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Fruit and cyanide

Cyanide and Fruit

I came home from work to discover that my wife had spotted a question by another woman asking if elderberries contain cyanide, and is it safe to give elderberries juice to her children? This question made me think, I hold the view that the majority of the public have heard a large number of tales about cyanide and as a result the “C word” does tend to get their attention. Ask a member of the public about aflatoxin or beryllium and they are unlikely to have heard of these,[1] but ask them about “cyanide” and they will have heard about it.

A quick yahoo search on “Agatha Christie” and cyanide reveals “Sparkling Cyanide” and “And Then There Were None” which are both tales, which include murders committed with cyanide. It has been carried by spies, terrorists and solders as part of a suicide kit. Lastly hydrogen cyanide was used by the Nazis in some of their genocidal crimes and it has been used for fumigation so many of the general public have heard of the poison.

Rather than just dismissing the question of the member of the public, I thought about the question. Many plants try to protect themselves by forming their own agrichemicals to keep pests at bay. These chemical defences vary greatly, azadirachtin (from the neem tree) acts as a very strong appetite suppressor in insects[2], nicotine kills insects and it is found in a plant and many plants contain cyano-sugars. The cyano-sugars can harm animals which eat the plant. I feel that in modern times that a poison in a plant can be either very good or very bad for the survival of the plant. If the poison is a substance of use to humans then people will tend and care for the plants and make sure that they survive at least until harvest time, while if the poison has no use in society then the plant will be viewed as a nasty weed which humans might attempt to kill by uprooting it, burning it, or poisoning it with herbicides.

The classic plant in my mind with cyano-sugars is cassava, which is an African vegetable which contains linamarin and lotaustralin which can decompose into glucose, acetone (or methyl ethyl ketone) and hydrogen cyanide. The cyano-sugar can be thought of as cyanide with a special delivery system, which allows it to reach the sensitive parts of animals with greater ease. Some time ago in Reading one of the PhD students working on metal cyanides discovered that he could buy the cassava from a specialist grocers shop, sadly, during my time there he never worked out how to prepare the cassava for eating so I have no idea of how to cook it or how it tastes.

Figure 1. The molecular structure of linamarin

Back to topic of Elder trees, I checked the literature and the trees do contain sambunigrin which is a cyano-sugar. While this is a substance which can decompose to glucose, benzaldehyde and hydrogen cyanide the mere fact that it can be detected in the plant does not mean the whole of the plant is dangerous. For example the stem of rhubarb is quite safe to eat while the leaves are very unsafe due to their high oxalic acid content. Also just because something can be detected does not mean it is at a dangerous level, as Paracelsus commented “the dose makes the poison” which means that for some things a small dose might be harmless while a large dose is very harmful to health.

I have already asked the Health Protection Agency and some botanists where in the plant the cyano-sugars are present; when they answer me I will pass on their words of wisdom. Below is shown the shape of a molecule of a cyano-sugar (taxiphyllin).

Figure 2. A molecule of taxiphyllin.

After seeing the molecular structure of substances such as taxiphyllin, I asked myself how it forms. The chemical literature revealed that an amino acid such as phenylalanine is decarbonylated and converted into an oxime. The oxime is then dehydrated to form the cyanide group. After hydroxylation, the cyanohydrin is then attached to a sugar such as glucose to finish off the formation of the molecule.

Figure 3. The biosynthesis of Sambunigrin.

It is important for me to give you one warning, some of the food chemists seem to speak a slightly different langauge to other chemists. They equate cyanide and cyanogen.[3]Cyanogen is a pseudohalide which is dinitrile.Cyanogen is formed when hydrogen cyanide is passed through a pad of silica which contains copper(II) salts (cupric copper in old langauge).

 

 


 

 

[1] I do not like the idea of ingesting any poison, but after considering the evidence both beryllium and aflatoxins seem particularly vile and odious substances.

[2] J.N.Bilton, H.B.Broughton, P.S.Jones, S.V.Ley, Z.Lidert, E.D.Morgan, H.S.Rzepa, R.N.Sheppard, A.M.Z.Slawin, D.J.Williams, Tetrahedron, 1987, 43, 2805.

[3] M.R. Haque and J.H. Bradbury, Food Chemistry, 2002, 77, 107.

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2 Responses

  1. The seeds of peaches and related fruits contain the cyanohydrin of benzaldehyde. So just eat the peach and not the seed!

    As you have written there are quite a few natural cyanohydrins.

  2. True the flesh of the peach is the part you should eat and not the stone at the middle. I know that apple pips contain some cyanide. I have on a few times taken an apple pip and crushed it with a tea spoon against a ard surface. I can oftein smell HCN when I do this.

    About 30 % of the population can smell HCN, I am one of them. I would say that HCN does not smell like almonds to me. I once had a bunch of undergrads who were working with benzaldehyde. They started yelling that they could smell cyanide as they had read that it smells like almonds. PhCHO smells like almonds. I soon explained that they were not being exposed to HCN.

    One problem is that I can not experience the world through someone else’s senses, so while I can describe how something smells to me I can not know how it smells to you.

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