Onions are a natural part of the daily diet for most of the world’s population. Tropea Red onion (Allium cepa L.) is an important component of the Mediterranean diet and is commonly consumed uncooked, minimizing loss of antioxidant compounds. Consistent with literature reports, onion phenol compounds are known to be potent free radical scavengers and antioxidants; they are considered to be protective against cardiovascular diseases and to contribute in the prevention of colorectal cancers in humans. It was observed that onion extracts and some flavonols were effective in inhibiting peroxidation of rat hepatocytes. It was also reported a good correlation between radical-scavenging activity and protection against oxidation. Polyphenols are potential protecting agents against the lethal effects of oxidative stress and offer protection of DNA by chelating redox-active transition metal ions. Quercetin effectively protected DNA strand scission from tertbutylhydroperoxide. Therefore, in the red variety presence of high quantities of quercetin might be responsible for better protection of DNA. The red variety with high phenolic content showed better protection compared to the others, indicating that protection was directly proportional to the concentration of total phenolic contents. Methods The flavonols and anthocyanins content was evaluated in the outer layers and in the edible portion of Tropea Red Onion. Results and discussion/conclusions Two flavonoid subgroups are found in Tropea Red onion, the anthocyanins, which give a red/purple colour to some varieties and flavanols such as quercetin and its derivatives responsible for the yellow and brown skins of many other varieties. At least 6 different flavonols have been characterised in this work, being quercetin and quercetin derivates the most predominant pigments. Quercetin-4’-glucoside and quercetin-3,4’-diglucoside were the main flavonols of Tropea Red onion. Also have been detected the anthocyanins cyanidin 3-glucoside, cyanidin 3-6”-malonylglucoside and cyanidin 3-6”-malonyl-3”-glucosylglucoside. In addition, minor variants of flavonoids and anthocyanins have been detected. Onions are widely used all over the world and their outer dry layers go to waste. This study shows that just in the outer layers of red onions are concentrated nutraceutical substances, mainly phenols, with promising antioxidant and free radical scavenging activities and ability to provide protection against DNA damage caused by reactive oxygen species. This study together with previous work suggests the double synergistic action of phenols in scavenging Reactive Oxygen Species (ROS) and repairing DNA radicals. Given the genetic diversity within onions, a detailed chemical profiling of their compositions is required in order to understand the range of variation which may exist and to identify further components which may offer enhanced health benefits.
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