I have to publish my first major retraction today. I can't believe I didn't know about this before, but...do NOT follow my old slow cooker beans technique when cooking beans, especially kidney beans, cannelini beans, or broad beans.
Do this instead:
Put the beans in a pot, cover with 2 inches of water, and bring to a boil. Boil for ten minutes, then drain. Put the drained beans into the slow cooker and add 8 cups of water. Cook on low for 8 hours.
Red kidney beans contain high levels of a toxin that occurs naturally in beans, Phytohaemagglutinin (also known as Kidney Bean Lectin.) If raw or undercooked red kidney beans are eaten...well, bad things happen. Let's just say you'll be getting to know the pattern of tiles in your bathroom very well.
For all the gory details, check here.
With normal cooking, this isn't a problem. The toxin is destroyed after ten minutes of boiling (cooking at 212*F or 100*C.), and most stovetop recipes easily take care of the toxin with their hours of simmering at that temperature. Raw, soaked beans are bad; eating four of them is enough to cause symptoms. Now, here's the problem with slow cookers: undercooked beans (cooked at 80*C, roughly 170*F) increase the toxicity fivefold. And, most slow cookers have their "low" setting at about 180*F. If your slow cooker runs a little cooler than most... England tracks this more carefully than we do here in the US. They had seven outbreaks of kidney bean poisoning between 1976 and 1979, caused by raw, soaked kidney beans or kidney beans cooked in a slow cooker.
The toxin occurs in red kidney beans, and to a lesser extent in white kidney beans (cannellini beans) and broad beans. Other beans contain the toxin, but at much lower levels.
How did I miss this? I never had, um, gastrointestinal issues from slow cooked kidney beans. My slow cookers must run hot enough that the toxin was destroyed. Also, I prefer black or pinto beans to kidney beans, so those are the beans I'm usually slow cooking. In the future, I am always going to boil my beans for at least 10 minutes before cooking them in a slow cooker. This takes some of the simplicity out of the "dump and cook" technique that I use, but it's still pretty simple. And the alternative is not pretty.
I want to extend my sincere apologies to anyone who followed my technique. I hope I didn't cause you any...problems.
References: Phytohaemagglutinin (Kidney Bean Lectin) [fda.gov]
Related posts: Basic Technique: Slow Cooker Dried Beans