Toxins
Some kinds of raw beans, especially red and kidney beans, contain a harmful toxin (lectin phytohaemagglutinin) that must be removed by cooking. A recommended method is to boil the beans for at least ten minutes; undercooked beans may be more toxic than raw beans. Cooking beans in a slow cooker, because of the lower temperatures often used, may not destroy toxins even though the beans do not smell or taste 'bad' (though this should not be a problem if the food reaches boiling temperature and stays there for some time).
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.
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