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?
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.
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/
