Onion is a major source of flavonoids and is cooked in various ways in the world. The major flavonoids in onion are two quercetin glycosides, quercetin 4'-O-β-gluco-side (Q4'G) and quercetin 3, 4'-Oβ-diglucosides (Q3, 4'G), which are recognized as bioac-tive substances that are good for our health. We have investigated the effect of cooking pro-cedures on the content of antioxidants. We selected quercetin conjugates, total phenol com-pounds, and ascorbic acid to estimate the amount of flavonoid ingestion from onion. We ex-amined the following cooking methods; boiling, frying with oil and butter, and microwave cooking. Various cooking methods do not consider the degradation of quercetin conjugates when cooking onion. Microwave cooking without water better retains flavonoids and ascor-bic acid. Frying does not affect flavonoid intake. The boiling of onion leads to about 30% loss of quercetin glycosides, which transfers to the boiling water. At that time, the effect of addi-tives on the quercetin conjugates is different according to the compounds. The hydrolysis of quercetin glycosides for daily cooking might occur with the addition of seasonings such as glutamic acid. Additional ferrous ions accelerated the loss of flavonoids.
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