varieties except 1015Y and Contessa contain moderate amount (2.5-16 mg/kg fresh weight) of free quercetin. Kadavan contain the highest amount of free quercetin (20.64 g/kg dry weight).