What you want to ask is what causes apples to turn black when they are cooked in a health pot? The reason is that phenolic compounds in apples combine with iron, which leads to oxidative discoloration. The reason why apples turn black when cooked in a health pot may be that the phenols in apples combine with iron, and there are a lot of phenols in apples, which will produce brown or black sediments after combining with iron ions, which will lead to the blackening of apples.