Raw dried beans contain a substance called phytohemagglutinin. Heat breaks it down, but the water must actually boil; warm cooking does not destroy it. Soak the beans for at least 5 hours, discard the soaking water, boil them vigorously in fresh water for at least 10 minutes, and then cook them until tender. The amount varies by type and is highest in red kidney beans.
Dried beans contain a protein the plant produces for its own protection; it is called phytohemagglutinin. It remains active in raw or undercooked beans and can cause nausea, vomiting, and abdominal pain soon after they are eaten. You do not need to eat a large amount; even a few beans may be enough. Effects usually begin within a few hours and pass on their own, but the illness can be serious. For this reason, do not treat dried beans as an ingredient whose doneness can be judged by tasting.
Heat destroys this substance, but not just any heat. The water must reach a rolling boil for it to break down; temperatures below the boiling point are not enough. The practical rule is to drain soaked beans, cover them with fresh water, and boil them vigorously for at least 10 minutes. These 10 minutes do not soften the beans; they only deactivate the substance. Continue cooking normally until tender. After the water starts boiling, leave the lid ajar and stay by the pot, because beans can easily boil over.
The most dangerous mistake is assuming that cooking at a low temperature is sufficient. In beans held for a long time below the boiling point, the substance does not break down; its amount has also been reported to increase in this temperature range. In other words, slow, prolonged cooking is not safer than a brief, vigorous boil. For this reason, never start dried beans over low heat; bring them to a boil on the stove first. For the same reason, do not cook beans by leaving them overnight in a thermos or a container of hot water.
The amount varies by type. It is notably high in red kidney beans; white beans, cranberry beans, and other dried bean varieties contain less, but still have some. Lentils and chickpeas do not present the same degree of concern. Still, the safest approach is to handle them all the same way: cook every dried bean by boiling, and never taste it raw or partially cooked. Do not let children play with raw beans in the kitchen either; even a small amount may have an effect.
Canned beans are outside this discussion. They have already been heat-treated during canning, so the substance is inactive; they can be rinsed and added directly to a salad. The same applies to frozen cooked beans. For dried beans, check doneness by texture rather than appearance: if a bean crushes easily between two fingers, it is cooked; if a firm center remains, continue cooking. If cooked beans will be stored, cool them quickly, refrigerate them, and use them within a few days. Repeat this check with every batch, because the age of the legumes can noticeably change their cooking time.