Chemical composition of onion

Onions possess strong characteristic aromas and flavors, which have made them important ingredients in food. It has been shown that bioactive compounds are present in every part of onion bulb. Onions are an important source of several phytonutrients as flavonoids, fructooligosaccharides (FOS), and thiosulfinates and other sulfur compounds, recognized as important elements of the Mediterranean diet.

Onion has been found to contain quercetin, fructose, quercetin-3-glucoside, isorhamnetin-4-glucoside, xylose, galactose, glucose, mannose, organosulfur compounds, allylsulfides, flavonoids, flavenols, S-alk(en)yl cysteine sulfoxides, cycloalliin, selenium, thiosulfinates, and sulfur and seleno compounds.

Flavonoids are the major phenolics in onions, which can be classified to different subclasses (flavones, flavanones, flavonols, isoflavones, flavanonols, flavanols, chalcones, and anthocyanins) on the basis of the degree of unsaturation and the degree of oxidation of the central ring. The most abundant flavonols found in onions are quercetins, namely quercetin-40-monoglucoside and quercetin-3,40-diglucoside, which account for more than 85% of the total flavonoid content.

Quercetin has demonstrated antioxidant and free radical scavenging power and its capability to protect against cardiovascular disease. Onion skins contain higher concentrations of quercetin aglycon than the flesh.

In addition to flavonoids, onions, especially the red varieties, are a rich source of anthocyanins. The most frequently reported anthocyanins in red onions are cyanindin derivatives, although minor amounts of peonidin, petunidin, and delphidin derivatives have also been identified.
Chemical composition of onion