Don’t eat the core, it’s poisonous: it's something parents often say to their children before they eat their first peach. Peach pits, which are hidden inside the nut-like husk, do in fact contain amygdalin, a substance which can degrade into hydrogen cyanide in the stomach.

But peaches, apricots and almonds didn’t develop this defence system to keep children from enjoying their fruit. It is actually nature’s way of protecting plant seeds from being eaten by insects.

Chemists from Wendelin Stark’s research group at ETH Zurich have drawn inspiration from this and copied the defence system of bitter almonds and other members of the Prunus family in the lab. They are developing a coating for seeds that functions in the same way as this natural model and is just as effective, but doesn’t impair seed germination and is also biodegradable. The study accompanying their work was recently published in the Journal of Agricultural and Food Chemistry.

Cyanide as a by-product of insect snacking

To determine the efficacy of a coating, the researchers tested different layer sequences. The sequence that finally emerged as the most effective consists of several layers of polylactic acid (PLA), a substance that is harmless to both humans and the environment. The innermost layer contains an enzyme. On top of this lies a layer of pure polylactic acid, followed by two layers in which the hydrogen cyanide precursor amygdalin is embedded – the same substance found in the husks of bitter almond seeds. A final layer is composed of pure PLA.

If an insect larva chews through these layers, the amygdalin is released, followed by the enzyme. The two substances mix together and the amygdalin breaks down into hydrogen cyanide, which kills the insect larva’s appetite – or even the creature itself.