Most people believe that as long as food is refrigerated, it’s safe. But some ingredients change in ways you can’t see or smell. Mushrooms and wood ear fungus, for example, contain complex proteins and naturally high nitrates. After sitting overnight, especially if not cooled quickly, those nitrates can convert into harmful compounds, and reheating stresses the digestive system further. The safest choice is to cook only what you’ll eat that day and avoid “saving” mushroom dishes for tomorrow.

Leafy greens are another hidden risk. Spinach, lettuce, bok choy, and similar vegetables are rich in nitrates that can transform into nitrites when left too long at room temperature or repeatedly reheated. This process is especially concerning for children and people with sensitive stomachs. To protect your health, serve leafy greens fresh, refrigerate promptly, and discard any that have been sitting out or stored for more than a short time.