17 February 2016

Red kidney bean poisoning

Many people are unaware that kidney beans, if consumed raw, contain a dangerous toxin... The toxin is named phytohemagglutinin (PHA), a member of a very common class of proteins called lectins. Lectins are glycoproteins that are present in a wide variety commonly-consumed plant foods, particularly in the seeds. In most cases, they are not harmful and possibly beneficial, but some lectins are known to be toxic. One of the most dangerous poisons known, ricin, is a lectin derived from the seeds of the castor bean Ricinus communis (not a true bean and totally unrelated to the legume family Fabaceae); this is not, however, the same lectin found in beans and other legumes...

PHA is known to be an insecticide, and plants probably developed it to keep their seeds from being destroyed by pests. In humans and other susceptible mammals (those of us without compound stomachs) PHA attacks and disables the epithelial cells lining the intestine. The body reacts to the threat by emptying the entire digestive tract as rapidly and completely as possible, to rid itself of the toxic substance...

For the safest results in cooking dried kidney beans, they should first be soaked for several hours, the soaking water discarded, then brought to the boil in fresh water and cooked for at least ten minutes...

Gardeners, in particular, should be a aware of the potential dangers of raw beans if they like to "graze" in their gardens, eating the fresh raw seeds directly from the shell. The level of PHA in all varieties is not known.

Green beans (snap beans) are a questionable matter. Many people do like to eat young green beans raw, and overcooking green beans until mushy is widely regarded as a sin against the vegetable. I can find no clear evidence that raw green beans have PHA levels high enough to make them unsafe for most of the population. It should be noted that the level of PHA is highest in seeds, and green beans are usually consumed for the sake of the green fleshy pod, at a stage when the seeds are only beginning to develop. 
And from another source:
The syndrome is usually caused by the ingestion of raw, soaked kidney beans, either alone or in salads or casseroles. As few as four or five raw beans can trigger symptoms. Several outbreaks have been associated with "slow cookers" or crock pots, or in casseroles which had not reached a high enough internal temperature to destroy the glycoprotein lectin.  It has been shown that heating to 80 degrees C. may potentiate the toxicity five-fold, so that these beans are more toxic than if eaten raw. In studies of casseroles cooked in slow cookers, internal temperatures often did not exceed 75 degrees C.


  1. The quote in your last paragraph got screwed up...

    1. That sometimes happens when I insert a picture and then move it by dragging. Thanks for the heads-up, Swift.

  2. weren't those bug infested beans you posted about a couple of weeks ago kidney beans? and the bugs ate the beans, and they were all dead?


  3. In my youth, I spent several summers living (and working) on a relative's farm in Western Kentucky. They always warned me not to eat the lima beans in the fields because they were toxic when not cooked. I thought this to be an old wives tale, but still did not eat many. Do you know if lima beans contain this PHA?

  4. The belief that indigestible proteins in the covering of many beans and grains cause systemic inflammation is behind the "paleo for immunity" diet that people are using to treat many autoimmune disorders, including arthritis, diabetes, Hashimoto thyroiditis, and some forms of depression.

  5. We often eat green beans out of the garden when they are young and tender. My daughter has eaten bushels of them. So far, we haven't died. :)

  6. OK, fair warning: Pedantry ahead.

    FTA: "One of the most dangerous poisons known, ricin, is a lectin derived from the seeds of the castor bean Ricinus communis (not a true bean and totally unrelated to the legume family Fabaceae);"

    OK. This is one of my pet peeves. There are absolutely no "unrelated" organisms on this planet (unless I missed an alien landing). I try to be strong when, in the common parlance people with the same last name are noted as having "no relation" which is intended to mean roughly "a relation more distant than second cousin". I'm working on it I really am.

    On the other hand when the author writes an ostensibly science related story about plant species more rigor is in order. And did he have to add the intensifier "totally unrelated"?!?! Anyway they are in fact related.

    * Both are plants
    * Both are flowering plants (Angiosperms)
    * Both are eudicots (they have tricolpate pollen grains)
    * Both are Rosids (a group defined mostly their DNA since their morphology is so varied)

    They diverged from one another about 90 million years ago (at the end of the Mesozoic Era) just after the first bees evolved (which may be the cause of the rapid diversification of angiosperms around that era). That is near the origin of mammals and before the origin of ants. Dinosaurs had about 24 million years left in them before a giant rock would blow the Yucatan / Gulf of Mexico apart.


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