Subcritical water extraction of quercetin and derivatives from onion skin wastes (Allium cepa cv. Horcal): effect of temperature and solvent properties

Abstract

Nowadays, the society is immersed in the transition from a linear to a circular economy, in which the value of products, materials and resources is maintained in the economy for as long as possible and the generation of waste is minimized. This means that the byproduct of a process becomes the input of a new one where it acquires new value. Among other agricultural by-products, onion (Allium cepa L.) skin wastes offer great potential for valorization. Onion is the second most important horticultural crop worldwide. More specifically, Spain, in the year 2018 produced 1.27 Mt [1]. The onion industry generates every year more than 0.5 Mt of Onion Skin Wastes (OSW) worldwide [2], including skins (the outermost layers), roots and bulbs unfit for consumption. The non-edible brown skin and external layers of onions are rich in phenolic compounds,

mainly flavonoids such as quercetin (QC) [3] and its derivatives: quercetin 4’-O-β- glycoside (QC4’), quercetin 3,4’-O-β-diglycoside (QC3,4’) and quercetin 3-O-β- glycoside (QC3). All of them are high-added value natural antioxidants [2]. The main

drawback of quercetin and quercetin derivatives is the limited solubility in water, which limits their oral bioavailability [4] and extractability, and forces the use of an organic solvent to successfully extract them. Alternatively, Subcritical Water (SubW) can be used to extract flavonoids from onion skins. SubW refers to water at temperatures ranging from 100 °C (boiling point) to 374 °C (critical point) which remains in a liquid state due to the application of pressure. Changes in the working conditions change the properties of the SubW (among them, viscosity, surface tension and dielectric constant, which can be similar to those of some organic solvents), enhancing mass transfer and the extractability of barely water-soluble bioactive compounds, as summarized by Benito-Roman et al. [5], as SubW favors the hydrolysis of the bonds between phenolic compounds and the vegetable matrix. In this work the extraction, identification and quantification of phenolic compounds from OSW has been studied using SubW in a semicontinuous extractor (flow rate constant and equal to 2.5 mL/min; temperatures up to 180 °C with working pressure of 5 MPa, to keep water in liquid state). The extraction of flavonoids resulted to be fast (<30 min) and temperature sensitive (maximum at 145 °C; total flavonoids, 27.4±0.9 mg/g OSW). Further increases of temperature decreased the number of flavonoids recovered. The experimental results were fitted to the Weibull model. The influence of the solvent properties on the flavonoids quantification was found to be critical. A precipitate was formed once the extracts cooled down. If removed, a significant fraction of the high temperature extracted flavonoids (as much as 71%, at 180 °C) was lost. This fact affected especially to those compounds that show extremely low solubility in water at room temperature, whereas quercetin glycosylated derivatives were less affected by the polarity change of the medium induced by the temperature change. It has been demonstrated that it is necessary to re-dissolve the subcritical water extracts by the addition of ethanol, which led to a medium with a polarity equivalent to that obtained with water at high temperature. At 145 °C, quercetin (15.4±0.4 mg/g OSW) and quercetin-4’-glucoside (8.4±0.1 mg/g OSW) accounted for the 90% of the total flavonoids identified by HPLC, according to the method described by Benito-Román et al. [6]. All in all, the obtained extracts resulted to be rich in flavonoids, which makes them suitable to be used as food additives in order to replace other synthetic antioxidant compounds.