Posted on Aug 10, 2011, 6 a.m.
The brown skin and external layers of onions are rich in fiber and flavonoids, while the discarded bulbs contain sulphurous compounds and fructans.
Onion production has risen over recent years in-line with the growing demand spawned by studies that assert cardiovascular and anti-cancer benefits of consuming the vegetable. As a result, onion waste -- the dry brown skin, the outer layers, roots and stalks, as well as onions that are not big enough to be of commercial use, or onions that are damaged -- has risen dramatically. Vanesa Benítez, from Autonomous University of Madrid (Spain), and colleagues identified the diverse substances and possible uses of each discarded part of the onion. The team submits that the brown skin could be used as a functional ingredient high in dietary fiber (principally the non-soluble type), noting that fiber consumption can help to reduce the risks of cardiovascular disease, gastrointestinal complaints, colon cancer, type-2 diabetes and obesity. As well, onion skin is rich in phenolic compounds such as quercetin and other flavonoids (plant metabolites with medicinal properties), which other studies suggest may help to prevent coronary disease and exert anti-carcinogenic properties. The two outer fleshy layers of the onion also contain fiber and flavonoids. As well, the dry skin and the outer layers of the bulbs are high in phenolic compounds. The internal parts and whole onions that are thrown away could be a source of fructans, a type of prebiotic with potential gastrointestinal benefits, and sulphurous compounds, which may help to reduce the accumulation of platelets, thereby improving blood flow and cardiovascular health in general.
Vanesa Benitez, Esperanza Molla, María A. Martín-Cabrejas, Yolanda Aguilera, Francisco J. Lopez-Andreu, et al. “Characterization of Industrial Onion Wastes (Allium cepa L.): Dietary Fibre and Bioactive Compounds.” Plant Foods for Human Nutrition, 2011, Volume 66, Number 1, Pages 48-57.