Enhancement of antioxidant activity of onion powders by browning during drying process

Drying process was applied to increase the antioxidant activity of onion powder: freeze-drying or air-drying at 50, 70, and 90°C and onion extracts were obtained from each powder using water or aqueous ethanol (50%) at 25°C and 60°C. In the color analysis, the freeze-dried powders showed higher L^∗ and lower a^∗ and b^∗ values than did the air-dried ones. The browning index of powders air-dried at 90°C was significantly higher than that of freeze-dried powders or those air-dried at temperatures below 90°C. Phenolic content in the extracts was 4.02-23.12 mg gallic acid equivalent/g sample, and was the highest in the extract from the sample air-dried at 90°C, regardless of the extraction condition. The highest antioxidant activity, measured by 2,2-azinobis-3-ethylbenzothiazoline-6-sulfonic acid and 1,1-diphenyl-2-picrylhydrazyl methods, was found in the powder air-dried at 90°C, which induced browning. These findings indicate that antioxidant activity depends more on browning during drying than on extraction conditions.