Semi-continuous hydrolysis of onion skin wastes with subcritical water: Pectin recovery and oligomers identification

The semi-continuous hydrolysis of onion skin wastes (OSW) using subcritical water (SubW; working conditions: 105–180 °C; 5 MPa; 2.5 mL/min) has been studied in this work. Liquids after hydrolysis showed a sharp increase in total organic carbon (>30 mg/g OSW) at temperatures above 145 °C, when SubW began to promote the partial hydrolysis of the structural components of OSW. Among them, pectin was one of main components recovered (extraction yield up to 9% at 145 °C), whereas cellulose was barely hydrolyzed in the range of temperatures studied. The composition of pectin demonstrated that SubW promoted the recovery of the valuable RG-I (21.1 ± 1.1 mol%) regions compared to acid water extraction processes (10.8 ± 0.9 mol%). The control of the hydrolysis conditions was found to be critical, since high temperatures and long hydrolysis times led to the formation of organic acids (acetic, formic and levulinic) and degradation products such as furfural (up to 0.8 mg/g OSW) from the C5 sugars. Finally, the High Heating Value (HHV) calculated for the solid residue obtained at 180 °C (16.4 ± 0.2 MJ/kg) indicated the potential use of this residue as a fuel, once the valuable compounds (phenolics and pectins) have been recovered. [Display omitted] •Onion skin wastes were hydrolyzed using subcritical water.•Pectin (galacturonic acid) was the main structural component recovered (9% yield) at 145 °C.•Subcritical water induced the recovery of valuable hairy pectin regions (RG-I).•Harsh experimental conditions led to the formation of organic acids and furfural.•Solid after hydrolysis shows high HHV and potential to be used as fuel.