Abstract
The effect of gamma irradiation on the content of total phenolic compounds, especially quercetin (Q), in onion (Allium cepa L.) skin was investigated. Onion skin extracts contained two predominant flavonoid compounds, Q and quercetin-4′-glucoside (Q4′G). After 10kGy gamma irradiation, the yield of Q in the extracts increased significantly from 36.8 to 153.9μg/ml of the extract, and the Q4′G content decreased slightly from 165.0 to 134.1μg/ml. In addition, the total phenolic compound content also increased after irradiation at 10kGy, from 228.0μg/g of fresh weight to 346.6μg/g; negligible changes (237.1–256.7μg/g) occurred at doses of up to 5kGy. As we expected, radical-scavenging activity was enhanced remarkably (by 88.8%) in the 10kGy irradiated sample. A dose-dependent increase in the peak intensity of the electron paramagnetic resonance (EPR) spectra was observed in all irradiated samples, with a maximum increase at 10kGy. The intensity relative to that of the control was 0.15, and it increased to 1.10 in 10kGy irradiated samples. The optimum gamma irradiation dose, which is sufficient to break the chemical or physical bonds and release soluble phenols of low molecular weight in onion skin, is about 10kGy.
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