Why does a cooked onion taste sweet and why does it not make the eyes water?

onionHave you ever wondered why a cooked onion tastes sweet and why cutting a cooked onion does not make the eyes water? Onion chemistry is extremely fascinating and extremely complex! We’ve been intrigued by this vegetable ever since our prehistoric ancestors gathered and cooked wild onions. By the time of the First Egyptian Dynasty some 5000 years ago, onions were widely consumed for flavor and for their supposed medicinal properties. At various times they have been associated with the prevention of colds, loosening of phlegm, correction of indigestion, inducement of sleep, stimulation of appetite, disinfection of wounds and driving parasites from digestive tract. In ancient times people believed that onions were a symbol of eternity because of the concentric circles that make up their structure. For this reason, onion shaped towers became popular in Russia and Eastern Europe; the idea was that these buildings would stand forever.

Onions may not make us live forever but some of their components may indeed have medical benefits in reducing cholesterol, blood pressure and perhaps even the risk of cancer. That’s why their chemistry has received a great deal of attention. We now know that slicing an onion sets off a series of reactions that begins with a compound called propenecysteine sulphoxide. The physical action of slicing liberates an enzyme called alliinase which converts it to propenesulphenic acid. This is highly reactive and converts to propanethial oxide, which is the compound that irritates the eyes and makes them water.

Frying the onion causes yet another reaction, resulting in the formation of bispropenyl disulfide which has a sweet smell and a sweet taste. Dozens of other chemical reactions also take place upon the application of heat, possibly forming other sweet substances and causing the destruction and evaporation of the strong tasting and smelling components. The reason that cooked onions do not cause us to cry is that they no longer contain any propanethial oxide. Onion research has more than a culinary goal. For example researchers in West Germany have discovered that compounds known as thiosulphinates in onions can relieve asthma. And Bela Karoly, the highly successful former Olympic gymnast coach swears by the old Transylvanian remedy of applying a cooked onion to an inflamed joint. And the only tears he had to worry about were those of his protégés.

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