Temperature-dependent studies on the total phenolics, flavonoids, antioxidant activities, and sugar content in six onion varieties

Heating effect on total phenol, flavonoids, antioxidant activity, and sugar content of six onion varieties has been quantitatively investigated to explore the effect of different temperatures. The onion varieties comprised one red-skinned variety, two white-skinned varieties, and three yellow-skinned varieties. The heating temperature was scanned at 80 degrees C, 100 degrees C, 120 degrees C, and 150 degrees C for 30 minutes each, and quantitative analysis was performed relative to the powdered onion at ambient temperature. Quercetin, glucosides and sugar content were analyzed using high-performance liquid chromatography. The total phenolic and antioxidant content increased in all six varieties. The total flavonoid levels showed a considerable change. On heating the onion samples at 120 degrees C for 30 minutes, the red-skinned variety showed the highest level of total phenolic content [13712.67 +/- 1034.85 mu g of gallic acid equivalent/g thy weight (mu g GAE/g DW)] and total flavonoids [3456.00 +/- 185.82 mu g of quercetin equivalents/g dry weight (mu g Q/g DW)], whereas the content of total phenolics and total flavonoids were 13611.83 +/- 341.61 mu g GAE/g DW and 3482.87 +/- 117.17 mu g Q/g DW, respectively, for the yellow-skinned (Sunpower) variety. Quercetin and its glucoside contents increased up to 120 degrees C and then decreased at 150 degrees C, whereas the sugar content continuously decreased with heating. All cultivars showed the same pattern in the heating effect, and the predominant flavonoids were destroyed at higher temperatures. Therefore, it is improper to expose onion powder to a temperature higher than 120 degrees C. Copyright (C) 2015, Food and Drug Administration, Taiwan. Published by Elsevier Taiwan LLC. All rights reserved.