When you get a package of dry beans examine the cooking instructions on the side. Most likely they’ll stress that after you’ve soaked your beans in water for an hour or two, you should discard the water before cooking the beans very, very well. That’s not just a culinary tip. Undercooking your beans can cause extreme, painful, and occasionally long-term reactions.
What you’re trying to remove with the soaking and kill with the cooking is a compound called lectin. It’s present in most beans, but especially in kidney beans. Lectin, once in your body, will seek out any cell it comes into contact with and alter that cell membrane’s ability to let anything into or out of the cell—especially proteins. Lectin is even nastier if it’s in the bloodstream. Lectin is sometimes known as hemagglutinin, because it tends to stick mammalian blood cells together in a big bundle. This is exactly what blood cells are not meant to do. The toxicity of beans is measured in hemagglutinating units, or hau. Even fully-cooked kidney beans contain about 200 hau, but raw ones contain about 70,000, which is why only five beans can make you sick.
Undercooked beans might actually be worse than raw ones. In one study, heating beans to under 80°C caused the lectin to be more active, not less. The beans know you’ve made an attempt on their life, and they don’t like it.
But don’t get the idea that beans will only make you erupt from both ends while doubling up in pain. (Although they will. The symptoms start from one to four hours after eating, and can keep going for a few hours or days if you’re very unlucky.) Lectin has one more trick up its sleeve. It triggers cell mitosis. Lectin causes cells to split again and again, very fast. Biologists actually use it, sometimes, to start up mitosis. The problem is, rapid and repeated mitosis can weaken an entire organism, leaving it vulnerable to infection and parasites. This, to be fair, has mostly been studied in the root systems of other plants, and not in animals. Still, beans not only contain something that causes their predators pain, they contain a compound that forces their competition to grow too fast so that they sicken and die.
Nasty little legumes.