ABSTRACT
World onion production has increased at least 25% over the last 10 years with current production being over 44 million tonnes making it the second most important vegetable crop after tomato. Because of the onion-storage characteristics and durability for shipping, onions have always been traded more widely than most other vegetables. Onions are versatile and are often used as an ingredient in many dishes and are accepted by almost all traditions and cultures. Onion consumption is increasing signifi cantly, particularly in the U.S.A., and this is partly because of heavy promotion that links fl avor and health. Onions are rich in two chemical groups that have perceived benefi ts to human health. These are the fl avonoids and the alk(en)yl cysteine sulfoxides. In onion two fl avonoid subgroups are found, the anthocyanins which impart a red/purple color to some varieties, and quercetin and its derivatives responsible for the yellow and brown skins of many other varieties. The alk(en)yl cysteine sulfoxides are the fl avor precursors, which, when cleaved by the enzyme alliinase, generate the characteristic odor and taste of onion. The downstream products
are a complex mixture of compounds which include thiosulfinates, thiosulfonates, and mono-, di-and tri-sulfi des. Compounds from onion have a range of health benefi ts which include anticarcinogenic properties, antiplatelet activity, antithrombotic activity, antiasthmatic, and antibiotic effects. Ioku et al. (2001) investigated the effect of cooking procedures on the contents of antioxidants and selected quercetin conjugates, total phenol compounds, and ascorbic acid to estimate the amount of fl avonoid ingestion from onion. Microwave cooking without water retains fl avonoids and ascorbic acid. Frying does not affect fl avonoid intake, but boiling of onion leads to about 30% loss of quercetin glycosides, which leach in the boiling water. Quercetin 3,4-diglucoside and quercetin 4’-glucoside are virtually unaffected by chopping. Boiling for an hour has more severe effects as it causes overall 20.6% fl avonol loss. Chopping tissues does not considerably infl uence the antioxidant capacity, but boiling does provoke notable changes (Makris and Rossiter, 2001). Processing techniques have also an impact on the fl avonoid structure, resulting in changes in bioavailability and activity of the fl avonoids. Rohn et al. (2007) studied the stability of selected model and onion quercetin glycosides under roasting conditions (180°C). Large amounts of quercetin glycosides subject to thermal processing in food production were isolated using counter current chromatography and roasted. The thermal treatment led to degradation of the quercetin glycosides; the main product being glycone quercetin, which remained stable during further roasting. During the roasting process, quercetin diglucoside isolated from onion and formation of a monoglycoside as an intermediate product was observed.
