Effect of Oven and Microwave Heating on the Total Antioxidant Capacity of Dietary Onions Grown in Turkey

Abstract

The active components of largely consumed dietary onions (Allium species) in Turkey were extracted with water and ethanol, separately heated in a drying oven (at 50°C, for 1–4 h) and in a microwave oven (at 90 W, for 1–4 min), and their total antioxidant capacity was determined with different electron transfer-based assays. Five different onion species/aerial parts, namely yellow, red, white, fresh green leaf, and fresh green root, were measured for total antioxidant capacity with different methods, the hierarchic order in aqueous extracts being: CERAC: yellow-skinned > red-skinned > spring-root > spring-leaf > white-skinned onion; CUPRAC: spring-leaf > red-skinned > spring-root > yellow-skinned > white-skinned onion; Folin-Ciocalteau method: spring-leaf > spring-root > red-skinned > yellow-skinned > white-skinned onion. Using all three methods, white onion, showed the lowest total antioxidant capacity, while with respect to two methods (i.e., CUPRAC and Folin-Ciocalteau), spring onion-leaf showed the highest value. In ethanolic extracts, white onion exhibited the lowest total antioxidant capacity using two methods (CUPRAC and Folin-Ciocalteau). Of the heat-processed onions, the highest CERAC and Folin-Ciocalteau total antioxidant capacity values were obtained for red-skinned onions, while the highest CUPRAC value was for spring onion leaves. All three assays marked white-skinned onion as the lowest total antioxidant capacity content of heat-processed products. The change in total antioxidant capacity caused by both heating processes was not drastic; spring onion leaves essentially maintained its total antioxidant capacity level after 4 min microwave or 4 h drying oven heating. Onion processing by heat treatment did not cause a drastic loss in antioxidant values, favourable for traditional cooking practices.