Abstract
There is increasing concern for reduction of the ecological impacts of industrial waste caused by fruits and vegetables. To reduce costs of onion waste disposal while obtaining value-added products, onion skin can be used to extract quercetin, a natural flavonoid with antioxidant, anti-inflammatory and anti-cancer effects. The aim was to optimize quercetin extraction from brown onion (Allium cepa L.) skin through investigation of the effects of different parameters on quercetin yield. Operational parameters for conventional maceration extraction and for ultrasound-assisted extraction were compared: solvent type, mass-to-liquid ratio, extraction time and temperature. Antioxidant capacity was determined using DPPH· radical scavenging assays and quercetin yield using HPLC/DAD. Anti-α-amylase activity of onion skin extracts was investigated using α-amylase inhibition assays. Optimal extraction conditions of quercetin from onion skin were obtained with maceration extraction, 50% ethanol, 1:100 mass-to-liquid ratio, 25 °C, for 15 min. Under these conditions, the antioxidant capacity (expressed as quercetin equivalents) was 18.7 mg/g and the mass fraction of quercetin was 7.96 mg/g. The onion skin extracts showed a dose-dependent relationship between dry extract concentration and α-amylase inhibition, which confirms that this onion skin extract can be considered as an anti-diabetes agent.
Highlights
Brown onion, which is known as yellow onion (Allium cepa L.), is a biennial herbaceous plant that originated from the territories of western and central Asia
The data in the present study demonstrate that these onion skin extracts that contain quercetin have a direct impact on the α-amylase activity and pointing out the fact that other substances, present in these extracts may have important impact on the overall anti-α-amylase activity, which was not shown by other studies
The optimal extraction conditions for quercetin from onion skin according to the quercetin mass fraction determined by this study, were obtained using conventional maceration extraction (CME) with 50% ethanol as solvent, 1:100 mass-to-liquid ratio, 15 min extraction time and 25 ◦C extraction temperature
Summary
Brown onion, which is known as yellow onion (Allium cepa L.), is a biennial herbaceous plant that originated from the territories of western and central Asia. Quercetin has beneficial effects on human health because of its antioxidant, anti-inflammatory, antimicrobial, antiviral, anti-allergic, cardioprotective, vasodilatory and anticancer activities [2,3,4]. It stabilizes cell membranes, inhibits the aging process of the skin, cornea and myocardium, and has positive effects on the function of the cardiovascular system [5]. Quercetin is found in many medicinal plants and in fruit and vegetables [5], and it is known that the dry outer skin of brown onion is one of the richest sources of free quercetin [6]. Quercetin often occurs in nature in its free form, and in the form of its glycosides, the most common of which is rutin [5]
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