Cyanogenic glycosides and HCN

I'm researching cyanide for a biology project and noted that one of the largest sources of carbohydrates in the tropics is the cassava root -- which I previously researched as a food that contains a cyanogenic glycoside that is metabolized into hydrogen cyanide.

Most of the sources I reviewed claimed that, while boiling the cassava, the cyanogenic glycosides were broken down into components such hydrogen cyanide, which made the plant safer to consume. I have a limited understanding of chemistry and can't resolve why this is considered a safe way to cook the plant.

My research indicates that inhalation of hydrogen cyanide is significantly more toxic than the oral route, so why would this method be considered beneficial? Not sure why this is considered safer, unless the information I've found is not correct. Any help in putting this together would be appreciated. Thanks!

2 Answers

OK, I was too lazy to look up LD50 and LC50 of $\ce{HCN}$ for you ;)

But if you have a chance, have a look at

G. Padmaja & Keith H. Steinkraus,
Cyanide detoxification in cassava for food and feed uses, Critical Reviews in Food Science and Nutrition, 1995, 35(4), 299-339 [DOI]

Seemingly, the processing of cassava (manioc) involves several steps, such as drying in the sun, grating, watering and cooking, each of which either facilitates the enzymatic or thermal cleavage of the linamarin (i.e. the toxic compound) to glucose or supports the release of $\ce{HCN}$.

In total, the processing minimizes the risk of exposition through inhalation. None of the steps is perfomed in confined space, watering helps to wash out the $\ce{HCN}$ which in water dissociates a bit ($\ce{HCN}$ is a weak acid).

Thus, digestion of the untreated roots is likely to be fatal, while the slow processing is apparently not.

Ok, I digged a bit and maybe you have you have access to this one:

Bala Nambisan

Strategies for elimination of cyanogens from cassava for reducing toxicity and improving food safety Food and Chemical Toxicology, 2011, 49, 690–693 [DOI]

• Thanks for the great answer. I hadn't seen any reference to the drying and grating process which would most likely release the HCN. I don't have access to this article and the ones I've read referenced the boiling process. It does make me wonder whether the boiling process would be enough to facilitate the thermal cleavage of HCN, since the enzymatic compounds may not be available to the linamarin without the grating process. Anything from the article give insight on that question? Thanks again for your answer. – user1457323 Jul 17 '12 at 19:04
• My pleasure :) I'm afraid i don't have full access to the article either. – Klaus-Dieter Warzecha Jul 17 '12 at 19:44
• But everybody has access to Wikipedia – Georg Jul 17 '12 at 21:28
• Thanks Georg. If you can find any relevant information on Wikipedia that would be helpful, please feel free to include it here. I've already read the pages on hydrogen cyanide, linamarin, and cassava, but if there's anything I'm missing let me know. – user1457323 Jul 18 '12 at 19:58
• If You read the forum rules, You will find that asking questiones easily anwered in wiki as well as links/quotes from Wiki are not welcome. You read the article on Cassava in English? I doubt that! – Georg Jul 19 '12 at 9:44

And we can use google too @Greg !!

"The cyanide ion (CN-) is the toxic moiety in hydrogen cyanide. The toxicity of simple cyanide salts, such as potassium and sodium cyanide is, therefore, similar to that of hydrogen cyanide." and from link "Toxicities: Cyanide is the least toxic of the "lethal" chemical agents. The LCt50s of AC and CK by inhalation have been estimated to be 2500-5000 mg"min/m3 for AC and about 11,000 mg"min/m3 for CK. LD50s for hydrogen cyanide have been estimated to be 1.1 mg/kg for intravenous administration and 100 mg/kg after skin exposure. The oral LD50s for sodium and potassium cyanide are about 100 and 200 mg/kg respectively." AC - the term refers to the anion CN-, or to its acidic form, hydrocyanic acid (HCN). Have a look at link absorption, metabolism and toxicity of cyanides

• Thanks, I appreciate your response. Like I mentioned in my question, I don't have a background in chemistry so I'm not familiar with some of the terminology, such as the LCt50 ratings. The toxicity thresholds mentioned in the second link suggest 150 mg/m3 as a lethal exposure via inhalation to HCN gas. I haven't found data on the oral LD50 of linamarin in humans -- but in rats it is around 500 mg/kg, so I think we can assume it is substantially higher in humans. I'll look into estimating the concentration of the vapors in a typical kitchen and see if I can find the answer there. – user1457323 Jul 18 '12 at 19:55
• This "answer" is off topic, nobody spoke of toxicity of various cyanic chemicals. Who is that "Greg"? Me? – Georg Jul 19 '12 at 9:47
• The answer is as actual as could be - If u read all the resources provided in links u would realize that, but who cares, the better way is to send one answer down. is that correct Georg ? – java_xof Jul 20 '12 at 5:29