Adding value to onion (Allium cepa L.) waste by subcritical water treatment

In this study, the treatment of brown type onion (Allium cepa L.) waste was investigated under subcritical water conditions. Experiments were carried out in a wide temperature range from 373 to 593 K, at a residence time of 5 min. Several phases were segregated after the reaction, including the aqueous phase, solid residue, and hexane and acetone soluble phases. In the aqueous phase, significantly increased amounts of total organic carbon and total nitrogen were identified from the decomposition of mainly the carbohydrate and protein parts of the onion. In addition, very promising amounts of water-soluble sugars were also quantified in the aqueous phase (65% of the dry matter for onion bulbs and 7% for skins). For the remaining solid phases obtained at different temperatures, higher heating values (HHVs) were calculated and compared, based on the ultimate analysis. Generally, the remaining solids showed increased amounts of carbon content, which led to increased HHVs and energy densities of the solids. Acetone and hexane soluble fractions were also evaluated. Very small amounts of hexane and acetone soluble compounds were obtained. Furthermore, two valuable products – fructooligosaccharides and quercetin – were successfully obtained from onion bulbs and skins, respectively.