Onions. Many people take onion "helpless", with iron pan fried will be discolored, with aluminum pan fried will be discolored. This is because onions contain a substance called flavonoids, flavonoids meet iron, aluminum and other metals will generate brown, blue, black and other compounds, so that the color of the onion is not translucent. The best solution is to cut the onion dipped in some dry flour and then stir-fried, so as to avoid flavonoids directly in contact with iron and aluminum, onions will not be discolored, the taste is also more crispy.