Why does a cooked onion taste sweet?

Answer 1

You might notice that a variety of foods taste sweeter or juicier after cooking. In heating these foods, you break down the large, complex molecules within them and create smaller molecules that are easier to digest. Your tongue is more sensitive to these smaller molecules; after all, if your body has the choice, it prefers them to their larger, less digestible analogues. Heating an onion is no different: since the onion bulb stores nutrients for the plant, it is full of chains of fructose called "fructan". When these break apart, they taste very sweet.

Answer 2

well. see the thing is when onions are cooked, the chemicals that cause the tears jut burn away with the fire.

AND if you have noticed, onions are never fried alone. there is always an oil, butter or some other marinate that helps cook them.

AND the fact that when onions are burned, its chemical burns down to simple constituents and the bitterness goes away. simple