Raw green onions are spicy, but heated green onions are sweet. Why is this? More specifically; what change in the raw green onion's organic chemicals makes it no longer spicy when the green onions are slightly burnt, and is there some chemical change in the burnt green onions that also make it sweeter, or is it just the removal of the spicy compound?

Korean original text: 생 파는 맵지만, 구운 파는 달다. 이것은 왜인가? 더 자세히 말하자면, 생 파에서 무슨 화학변화가 일어나서 더 이상 안 매워지는것이고, 구운 파에서 어떤 화학 변화가 일어나서 달아지는것인가, 아니면 매운 성분만 제거되는것인가?

Raw green onions are spicy, but heated green onions are sweet. Why is this? More specifically; what change in the raw green onion's organic chemicals makes it no longer spicy when the green onions are slightly burnt, and is there some chemical change in the burnt green onions that also make it sweeter, or is it just the removal of the spicy compound?