Most cooking mistakes with beans cost nothing worse than a tough texture or a long wait. Red kidney beans are the exception. Raw or undercooked, they contain enough of a natural toxin to make people ill. The rule for handling them safely is short, and it comes from the US Food and Drug Administration.
What the toxin is
The FDA's page on natural toxins in food names it phytohaemagglutinin, often shortened to PHA. PHA is a lectin. The FDA describes lectins as proteins that bind to carbohydrates, and explains that some plants make them as a natural defense. PHA is found in raw or undercooked beans.
Utah State University Extension adds two points about how PHA behaves. It is very heat stable, which is why gentle heat alone may not destroy it, and acid, such as tomatoes in a sauce, makes it more stable still. Both points explain the shape of the safety advice below: soak, throw away the soaking water, then boil hard in fresh water before any slow or acidic cooking.
Which beans carry the most
The FDA's warning is written about kidney beans. Utah State University Extension says that among the dry beans commonly sold in the United States, dark red kidney beans and black, or turtle, beans contain the most PHA, and that the other regularly eaten dry beans contain negligible amounts.
For a home cook that suggests a simple habit. Give red kidney beans the full FDA treatment every time. Give black beans the same care when they are going into a slow cooker or an acidic dish, as Utah State advises for dry beans generally. And cook every dry bean until it is fully tender, which is good practice anyway.
What it can do
The FDA says the low levels of PHA in canned and properly cooked kidney beans will not affect you, but that the high levels in raw beans can cause nausea, severe vomiting and diarrhea. Utah State University Extension describes the same digestive symptoms and warns that even small amounts can be harmful.
This guide does not describe how many beans it takes to cause illness, how quickly symptoms start or how they are treated. Those are medical questions, and the sources used here do not give figures a home cook could rely on. The point is to avoid the exposure in the first place.
The FDA soak-and-boil rule
The rule for dry red kidney beans
The FDA says that soaking the beans for a minimum of 5 hours, and then boiling them in fresh water for at least 30 minutes, will remove and destroy the toxin.
In practice that means three steps.
- Soak for at least 5 hours. The overnight method in the soaking guide meets this easily. A one-hour quick soak does not.
- Discard the soaking water. Drain the beans, throw the water away and rinse them.
- Boil in fresh water for at least 30 minutes. A real boil, with the water bubbling hard, not a gentle simmer. After that the beans can be simmered until tender, or added to a recipe.
Some extension publications give shorter boiling times; Utah State, for example, describes boiling dry beans on the stove for 10 minutes before they go into a slow cooker. This guide follows the FDA's longer figure for red kidney beans, because it is the federal food-safety guidance and it leaves a margin. Where two sources differ on a safety step, the more cautious one costs only time.
Slow cookers and microwaves
Slow cookers need particular care, because they are designed to cook gently. Utah State University Extension warns that slow cookers may not get hot enough to cook dry beans fully, and says dry beans should be boiled on the stove before they go into one. For red kidney beans, do the full FDA soak and 30-minute boil first, then add the beans to the slow cooker.
Utah State also does not recommend cooking dry beans in a microwave, because microwaves heat unevenly. If a recipe uses a microwave, the beans should be soaked and boiled on the stove first.
Example: a slow-cooker chili
A recipe says to put dry red kidney beans, tomatoes and spices into a slow cooker and leave it on low all day. Following the FDA rule, the cook soaks the beans overnight, drains and rinses them, boils them in fresh water for at least 30 minutes on the stove, drains them again, and only then adds them to the slow cooker with the other ingredients. The tomatoes go in after the boil, since acid makes PHA more stable. This example describes no real recipe.
Tomatoes and other acids
Many bean dishes include tomatoes, vinegar or citrus. Utah State University Extension says acid makes PHA more heat stable and also slows the beans' uptake of water, and advises boiling or soaking beans before acidic ingredients are added. The University of Nebraska-Lincoln, for texture reasons, advises adding acidic ingredients after the beans are cooked. Both point the same way: cook the beans first, then add the acid.
Canned kidney beans
Canned kidney beans are a different case. The FDA says the low levels of PHA in canned kidney beans will not affect you. Utah State University Extension explains that commercially canned kidney beans are processed at high temperature and pressure, which destroys most of the PHA, and that they can be used in any recipe, including slow-cooker dishes and dishes with tomatoes. Canned beans are therefore the simplest choice when there is no time for the soak and boil.
Canned goods have their own safety checks, such as throwing away bulging or deeply dented cans. Those are covered in the storage guide.
After cooking
Once kidney beans are properly cooked, they are handled like any other cooked food. The USDA Food Safety and Inspection Service (FSIS) calls the range from 40°F to 140°F (4.4°C to 60°C) the danger zone, because bacteria grow most rapidly there, and advises not leaving perishable food out for more than 2 hours, or 1 hour when the air is above 90°F (32.2°C). Cooked beans should be eaten, kept hot, or cooled and refrigerated within that time.
If someone becomes unwell
Food poisoning can be serious. If someone becomes ill after eating beans, or any food, contact a doctor or other qualified health professional, and seek emergency care for severe symptoms. A website cannot assess an illness.
Next from the pantry: Canned beans, cooked leftovers and pantry storage