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Toxins

Some kinds of raw beans contain a harmful, tasteless toxin: the lectin phytohaemagglutinin, which must be removed by cooking. Red kidney beans are particularly toxic, but other types also pose risks of food poisoning. Even small quantities (4 or 5 raw beans) may cause severe stomachache, vomiting, and diarrhea. This risk does not apply to canned beans because they have already been cooked. A recommended method is to boil the beans for at least ten minutes; under-cooked beans may be more toxic than raw beans.

Cooking beans, without bringing them to a boil, in a slow cooker at a temperature well below boiling may not destroy toxins. A case of poisoning by butter beans used to make falafel was reported; the beans were used instead of traditional broad beans or chickpeas, soaked and ground without boiling, made into patties, and shallow fried.

Bean poisoning is not well known in the medical community, and many cases may be misdiagnosed or never reported; figures appear not to be available. In the case of the UK National Poisons Information Service, available only to health professionals, the dangers of beans other than red beans were not flagged as of 2008.

Fermentation is used in some parts of Africa to improve the nutritional value of beans by removing toxins. Inexpensive fermentation improves the nutritional impact of flour from dry beans and improves digestibility, according to research co-authored by Emire Shimelis, from the Food Engineering Program at Addis Ababa University. Beans are a major source of dietary protein in Kenya, Malawi, Tanzania, Uganda and Zambia.