The valorization of onion skin wastes (OSW) through the extraction, identification,
and quantification of phenolic compounds was studied in this work, using subcritical water in a
semicontinuous extractor (2.5 mL/min; 105–180 C; 5 MPa). 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
dry OSW (DOSW)). The experimental results were fitted to theWeibull 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. Such a condition a ected especially those compounds
that show extremely low solubility in water at room temperature, whereas quercetin glycosylated
derivatives were less a ected by the polarity change of the medium induced by the temperature
change. It was 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 DOSW) and quercetin-40-glucoside
(8.4 0.1 mg/g DOSW) accounted for the 90% of the total flavonoids identified. By recovering high
added value bioactive compounds from OSW the principles of circular economy were fulfilled,
providing a new use for this agricultural waste.
