Next time around while you're in the kitchen don't throw away the dry onion skin, as it are 'rich in compounds that are beneficial for human health', according to scientists.
According to the researchers, the brown skin and external layers are rich in fibre and flavonoids, while the discarded bulbs contain sulphurous compounds and fructans.
"One solution could be to use onion waste as a natural source of ingredients with high functional value, because this vegetable is rich in compounds that provide benefits for human health", said Vanesa Benitez, a researcher at the Department of Agricultural Chemistry at the Autonomous University of Madrid (Spain).
Benitez''s research group worked with scientists from Cranfield University ( United Kingdom) to carry out laboratory experiments to identify the substances and possible uses of each part of the onion.
According to the study, the brown skin could be used as a functional ingredient high in dietary fibre (principally the non-soluble type) and phenolic compounds, such as quercetin and other flavonoids (plant metabolites with medicinal properties). The two outer fleshy layers of the onion also contain fibre and flavonoids.
"Eating fibre reduces the risk of suffering from cardiovascular disease, gastrointestinal complaints, colon cancer, type-2 diabetes and obesity", added the researcher.
Phenolic compounds, meanwhile, help to prevent coronary disease and have anti-carcinogenic properties. The high levels of these compounds in the dry skin and the outer layers of the bulbs also give them high antioxidant capacity.
The study is detailed in the journal Plant Foods for Human Nutrition.