By Reed Mangels, PhD, RD

QUESTION: My question is regarding the soaking of legumes to reduce phytate content in order to make iron more available. What should I do with the soaking water after removing the legumes? Discard it? Are the phytates in the water or do they change their chemical structure and the water can be otherwise used in recipes?

T.M., via email

ANSWER: Dried beans contain substances called phytates. When the beans are eaten, these phytates can bind up iron, zinc, and other minerals in our intestines. The minerals cannot be absorbed when they are bound to the phytates and are excreted. One way to reduce the effect of phytates is to soak dried beans for several hours or overnight and discard the soaking water. The phytates dissolve in water so when the beans are soaked, the phytates move into the soaking water and can be discarded. Then, when the beans are eaten, there is less phytate to interfere with mineral absorption. I'd recommend discarding the soaking water and not using it in other recipes.

QUESTION: I like to cook dried beans in my slow cooker. Someone said that I shouldn't cook dry kidney beans this way, even if I soaked them beforehand. Is there any truth to this?

K.B., via email

ANSWER: Both red kidney beans and white kidney beans contain relatively high levels of phytohaemagglutinin, a substance that can cause extreme nausea, severe vomiting, and diarrhea. These uncomfortable effects have been seen in people eating as few as four or five uncooked beans. Proper cooking destroys the toxic substance. The Food and Drug Administration (FDA) suggests that kidney beans should be soaked in water for at least five hours, the water poured away, and the beans boiled in fresh water for at least 30 minutes.1 The problem with slow cookers is that they don't reach a high enough temperature to destroy the toxic substance. A number of incidents have been reported where people eating dry kidney beans cooked in a slow cooker have gotten sick.2 It's also not a good idea to soak dry red or white kidney beans and then grind the soaked, uncooked beans to use in baked goods. If you like making recipes with kidney beans in your slow cooker, either cook the beans on the stove first following the FDA's instructions or use canned kidney beans.


  1. Food and Drug Administration. Bad Bug Book, Foodborne Pathogenic Microorganisms and Natural Toxins. Second Edition. 2012.
  2. Rodhouse JC, Haugh CA, Roberts D, Gilbert RJ. Red kidney bean poisoning in the UK: an analysis of 50 suspected incidents between 1976 and 1989. Epidemiol Infect. 1990;105:485-91.