If vegetables are stored for several days before consumption, it is very dangerous. The danger comes from the nitrates contained in vegetables. Nitrates themselves are non-toxic, however, after being stored for a period of time, due to the action of enzymes and bacteria, nitrates are reduced to nitrites, which are a toxic substance. In the human body, nitrites combine with protein-like substances to form nitroso compounds, which are highly carcinogenic.
The nitrates in vegetables come from fertilizers. Fertilizers themselves are not toxic, and there is nothing wrong with fertilizing vegetables. The problem is that people often apply large amounts of a single type of nitrogen fertilizer, exceeding the plant's needs, or they fertilize just before harvesting. Because the plants do not have enough time to use all of it to synthesize nutrients, they remain in the vegetables in the form of nitrates, becoming a hidden danger. The nitrate content in nutrient-rich leafy vegetables is higher than in root and solanaceous vegetables. Storing vegetables not only produces harmful substances but also leads to a loss of nutrients. Experiments have shown that when leafy vegetables are stored for several hours in a room at 30 degrees Celsius, almost all of the vitamin C is lost, while the nitrite content increases several dozen times.
Therefore, when buying vegetables at the market, one should choose the freshest ones and should not buy withered or yellowed vegetables just because they are cheap. The storage period for fresh vegetables in the refrigerator should not exceed 3 days. Any vegetables that have already turned yellow, withered, water-soaked, or started to rot must not be eaten.