Quantification of Quercetin Glycosides in 6 Onion Cultivars and Comparisons of Hydrolysis-HPLC and Spectrophotometric Methods in Measuring Total Quercetin Concentrations

ABSTRACT: This study was performed to purify and quantify quercetin glycosides (QG) and aglycone (free) quercetin (Q) in 6 selected onion cultivars and to compare analytical approaches based on high‐performance liquid chromatography (HPLC) and spectrophotometry for the quantification of total quercetin (TQ) concentrations. Individual mono‐ and di‐glycoside Q compounds were purified using a semipreparative HPLC and identified by comparing spectral data and by confirming corresponding peaks of QG and Q after incomplete enzyme‐hydrolysis. Purified QG were quantified as Q by enzyme‐hydrolysis/HPLC. TQ concentrations obtained from 20 onion bulbs with enzyme‐hydrolysis/HPLC, no‐hydrolysis/HPLC, and a spectrophotometric method without prior hydrolysis were significantly correlated ( r 2 = 0.99) and were about 15% higher, identical, or 10% less than those concentrations by a standard acid‐hydrolysis/HPLC method, respectively. During enzyme‐hydrolysis of onion extracts, progressive reduction of the QG and formation of the corresponding mono‐glycosides and Q were monitored using an analytical HPLC. TQ ranged from 83 to 330 μg/g F.W. in 6 selected cultivars of long‐day or short‐day onions. Q3,4′G and Q4′G were the 2 major compounds and comprised approximately between 94% and 97% of TQ in onions.