| Boiling Point No. 15 - April 1991 |
Question: Recipes including red kidney beans always say the beans must be soaked and boiled and the cooking water discarded before use. What is the toxin in red beans and is it destroyed by heat or dissolved in the water?
Answer: Red kidney beans contain compounds called lectins (haemagglutinins) which form part of the plant's natural defensive systems, offering protection against insect and herbivore predators. Lectins are wholly or partly protein in structure and thus their biological effects are destroyed by moist heat. The beans should be thoroughly soaked to allow the water to fully penetrate and swell the tissue. Boiling is essential because at lower temperatures not all of the lectin will be inactivated and some toxicity will remain. When it was fashionable partially to cook red kidney beans, for example, for use in salads, many cases of gastro intestinal disturbance and "food poisoning" were reported. While little of the lectin will be removed by soaking alone, this process may remove very much smaller compounds such as saponins (which give the washing water its "soapy" feel) and oligosaccharides - which are a cause of inteslinal gas and social discomfort.
Lectins are found in many different legumes, the seeds generally being the richest source. Red, white, brown and black kidney beans, runner beans and tepary beans are among the most toxic in the raw state. All lectins bind to sugars in the cells of the intestinal wall, they interfere with nutrient absorption and cause reduced growth, diarrhoea, intestinal discomfort and increased incidence of bacterial infection.
GR Fenwick, Institute for Food Research, Norwich .