Distribution of quercetin content in different rings of various coloured onion (Allium cepaL.) cultivars

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Abstract
Representative cultivars of different coloured onions were compared to determine the extent of differences in the distribution of free and total (bound and free) quercetin among different onion rings. The dry skin, outer rings, and inner rings were separated and extracted with ethanol to obtain quercetin glycosides (bound form) that were then hydrolyzed to free quercetin. 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 the 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 concentration was observed in the dry skins of ‘Red Bone’ (30.66 g kg–1 dry weight) while ‘Contessa’ contain the least amount (0.094 g kg–1 dry weight). Total quercetin content in outer rings (1–2) in ‘Kadavan’ was high highest (345.51 mg kg–1 fresh weight); however, trace amounts were observed in ‘Contessa’. Inner rings (5–6 and 7–10) contain less total quercetin in all the cultivars. Outer rings (cataphylls) of all the cultivars except ‘Texas Grano 1015Y’ and ‘Contessa’ contain moderate amounts (2.5–16 mg kg–1 fresh weight) of free quercetin. ‘Red Bone’ skin contain the highest amount of free quercetin (20.64 g kg–1 dry weight). Quercetin distribution based on fresh and dry weight method was also evaluated.

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