Subcritical solvent extraction and microencapsulation of flavonoids from onion skin waste

• SSE is an effective method of recovering flavonoids from OSW. • Flavonoids in the OSW extracts decrease during storage. • Microencapsulation enhanced storage stability and handling properties. • Fine starch is a novel option of microcapsule wall material. Large amounts of onion skin are currently discarded as food waste despite containing many useful flavonoids such as quercetin. Subcritical solvent extraction methods and the storage stabilities of their extracts were investigated to develop effective uses for onion skin. In a 12-week storage experiment (40°C in darkness), the total flavonoid content of the extract decreased by 42.0%. Microencapsulation of the extract was examined to assess storage stability and handling properties. Dextrin hydrate and subcritical-water-treated fine waxy rice starch were used as wall materials. Drying of the extract was achieved by freeze-drying or spray-drying. Microcapsules prepared by freeze-drying showed higher encapsulation efficiency (66.6%–78.0%). In a storage experiment, all microcapsules showed high retention of flavonoids after 8 weeks (89.4%–98.8%). Overall, this study showed that microencapsulation enhances storage stability and handling properties of flavonoids and that fine starch can be used as an alternative wall material to dextrin.