Changes in the antioxidant activity and metabolite profile of three onion varieties during the elaboration of ‘black onion’

Abstract This study aims to investigate whether the heat treatment applied during the production of black onion, a novel derived product made from fresh onion, produces changes in the content of flavonoids, organosulfur compounds, organic acids, water soluble sugars and amino acids in three onion varieties (‘Shallot’, ‘Chata’ and ‘Echalion’). The total flavonoid content decreased up to 12-fold in black onions compared with fresh onions while the quantities of isoalliin, the main organosulfur compound in black onions, drastically increased during the process. Moreover, the levels of fructose and glucose significantly increased during the elaboration process, contributing to the sweetness of black onions. The influence of heating on their antioxidant capacity showed a decreasing trend of the ORAC antioxidant activity of onion, while ABTS and DPPH did not show a clear tendency. These results present a comprehensive phytochemical characterization of black onions, highlighting the significant influence of the heating process on their phytochemical composition.