I'm attempting my first slow-cook today but, of course right after I started it, I read that there's some nasty chemical in uncooked kidney and cannellini beans that can cause vomiting. And apparently slow-cooking, even for eight hours, is not sufficient to deactivate it.

I'm confused as to whether this applies to just dried kidney beans, or if it is also true for canned beans one gets from a store (i.e. in water). Will these have been pre-cooked to remove this toxin?

  • 2
    What toxin would that be? Canned beans are pre-cooked, but usually as part of the canning process--that is to say, they are cooked in the can. The thing is, if canned beans were toxic, after many years of such beans being sold and consumed, it would have come to light by now.
    – SAJ14SAJ
    Jan 25, 2013 at 12:27
  • Disregard comment--did some googling. See answer below.
    – SAJ14SAJ
    Jan 25, 2013 at 12:33
  • All beans contain some level of the toxin. Common French green beans are at a low enough level to eat a normal serving raw. At the other end of the scale they must be cooked and the cooking water discarded
    – TFD
    Jan 26, 2013 at 5:37
  • 3
    @SAJ14SAJ your comment is still valid though, if those beans were poisonous people'd have died or fallen ill in their millions by now as a result of eating them.
    – jwenting
    Jan 26, 2013 at 5:39

2 Answers 2


If Wikipedia is to be trusted (and in this case, their source is the FDA), there is in fact a toxin in some raw beans, such as kidney beans.

The toxic compound phytohaemagglutinin, a lectin, is present in many common bean varieties, but is especially concentrated in red kidney beans. White kidney beans contain about a third as much toxin as the red variety; broad beans (Vicia faba) contain 5 to 10% as much as red kidney beans.3

Phytohaemagglutinin can be deactivated by boiling beans for ten minutes; the ten minutes at boiling point (100 °C (212 °F)) are sufficient to degrade the toxin, but not to cook the beans. For dry beans, the U.S. Food and Drug Administration (FDA) also recommends an initial soak of at least 5 hours in water, which should then be discarded.3

During the pressure-canning process beans undergo (they are fully cooked in the can), the toxin is certainly deactivated. Canned beans are ready to eat, even cold, although they probably taste better hot, and with some flavor from a sauce or accompanying dish.

The danger would be in slow-cooking dry beans of this variety, which have never been previously cooked.

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    The FDA in the 2012 edition of the Bad Bug Book says: "Studies done by British scientists suggest that beans should be soaked in water for at least 5 hours, the water poured away, and the beans boiled in fresh water for at least 30 minutes." pg 254 fda.gov/downloads/Food/FoodSafety/FoodborneIllness/…
    – Fisher
    Jan 25, 2013 at 17:10

Kidney beans have high levels of phytohaemagglutinin, which is a protein that can mess with cell walls and cell metabolism. If you don't cook the beans enough, this protein can make you really sick.

The good news is that boiling kidney beans for 10 minutes takes care of the problem. The protein breaks down and leaves your beans perfectly safe to eat. So canned beans and dried beans that have been simmered on the stove are fine, but slow cookers don't ever hit the boiling temperature. Since the beans never get hot enough to kill the protein, slow-cooked beans can still make you sick.

I just wrote about this on my food science blog. Check it out! http://www.fchem101.com/2014/09/kidney-bean-poisoning/

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