I've purchased yellow "butter beans" immature in the pods from a local farm stand. They are very good. But I just learned that lima beans have (or convert into) cyanide and so mature beans require long cooking to remove this "protective mechanism" that beans of this sort have evolved. My question is whether there are issues in cooking (4-5 minutes) "fresh" butter beans just like regular green beans or if there are risks or precautions?
Lima beans or butter beans (same species – Phaseolus lunatus L.) contain linamarin, a cyanogenic glycoside. Many other plants, particularly cassava, contain these types of compounds. When the plant is damaged (chewing) or at early stages of processing (crushing, stirring, etc), cyanide can be released – believed to provide a protective and evolutionary advantage to the plant.
Humans have the ability to detoxify small amounts of cyanide as long as it isn’t consumed frequently and that they have sufficient protein in their diets. Sufficient protein in the diet provides adequate sulfur for your liver and muscle tissues to support excretion through urine. There are parts of Africa that suffer from chronic cyanide poisoning (a disease called konzo) from having to live off of under-processed cassava during droughts.
The cyanide content of wild lima beans can be very high (Costa Rica, Mexico, Nigeria: 3,000-4,000 mg/kg) Fortunately, in the US, commercially grown lima beans must have <200 mg cyanide/kg. The cyanide content in US lima beans is usually 100-170 mg/kg. I could not find any data on cyanide concentration in fresh lima/butter beans; however, it is likely the cyanide concentration would be lower in fresh beans.
Cooking lima beans can destroy the enzymes that release the cyanide; however, total cooking time and method of cooking will have a different effect on cyanide removal. Most cooking research related to cyanide removal has been conducted on cassava. Boiling in water for long periods of time (>30 min) in a large excess of water is the most effective method for reducing cyanide (80% of the original cyanide will be removed). Soaking in water for 24-48 hours, draining water, and then boiling for a short period of time (<5 min) in fresh water is also effective. Steaming is less effective than boiling for reducing cyanide levels.
See these online resources:
- Processing Techniques to Reduce Toxicity and Antinutrients of Cassava for Use as a Staple Food
- Effects of Processing Techniques on Chemical, Functional and Microbial Properties of Two Varieties of Lima Beans and Maize
Unfortunately, we have no information on the effectiveness of cooking methods commonly used for fresh butter beans. However, given the relatively low cyanide level in US lima beans and the likely lower concentration in fresh beans, boiling or steaming beans for 5-10 minutes would cause a reduction in the amount of cyanide (as compared to eating fresh lima/butter beans out of the garden. Assuming you are getting enough protein in your diet and that your diet isn’t exclusively these beans, it is unlikely that you will have any problems from eating these beans.