Identification and extraction method of quercetin from flesh and skin of shallot (Allium ascalonicum) cultivated in Soc Trang province, Vietnam

Abstract

Quercetin is a bioactive compound that has many beneficial effects on human health. Due to a lack of information about quercetin and quercetin derivatives in the Vietnam shallot, this work aimed to identify the quercetin family in shallot skin and flesh by liquid chromatography-mass spectrometry. Effect of extraction method (conventional solvent extraction and ultrasound-assisted extraction), including ethanol concentration (40–80%) and extraction time (15 to 60 mins) on the yield of quercetin were also investigated in this study. Quercetin, quercetin-glucoside and methylated-quercetin-hexose were commonly found in the extract of shallot flesh and skin. A very small amount of quercetin aglycone (0.7%) was found in flesh extract, while it was the most abundant in the skin sample (46.3%). The high content of quercetin-mono-glycoside was obtained in shallot extract (43.8% and 35.3% in the shallot flesh and skin, respectively). The flesh extract also contained high level of quercetin-di-glucoside (38.9%). About 14-16% of methylated-quercetin (-glucoside) were found in shallot bulb (both skin and flesh). It was found that ethanol concentration and extraction time directly affected on the quercetin extraction yield. The highest quercetin content was obtained in the sample which was extracted in 60% ethanol in combination with ultrasound-assisted extraction for 15 to 45 mins. The ultrasound-assisted extraction method improved quercetin yield by 13.38-15.64% and 49.46-56.88% for shallot skin and flesh compared to conventional solvent extraction. This study proved that ultrasound-assisted extraction could successfully be used for extraction of quercetin from shallot (both skin and flesh).