The cooked food contains water, and the water reacts with iron to produce rust, which causes the soup to turn black. Iron can easily precipitate iron ions at high temperature and react with other salts and acids to form iron compounds, which generally occur in acidic foods, such as spinach and tomatoes.

It is not recommended to use iron utensils to make soup. The so-called soup takes a long time, and the longer it is at high temperature for a long time, the more iron compounds are produced, and it is toxic. Iron pots are easy to rust, and the human body absorbs too much iron oxide, that is, after rust, it will do harm to the liver. When using iron pots, it is necessary to abide by some principles in order to be healthy.

Extended information:

Iron pot is a traditional kitchen utensil for cooking food, which generally does not contain toxic substances, but it will oxidize when exposed to water for a long time. The main varieties of iron pot are printing pot, ear pot, flat pot, oil pot, pancake pot and so on. The main component is iron, but also contains a small amount of sulfur, phosphorus, manganese, silicon, carbon and so on.

Ordinary iron pots are prone to rust, and the human body absorbs too much iron oxide, which is harmful to the liver ... Therefore, when people use iron pots, they need to follow some usage principles to be healthy. These principles are:

1. After frying a dish, brush the pot once and fry another dish. After each meal is finished, the inner wall of the pot must be cleaned and the pot must be dried, so as to prevent the pot from rusting and producing iron oxide which is harmful to human health.

2. Try not to cook soup in an iron pot.

3. Don't use an iron pot to hold vegetables overnight, because iron will dissolve out in the iron pot under acidic conditions, which will destroy vitamin C in the vegetables.

4. Use as little detergent as possible when washing pots. If there is slight rust in the pot, it can be cleaned with vinegar.

5. The iron pot with serious rust, black slag and black skin should not be used again.

in addition, the iron pot is not suitable for decocting medicine, and it is not suitable for cooking mung beans with iron pot.