This substance is stewed with iron ions in an iron pot, and the color becomes darker. There is also a book that lotus root is rich in polyphenols, which produce a chemical substance under the action of polyphenol oxidase, and then combine with iron ions in an iron pot to turn black. Lotus root will slowly turn black after being cut, and gallic acid, one of these polyphenols, will form a blue-black substance after combining with iron ions, so this can explain why lotus root will turn black when cooking soup in an iron pot.
If you cook lotus root slices, you can use aluminum pot or stainless steel pot, put a little salt in the pot with water that can submerge the lotus root slices, and drop a little white vinegar, so that the cooked lotus root slices are crisp and refreshing, and the color is still white, which turns blue or dark green when it meets iron. Therefore, it is not suitable to cook in an iron pan. Bian Xiao suggested using a casserole, and it is best not to use a stainless steel knife when cutting. Moreover, the lotus root itself is easy to oxidize, and it will oxidize and turn black when cut in the air.
Pot-stewed lotus root soup is not acceptable, because contact between iron and lotus root will produce a chemical reaction. Stewed soup can't be cut with a knife. You can break it with a stick and then break it with your hands. You can use non-ferrous metal pots such as porcelain pots. It is also possible to burn in boiling water at 100 degrees for about 30 seconds. In addition, another reason for the discoloration of tannins is that they form dark tannins with metals, which will turn blue or dark green when they meet iron. It is for these reasons that it is better to avoid using iron pots and casserole when cooking lotus roots. It is best to cut lotus roots with a stainless steel knife.