Valorization of by-products Derived from Onions and Potato: Extraction Optimization, Metabolic Profile, Outstanding Bioactivities, and Industrial Applications

Huge quantities of vegetables and fruits by-products are discarded annually worldwide following the industrial food processing techniques. These biowastes were found to cause further environmental hazards. However, they could represent rich sources of numerous bioactive metabolites and substrates for high valued products. Specifically, onion (Allium cepa L.) and potato (Solanum tuberosum L.) are of economic importance since they are cultivated and found as chief components of most food recipes worldwide. Nevertheless, potato peels and the outer onion scaly leaves are major non-edible by-products. Both biowastes are rich in bioactive phenolic compounds, whereas potato peels are rich in chlorogenic acids and onion solid wastes in flavonoids, particularly flavonols (quercetin derivatives). Also, they are good sources of dietary fibers, fatty acids, starches, sugars and proteins. In addition, they are potential candidates for biofuels production. Hence, with the recent advances of bio-refinery concepts valorization of such treasures is highly recommended. The current review highlighted the major metabolic classes of onion and potato agro-industrial wastes and how we can utilize the available possibilities to maximize the recovery and benefits of metabolites found in these wastes.